Physics for Secondary Schools Student’s Book Form Four Tanzania Institute of Education Physics Form 4.indd 1 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
255 735 041 170 E-mail: [emailprotected] Website: www.tie.go.tz All rights reserved. No part of this textbook may be reproduced, stored in any retrieval system or transmitted in any form or by any means whether electronic, mechanical, photocopying, recording or otherwise without prior written permission of the Tanzania Institute of Education. Physics Form 4.indd 2 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
iii Table of Contents Acknowledgements.......................................................................................... v Preface.............................................................................................................. vi Chapter One: Waves........................................................................................... 1 Concept of wave............................................................................................... 1 Propagation of waves....................................................................................... 5 Behaviour of waves.......................................................................................... 7 Sound waves..................................................................................................... 20 Musical sounds................................................................................................. 25 Electromagnetic waves..................................................................................... 39 Chapter summary ............................................................................................. 47 Revision exercise 1........................................................................................... 48 Chapter Two: Electromagnetism....................................................................... 51 Concept of electromagnetism........................................................................... 51 Magnetic field due to a current carrying conductor ......................................... 52 Electromagnetic induction................................................................................ 60 Chapter summary ............................................................................................. 80 Revision exercise 2........................................................................................... 81 Chapter Three: Physics of the atom .................................................................. 86 Concept of an atom........................................................................................... 86 Natural radioactivity......................................................................................... 93 Artificial radioactivity ...................................................................................... 104 Radiation hazards and safety............................................................................ 105 Nuclear reactions.............................................................................................. 107 Chapter summary ............................................................................................. 112 Revision exercise 3........................................................................................... 113 Chapter Four: Thermionic emission ................................................................. 115 Concept of thermionic emission....................................................................... 115 Cathode rays..................................................................................................... 116 X-rays............................................................................................................... 120 Chapter summary ............................................................................................. 123 Revision exercise 4........................................................................................... 123 Physics Form 4.indd 3 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
iv Physics for Secondary Schools Student’s Book Form Four Chapter Five: Electronics................................................................................... 125 Concept of electronics...................................................................................... 125 Band theory in solid materials.......................................................................... 128 Classification of solid materials in terms of conductivity ................................ 129 Types of semiconductors.................................................................................. 132 Diodes .............................................................................................................. 140 Transistors ........................................................................................................ 146 Electronic amplifiers......................................................................................... 150 Chapter summary ............................................................................................. 153 Revision exercise 5........................................................................................... 154 Chapter Six: Elementary astronomy................................................................. 157 Concept of astronomy ...................................................................................... 157 Constellations................................................................................................... 161 The solar system............................................................................................... 167 Gravitational force............................................................................................ 175 The Earth and its Moon.................................................................................... 177 Chapter summary ............................................................................................. 183 Revision exercise 6........................................................................................... 184 Chapter Seven: Physics of the Earth and its atmosphere ............................... 186 The concept of the Earth and its atmosphere ................................................... 186 The structure and composition of the Earth ..................................................... 187 Earthquakes and volcanoes .............................................................................. 191 Structure and composition of the atmosphere .................................................. 203 The greenhouse effect and global warming...................................................... 206 Chapter summary ............................................................................................. 211 Revision exercise 7........................................................................................... 212 Answers to numerical questions ........................................................................ 214 Appendix 1: Periodic table ................................................................................. 216 Glossary................................................................................................................ 217 Bibliography ........................................................................................................ 223 Index ................................................................................................................... 224 Physics Form 4.indd 4 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
v Acknowledgements The Tanzania Institute of Education (TIE) would like to acknowledge the contributions of all the organisations and individuals who participated in designing and developing this textbook. In particular, TIE wishes to thank the University of Dar-es-Salaam (UDSM), the University of Dodoma (UDOM), Mkwawa University College of Education (MUCE), Tanzania Atomic Energy Commission (TAEC), School Quality Assurance (SQA) Department, Teachers’ colleges and secondary schools. Besides, the following categories of individuals are acknowledged: Writers: Ms Auguster F. Kayombo (TIE), Mr Shaban J. Baya (TIE), Dr Emmanuel D. Sulungu (UDOM), Mr Daudi T. Mazengo (UDOM) & Mr Mkomwa A. Mtiga (Loyola high school) Editors: Dr Ismael N. Makundi (UDSM), Dr Innocent J. Lugendo (UDSM), Dr Talam E. Kibona (MUCE), Dr Benard S. Mwankemwa (UDOM), Dr Suleiman A. Suleiman (TAEC), Mr Tungu C. Chagu (MOTCO) & Mr Alphonce A. Mbalwa (Marian girls’ secondary school) Designer: Mr Amani J. Kweka Illustrators: Mr Fikiri A. Msimbe (TIE), Ms Rehema H. Maganga (TIE), Mr Godlove S. Kyando (TIE) & Alama Art and Media Production Co. Ltd Coordinator: Ms Auguster F. Kayombo Furthermore, TIE extends its sincere appreciation to the United States Agency for International Development (USAID)-Tanzania for granting permission to use the materials from 2011 Physics for Secondary Schools, Forms 4, (Revised Edition) textbook. TIE also appreciates the participation of the secondary school teachers and students in the trial phase of the manuscript. Likewise, the Institute would like to thank the Ministry of Education, Science and Technology for facilitating the writing and printing of this textbook. Dr Aneth A. Komba Director General Tanzania Institute of Education Physics Form 4.indd 5 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
vi Physics for Secondary Schools Student’s Book Form Four Preface This textbook, Physics for Secondary Schools is written specifically for Form Four students in the United Republic of Tanzania. It is written in accordance with the 2007 Physics Syllabus for Ordinary Secondary Schools, Form I-IV, issued by the then Ministry of Education and Vocational Training. The book consists of seven chapters, namely Waves, Electromagnetism, Physics of the atom, Thermionic emission, Electronics, Elementary astronomy as well as The Earth and the atmosphere. Each chapter contains illustrations, activities, tasks and exercises. You are encouraged to do all the activities, tasks and exercises as well as other assignments that your teacher will provide. Doing so will enable you to develop the intended competencies. Tanzania Institute of Education Physics Form 4.indd 6 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
1 Waves Concept of wave If you drop a stone into a pond of still water, some disturbance will be created on the water surface. The disturbance, in form of ripples, spreads outward in a circular pattern as shown in Figure 1.1. Figure 1.1: Water ripples Introduction In your daily activities, you may have experienced phenomena such as ripples in a water pond, musical sound from a guitar and tremors caused by earthquakes. All these are wave phenomena. Devices such as television, cellphones, radios, microwave ovens and medical diagnostic machines such as X-ray and ultrasound make use of waves in their operations. Generally, waves are important in our daily activities. In this chapter, you will learn about the concept of waves, propagation of waves, behaviour of waves, sound waves, musical sounds and electromagnetic waves. The competencies developed will enable you to explore how waves are involved in various fields including communication and medicine. You will also acquire skills that will enable you to design and repair some musical instruments and other devices that apply wave principles. This happens because when the stone hits the water surface, the energy of the stone is transferred to the water molecules at the point of contact. The molecules then vibrate up and down, disturbing the neighbouring water molecules in the process. This results into transfer of energy which is observed as the movement of the disturbance outwards. Basically, the water molecules do not move outwards with the disturbance, only the disturbance moves. The movement of the disturbance is called a wave. A wave can also be observed when you shake one end of a rope up and down. Waves Waves Chapter One Ripples Physics Form 4.indd 1 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
2 Physics for Secondary Schools Student’s Book Form Four Task 1.1 produced by dropping a stone into a water pond or by shaking a rope require a medium for their travel. For the case of dropped stone, the medium is water while for the case of the rope, the medium is the rope itself. Generally, the medium can be a solid, liquid or gas. A wave is a periodically repeating disturbance that travels through a medium from one location to another, without a net movement of the medium or particles of the medium. Given a rope, small stone, basin, water, and slinky spring 1. Tie one end of a stretched rope to a fixed object. Hold the loose end of the rope and gently shake it up and down. Observe the wave motion of the rope. 2. Drop a small stone in a basin that is three quarter filled with water. Observe the wave motion in the water. 3. Place the slinky spring on a flat surface. Hold it on one end and allow one of your friends to displace the spring sideways by approximately one centimetre and then release it. Observe the motion. 4. Discuss your observations with your colleagues in class. Wave terminologies and parameters Different terminologies and parameters are used to describe waves. These terminologies and the wave parameters can be explained well using the displacement-distance graph shown in Figure 1.2. 1. Crest It is the point of maximum positive displacement of the medium particles from the equilibrium position. 2. Trough It is the point of maximum negative displacement of the medium particles from the equilibrium position (still position). 3. Amplitude The amplitude, represented by A, is the maximum displacement of the medium particles from the equilibrium position. It is the distance from the central line to the top of a crest or to the bottom of a trough. The SI unit of amplitude is metre (m). 4. Wavelength This is the distance between two successive crests or two adjacent troughs as shown in Figure 1.2. It is also the distance that the wave travels in one complete cycle WXY. The wavelength is represented by the Greek letter lambda, λ and its SI unit is metre (m). Figure 1.2: Features of a wave profile Distance (metre) W -2 -1 0 1 2 X Y Z A H A Displacement (metre) Wavelength, λ Wavelength, λ Crest Trough Crest Equilibrium position Physics Form 4.indd 2 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
3 Waves 5. Period This is the time taken for the wave to travel from a crest to the next crest or from a trough to the next trough. It is also the time taken to make one complete cycle WXY as illustrated in Figure 1.3. Period is represented by the letter T and can be calculated by dividing the time of travel, t by the number of complete cycles, n. That is, T = t n . The SI unit of period is second (s). Figure 1.3: Period of a wave 6. Frequency The number of waves that pass a certain point per specified amount of time is referred to as frequency, represented by the letter f. Frequency can be expressed as the number of wave cycles completed in one second. That is, From the definition of period, t = nT. Thus, f = n nT = 1 T Hence, f = 1 T . This is the relationship between the period, T and frequency, f of a wave. Since T is measured in seconds, then the SI unit of frequency is cycles per second. For example, if the end of the rope is moved up and down thrice in a second, three waves are produced in this time. Therefore, the frequency of the wave is 3 wave cycles per second. The SI unit of frequency is also known as Hertz (Hz). One Hertz is the same as one cycle per second. 7. Wave velocity This is the velocity at which the wave moves through a medium. It is commonly referred to as speed. It is the distance travelled by a wave per unit time. The SI unit of wave velocity is metre per second (m s-1 ). 8. Phase It refers to the angle-like quantity that represents the fraction of a cycle covered in a certain time, t. Phase is normally denoted by ‘phi’, φ . If we consider the points W, X, Y and Z, in Figure 1.3, the velocity of particles corresponding to points W and Y is the same in magnitude and direction. Thus, W and Y are said to be in phase. At points W and X, the particle has the same velocity in magnitude but it takes different directions. Therefore, at these points the wave is out of phase. Relationship between wave parameters When a wave completes one cycle, it has travelled a distance, l = λ. Since the time taken to complete one cycle is T, then wave speed can be determined from the Time (second) W T T -2 -1 0 1 2 X Y Z Displacement (metre) Equilibrium position Physics Form 4.indd 3 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
4 Physics for Secondary Schools Student’s Book Form Four Example 1.3 Example 1.2 Example 1.1 relation, v = λ T = λ × 1 T But f = 1 T Therefore, v = λ f . Determine the amplitude, period and frequency of the wave represented in the displacement-time graph shown in Figure 1.4. Figure 1.4 Solution The maximum position is measured from equilibrium 0 to 0.5 m. Therefore, amplitude of the wave is 0.5 m. The period of the wave, is the time taken to complete one cycle. In Figure 1.4, the time taken to complete one cycle is 0.2 s. Therefore, the period is 0.2 s. The frequency of the wave is given by: f = 1 T f = 1 0.2 s f = 5 Hz. The displacement-distance graph corresponding to a wave in example 1.1 is shown in Figure 1.5. Use the graph to determine the wavelength and velocity of the wave. Figure 1.5 Solution From the graph, the wavelength is the distance between two successive crests and its value is 2 m. The velocity of the wave, v is given by: v = f λ v =5 Hz × 2 m What is the wavelength of a wave whose speed is 4 m s-1 and frequency is 2 Hz? Solution Speed, v = 4 m s-1 Frequency, f = 2 Hz = 2 s-1 Then from, v = f λ Thus, λ = v f = 4 m s-1 2 s-1 = 2 m. 0 -0.25 0.25 -0.5 0.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Displacement (m) Time (s) 0 -0.25 0.25 -0.5 0.5 1 2 3 4 5 6 7 8 Displacement (m) Distance (m) Physics Form 4.indd 4 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
5 Waves Propagation of waves The movement of waves in space and time is referred to as wave propagation. Some waves require a medium to propagate, while others do not. Waves that require a material medium to propagate are called mechanical waves. Those waves which do not require material medium to propagate are called electromagnetic waves. Mechanical waves A mechanical wave is the wave that is produced when particles vibrate in a medium (solid, liquid or gas) in which the wave propagates. The propagation of a mechanical wave through a medium depends on the elastic and inertia properties of that medium. A typical example of a mechanical wave is sound wave. Other examples, of mechanical waves are water waves and waves on strings. As the mechanical wave propagates, particles of the medium vibrate about their equilibrium positions. Electromagnetic waves An electromagnetic wave is a wave that is created as a result of vibrations between an electric field and a magnetic field. The electric and magnetic fields propagate in phase and at right angle to each other. All electromagnetic waves travel through vacuum at the speed of 3×108 m s-1. Examples of electromagnetic waves include: visible light, radio waves, microwaves, and X-rays. Modes of wave propagation All waves can be classified in terms of their modes of propagation. There are two modes of wave propagation; transverse propagation and longitudinal propagation. Waves that propagate in a direction perpendicular to the direction of particle vibrations are called transverse waves. On the other hand, waves that propagate in a direction that is parallel to the direction of particle vibrations are referred to as longitudinal waves. Transverse waves In a transverse wave, the motion of particles make a right angle with the direction of propagation of the wave. For example, when the string or rope under tension oscillates up and down at one end, the disturbance moves along the rope. The particles in the rope vibrate perpendicular to the direction of the disturbance. Figure 1.6 illustrates the propagation of a transverse wave. Figure 1.6: A transverse wave in a string Displacement (m) Wave direction Particles of the medium Equilibrium position Time (s) Physics Form 4.indd 5 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
6 Physics for Secondary Schools Student’s Book Form Four Water waves are another example of transverse waves. The water particles move up and down while the waves move in a horizontal direction. That is why a boat on the ocean moves up and down while the waves themselves move towards the shore (Figure 1.7). Figure 1.7: Water waves Direction of wave motion Compression Rarefaction Fixed end λ Longitudinal waves In longitudinal waves, particles of a material medium vibrate in a direction parallel to the direction of the wave propagation. A longitudinal wave consists of regions of high and low particle density. These regions are respectively termed as compression and rarefaction regions. Compression regions are regions of high pressure and high density since particles are being compressed close together. Conversely, rarefaction regions are regions of low pressure and low density as particles are being spread further apart as illustrated in Figure 1.8. The distance between the centres of two consecutive regions of compression or adjacent regions of the rarefaction is the wavelength, λ. Figure 1.8: A longitudinal wave in a spring A sound wave is an example of a longitudinal wave. It is produced by the vibration of particles and travels through a medium such as air. The amplitude of a sound wave is the difference between the maximum pressure caused by the disturbance and the pressure of the undisturbed particles in a medium as illustrated in Figure 1.9. Physics Form 4.indd 6 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
7 Waves Task 1.2 Figure 1.9: Propagation of sound waves in air Note that, in some cases, waves are neither purely transverse nor purely longitudinal. For example, water waves at the surface of a large water body involve components of both longitudinal and transverse waves. For all electromagnetic waves, oscillations are perpendicular to the direction of wave propagation. Therefore, electromagnetic waves are classified as transverse waves. Use your own creativity to create longitudinal and transverse waves and share your experience with your classmates by demonstrating how you create those waves. Behaviour of waves In a medium of uniform properties, waves propagate at constant speeds. Any change in medium properties results in a change in the speed of a wave. For example, the propagation speed of sound depends on the type, composition, and temperature of a medium through which it propagates. A change in the speed of the wave means a change in its wavelength, since the frequency remains constant. Thus, in a medium of uniform properties the speed of a wave remains constant so the wavelength varies inversely proportional to the frequency. The amplitude of the wave depends on the amount of energy being transmitted. A decrease in the amplitude of a wave shows that the wave has lost some energy. Change of medium properties result to different behaviours of a waves. These behaviours include reflection, refraction, diffraction and interference. Reflection of waves A travelling wave may encounter a boundary between two media of different properties. If the boundary does not allow the wave to pass through, the wave bounces back to the medium in which it was propagating before striking the boundary. This phenomenon is referred to as reflection. For example, a wave travelling through a string which is fixed at one end will be reflected upon reaching the fixed end of the string. In this case, the fixed end of the string acts as the boundary. Consider the situation where a string is fixed to a rigid wall at its right end. This end is called a fixed end. When a wave is allowed to propagate through the string, the wave reaches the fixed end, and gets reflected. The reflected wave will be inverted as shown in Figure 1.10 (a). In this case, the Increased pressure Pressure of undisturbed air Decreased pressure Motion of air molecules Propagation of a sound wave Wavelength, λ Physics Form 4.indd 7 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
8 Physics for Secondary Schools Student’s Book Form Four amplitudes of both the incident and reflected waves are the same. If the right end of the string is tied to a ring, which can slide up and down on a rod without any friction, the end is termed as a free end. In this case, when the wave arrives at the free end, the ring moves up and down. The motion of the free Reflected wave Reflected wave Incident wave Incident wave Incident wave Inverted reflected wave Transmitted wave Less dense Denser end results into a reflected wave which is not inverted. The reflected wave will have the same speed and wavelength as the incident wave. However, the reflected wave will have a smaller amplitude as shown in Figure 1.10 (b). The decrease in amplitude indicates that the wave lost some of its energy at the boundary. (a) Fixed end (b) Free end Figure 1.10: Reflection of waves at fixed and free ends of a string When a wave encounters a boundary that allows it to pass through, part of the wave will be reflected and part will be transmitted into the new medium. Consider two ropes of different thicknesses tied together end-to-end and suppose that, a transverse wave is produced in the thinner rope. When the wave reaches the boundary between the two ropes, it will split into an inverted reflected wave and an upright transmitted wave. The reflected wave will have the same speed and wavelength as the incident wave. The transmitted wave will have a lower speed and a shorter wavelength than the incident wave. Each wave will have an amplitude less than that of the incident wave since the energy of the incident wave is split into the two waves. See Figure 1.11. Figure 1.11: Reflection of a wave travelling from a less dense medium to a denser medium If the new medium has a lower density, the reflected wave will not be inverted, as illustrated in Figure 1.12. It will have the same speed and wavelength as the incident Physics Form 4.indd 8 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
9 Waves wave. The transmitted wave will have a higher speed and longer wavelength. According to the principle of conservation of energy, when the wave breaks up into a reflected wave and a transmitted wave at the boundary, the sum of the energies of these two waves must be equal to the energy of the incident wave. Because the reflected wave contains only part of the energy of the incident wave, its amplitude must be smaller. Reflection and other behaviours of waves may be demonstrated using water waves. This is done using a ripple tank. Figure 1.12: Reflection of a wave travelling from a denser medium to a less dense medium Ripple tank A ripple tank is an example of an instrument used to demonstrate the behaviour of waves. The structure of a ripple tank is shown in Figure 1.13. It consists of a power supply used to run an electric motor. When the motor runs it makes the oscillating paddle attached to an elastic band to vibrate on the water surface. The vibration of the paddle Reflected wave Incident wave Transmitted wave Denser medium Less dense medium Power supply Elastic band Motor Wave pattens on a viewing screen Oscillating paddle Shallow tank of water Support Bulb generates parallel water waves (ripples). The oscillating paddle is used to transform mechanical energy generated by the motor to ripples in a shallow tank of water. A bulb/lamp shines light through the water and a shadow of the wave pattern is produced on a sheet of paper or glass placed under the tank. The paper or glass act as viewing screen. All behaviours of waves can be demonstrated with the aid of a ripple tank. Figure 1.13: Ripple tank Physics Form 4.indd 9 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
10 Physics for Secondary Schools Student’s Book Form Four Reflection of water waves can be observed by placing various obstacles in the tray of the ripple tank. The depth of the water can also be varied by laying glass plates of different thicknesses in the tray. This allows the observation of waves travelling from one medium to another. Aim: To observe the reflection of water waves in a ripple tank. Material: Ripple tank, barriers of different shapes (straight barrier, wooden block, concave and convex barriers), and droppers Procedure 1. Assemble the ripple tank as illustrated in Figure 1.13. 2. Set up the barrier in the ripple tank as shown in Figure 1.14 3. Switch ON the motor to run the oscillating paddle that generates some waves in the ripple tank. 4. Record your observations. 5. Remove the oscillating paddle. 6. Put some water into a dropper. 7. Hold the dropper about 2 cm above the water surface. 8. Let one drop of water fall at the middle of the ripple tank. 9. Observe how the water waves will be reflected after striking the barrier. Activity 1.1 Figure 1.14 10. Draw the incident and the reflected wave patterns in steps 4 and 9. 11. Change the angle of the barrier and repeat steps 3-9. Observe any changes in the reflected waves. 12. Replace the rectangular barrier with a curved one (concave) as shown in Figure 1.15, and repeat steps 3-9. Figure 1.15 13. Record all your observations. 14. Replace the concave barrier with a convex one and repeat steps 3-9. 15. Observe any changes in the reflected waves. Ripple tank Direction of waves propagation Incident wavefronts Barrier 45° Curved barrier Ripple tank Direction of the waves Incident wavefronts Physics Form 4.indd 10 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
11 Waves Questions (a) What happens to the incident water waves when they reach the rectangular barrier? (b) Describe the wave pattern produced when the incident water waves strike the concave or convex barriers. Reflection involves a change in the direction of waves when they fall on a barrier. The direction in which a wave is travelling is represented by an arrow. The arrow is called a ray and is drawn perpendicular to the wavefronts. Upon reaching the barrier placed within the water, water waves bounce off the barrier and head in a different direction. Regardless of the angle at which the wavefronts approach the barrier, the waves will always be reflected in such a way that the angle of incidence at the barrier with respect to the normal is equal to the angle at which the waves are reflected off the barrier (Figure 1.16). This is in accordance to the laws of reflection which states that: 1. Upon reflection from a straight barrier, the angle of the reflected ray, r is equal to the angle of incident ray, i with respect to the normal, N (a line that is perpendicular to the surface at the point of contact). That is, i = r. 2. The incident line of propagation, the normal line and the reflected line of propagation all lie in the same plane. Figure 1.16: Reflection of water waves When, a straight water wave strikes a curved barrier, the principles of reflection still apply, but the pattern becomes more complex. Consider a rubber tube having the shape of a parabola placed within the water. Upon reflection on the parabolic barrier, the water wave will change direction and head towards a point known as the focal point. This is the point at which the wave energy concentrates. After passing through the focal point, the waves spread out as shown in Figure 1.17. This is also the case when circular water waves strike a straight or an outward curved (convex) barrier. Note that the parabolic barrier focusses the water waves exactly at half the distance from the centre of curvature. Reflected wavefronts Direction of waves Incident wavefronts i r N Physics Form 4.indd 11 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
12 Physics for Secondary Schools Student’s Book Form Four Activity 1.2 Figure 1.17: Reflection of linear waves from curved barriers (a) concave barrier (b) convex barrier Applications of reflection of waves Reflection of waves is used in various human activities. Some of the applications of wave reflections are hereby described. 1. Reflection of light waves is used in designing mirrors. Light waves bounce upon striking a silvery surface of glass. 2. Sonar (sound navigation and ranging) systems rely on the reflection of sound waves to measure the distance and speed of underwater objects. 3. The reflection of sound is what makes a hearing aid operate. Sound waves are reflected into a smaller region in a hearing aid, which directs the sound to the ear. 4. The soundboard is built upon sound reflection. Here, sound waves are uniformly reflected in an auditorium. This aids in the enhancement of sound quality. 5. The working of a stethoscope is based on the reflection of sound. In the stethoscope, the sound of a patient’s heartbeat reaches a doctor’s ear by multiple reflections of sound. Refraction of waves When waves travel from one medium to another, they tend to change their travelling speed. This phenomenon is known as refraction of waves. Refraction of waves occurs because the speed of a wave depends on the medium through which the wave is travelling. If the medium is changed, the speed of the wave will also change. The change in speed results in a change in the wavelength of the wave. Aim: To observe the refraction of water waves in a ripple tank. Materials: Ripple tank and its accessories, rectangular glass plates and metre rule Procedure 1. Set a ripple tank as illustrated in Figure 1.13 2. Fill the ripple tank with water. 3. Gently place a rectangular glass plate in one part of the ripple tank to make it shallower than other parts (b) Convex barrier Focal point Linear waves Reflected waves (a) Concave barrier Focal point Linear waves Reflected waves Physics Form 4.indd 12 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
13 Waves of the tank. The glass plate should be placed at the end opposite to that of the vibrator. 4. Produce some water waves using the vibrator. 5. Measure the distance between successive crests in the deeper part, λ and then in the shallow part, λ1 . Questions (a) Work out the ratio λ λ1 ⋅ (b) What is the relationship between the ratio of wave velocities in the deep and shallow parts of the tank? When water waves travel from a deep part to a shallow part, the wavelength decreases as illustrated in Figure 1.18. However, the frequency of the waves does not change. Since velocity, v, is given by λf, the velocity of the waves decreases with decrease in wavelength. Figure 1.18: Change in the wavelength of water waves The ratio λ λ1 is equal to the ratio of the velocities of water waves in the deep water, v to that in the shallow water, v1 , that is, λ λ1 = v v1 ⋅ The velocity of water waves is higher in the deep water than in the shallow water. Refractive index The refraction of water waves can be further observed if the boundary between the deep and shallow regions is at a certain angle to the incident wavefronts. In this case the deep region acts as the first medium while the shallow region acts as the second medium. If in Activity 1.2, the glass plate was placed at some angle, water waves would be refracted as shown in Figure 1.19. Figure 1.19: Refraction of water waves Experimental observations have shown that water waves obey Snell’s Law of refraction. That is, η = sini sinr , where η is the refractive index of the second medium relative to the first medium, i is the angle of incidence and r is the angle of refraction. The refractive index, η is also the ratio of the velocity of the wave in the first medium to that in the second medium, that is; sini sinr = v v1 ⋅ Refracted waves Medium 1 Medium 2 Incident waves i i v r r v1 N N λ λ1 Glass plate Equilibrium water level Physics Form 4.indd 13 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
14 Physics for Secondary Schools Student’s Book Form Four Example 1.4 The speed of light is in water and in air. Determine: (a) The refractive index of light from air to water. (b) The angle of refraction in the water if the incident angle of light at the surface of water is 30°. Solution (a) Refractive index, η = Speed of light in air Speed of light in water η = 3 × 108 m s−1 2.25 × 108 m s−1 η = 1.33 Therefore, refractive index of water is 1.33. (b) η = sini sinr sinr = sini η ⇒ sin30° 1.33 r = sin−1 (0.3760) r = 22.08° Therefore, the angle of refraction in the water is 22.08°. Applications of refraction of waves As in the case of wave reflection, wave refraction is also useful in many activities. The following are some applications of refraction of waves: 1. Refraction is used in optical instruments which focus or spread light. These include cameras, microscopes and telescopes. 2. Spectacles worn by people with visual impairment use the principle of refraction of light. 3. Since every material has its own value of refractive index, purity of a material can be identified by determining the refractive index of the material. Interference of waves Consider two or more waves propagating in the same medium. If the waves are travelling in the same direction, they tend to add up forming a new wave of larger amplitude. If waves of the same frequency and amplitude travelling with the same speed but in opposite directions meet, they tend to cancel each other. That is, no new wave is formed. The addition of waves is called wave superposition and is governed by the principle of superposition of waves. The principle of superposition of waves states that, the resultant disturbances at any point is equal to the algebraic sum of the disturbances of individual waves at that point. When two or more waves combine to form a resultant wave, the waves are said to have interfered. Therefore, wave interference is a combination of two or more waves to form a resultant wave in which the particle displacement is either reinforced or cancelled. When two waves interfere and if a crest of one wave meets the crest of another wave at the same Physics Form 4.indd 14 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
1 -2 -1 -1 -1 -2 In constructive interference, we get lines of increased disturbance. These lines are called antinodal lines. In destructive interference, we get lines of zero disturbance. These lines are called nodal lines. Figure 1.22 shows circular waves from two dippers that are close together in a ripple tank. The waves cross through Maximum Amplitude Minimum Amplitude Source S1 and S2 A2 A1 S1 S2 A0 A1 A2 N2 N1 N1 N2 Crest Trough Physics Form 4.indd 15 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
16 Physics for Secondary Schools Student’s Book Form Four Activity 1.3 of maximum constructive interference are labelled A0 , A1 and A2 . Points on these lines move up and down with higher amplitudes much than they would if the waves came from one source alone. The lines labelled N1 and N2 represent bands along which there is maximum destructive interference. Points on these lines move up and down with much less amplitude than they would if the waves came from either source alone. Note that, if the two waves from S1 and S2 have the same amplitude, then the points along the lines A0 , A1 and A2 will have the amplitude twice the amplitude of the wave. On the other hand, the points along the lines N1 and N2 will have zero amplitude. Aim: To investigate the interference of water waves in a ripple tank. Materials: Ripple tank and its accessories Procedure: 1. Attach two-point sources to the wave generator. 2. Generate some water waves at a constant frequency. Observe the pattern created by the waves. 3. Increase the frequency of the wave generator and observe what happens to the pattern created by the waves. 4. Decrease the frequency and again observe what happens to the pattern of the waves. 5. While keeping the frequency constant, increase the distance between the two-point sources. 6. Record all your observations. Question Describe the pattern produced by the waves from the two-point sources in each case. Water waves from two identical point sources add up at certain points (where a crest meets a crest) and cancel out at certain points (where a crest meets a trough). Where the water waves add up, constructive interference occurs. The water waves have increased amplitudes along lines of constructive interference. On the other hand, destructive interference occurs where the water waves cancel out. Along the lines of destructive interference, the water is observed to be still. Applications of interference of waves 1. Wave interference is applied when creating holograms. A hologram is a photograph of an interference pattern which is able to produce a threedimensional image. 2. Destructive interference is used in noise-reduction systems such as earphones and car mufflers. The systems capture sound from the environment and produces a second wave, which interferes with the first wave destructively leading to the reduction in the loudness of the noise. 3. Another application of interference of waves is in Active Noise Control. This is based on the fact that the wave generated by a primary source (such as an engine) can be cancelled by the Physics Form 4.indd 16 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
17 Waves Activity 1.4 wave emitted by a secondary source (loudspeakers) driven at the same frequency as the primary source, so that the two waves cancel out each other. This technique is applied to reduce the annoying propeller noise inside the cabin of an aircraft. Diffraction Suppose water waves in a ripple tank are allowed to pass through a gap (aperture) formed by an obstacle (Figure 1.23). If the width of the gap is wider than the wavelength of the water waves, the waves appear to move in the same straight line as the incident wave. If the width of the gap is decreased to the order of the wavelength of water waves, then after the wave has passed through, it spreads in all directions. The spreading of the wave as it encounters an obstacle is called diffraction. The extent of spreading depends on the width of the aperture in comparison to the wavelength of the incident wave. A waves whose wavelength is comparable to or larger than the width of an aperture spreads out in all forward directions upon passing through the aperture. The bigger the width, the less the diffraction. Figure 1.23: Diffraction of water waves Aim: To investigate diffraction of water waves in a ripple tank. Materials: Ripple tank and its accessories, glass block and pencil Procedure 1. Add water to the ripple tank to a depth of approximately 1 cm. 2. Adjust the legs of the ripple tank to make the depth of the water as uniform as possible. 3. Place a glass block in the water across the tank. 4. Drop a pencil onto the water, just a few centimetres behind the glass block. Note: The pencil should be aligned in such that the generated waves travel in a direction perpendicular to the edge of the glass block. 5. Observe the waves as they move past the glass block. 6. Put some additional glass blocks in the ripple tank to create a barrier across the entire tank. Leave an opening of about 1 cm at the centre of the ripple tank. 7. Generate some straight waves using the oscillating paddle of the ripple tank. 8. Observe what happens to the waves as they travel past the barrier. 9. Increase the size of the opening, then repeat step 6. 10. Replace the straight wave source with a point source, then repeat step 6. 11. Record all your observations. Diffracted waves Narrow Wave direction aperture Incident wavefronts Physics Form 4.indd 17 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
18 Physics for Secondary Schools Student’s Book Form Four Questions (a) Explain what happens to the waves on passing through the single block barrier. (b) Explain what happens to the waves on passing through the narrow opening. (c) What was the effect of increasing the size of the opening on the waves passing through it? When straight waves are approaching a barrier, the barrier obstructs a part of the wave that strikes it and allows just that part of the wave which does not strike it to pass through. The wavefronts that pass the barrier spread into the shadow area of the barrier. The spreading takes place whenever a wave meets a barrier or an aperture (narrow opening). When the gap is narrow, the straight wavefronts are converted into circular wavefronts as illustrated in Figure 1.24 (a). These wavefronts appear to be produced by a new point source in the gap. They spread out round the edges of the opening in all directions. When the gap is wide as in Figure 1.24 (b), the waves emerge almost straight, apart from a slight curvature, and spread out at the edges. (a) Narrow gap (b) Wide gap Figure 1.24: Diffraction of water waves The amount of diffraction is maximum when the width of the opening is the same as the wavelength of the waves. Diffraction can occur with any kind of wave. For example, ocean waves diffract around obstacles just as sound and light can diffract around objects. The amount of diffraction depends on the size of the aperture and the wavelength of the incident wave. Since sound has longer wavelength than light, it diffracts more than light upon falling on an aperture. Applications of diffraction of waves Diffraction of waves has many applications. Some of these applications are: 1. The process of diffraction is significantly used in long-distance radio signal propagation. Despite the curved surface of the Earth and the presence of huge obstacles on it, radio signals from a transmitter can reach an observer on the other side of the Earth or obstacle. This is possible due to the diffraction of waves at the obstacle. The signal diffracts to fill the void after the obstacle and surfaces to travel to the observer. 2. Diffraction is used in a hologram to generate a three-dimensional impression of an image. Different Waves straight except for slight edge curvature Incident wavefronts Barrier Waves spreading out in all directions Physics Form 4.indd 18 01/06/2022 13:46 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
19 Waves versions of the image get diffracted and reach a lens from multiple sides, all together forming an interference pattern. This pattern is then made to fall on a holographic plate providing a three-dimensional image. 3. Diffraction is used in measuring the coefficient of thermal expansion, crystallite size and thickness of thin films. 4. X-rays diffraction is used to determine the distance between two consecutive atoms of a material. X-ray diffraction process is crucial in the meteorological, pharmaceutical, chemical, and other related industries because whenever researchers come across some unidentified materials, they need to figure out the details about their structure, starting with the alignment, distance, and other characteristics of their atoms. 1. If you want to hit a fish under water using a spear, should you aim below it, straight at it or above it? Explain. 2. A periodic disturbance in a lake creates waves which emanate outwards from its source to produce circular wave patterns. If the frequency of the source is 2 Hz and the wave speed is 5 m s-1, determine the distance between adjacent wave crests. 3. A wave whose speed in the first medium is 4.4 m s-1 enters a second medium. The wavelength changes from 2 m to 3 m. What is the speed of the wave in the second medium? 4. For a certain transverse wave, the distance between two successive crests is 1.2 m, and eight crests pass a given point along the direction of travel every 12 s. Calculate the wave speed. 5. A sinusoidal wave is travelling along a rope. The oscillator that generates the wave completes 40 vibrations in 30 s. Also, a given crest of the wave travels 425 cm along the rope in 10 s. Calculate the wavelength of the sinusoidal wave. 6. The velocity of light in water is 2.2×108 m s-1 and the velocity of light in glass is 2.0×108 m s-1. Calculate: (a) The relative refractive index as light passes from water to glass. (b) The angle of incidence in the water which would produce an angle of refraction of 30° in the glass. 7. A straight vibrator causes water ripples to travel across the surface of a shallow tank. The ripples travel a distance of 33 cm in 1.5 s and the distance between successive crests of the wave is 4.0 cm. Calculate the frequency of the vibrator. 8. Radio and light waves travel at a velocity of 3×108 m s−1 in air. Calculate: (a) The wavelength of radio waves when transmitted at a frequency 150 MHz; and Exercise 1.1 Physics Form 4.indd 19 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
20 Physics for Secondary Schools Student’s Book Form Four (b) The velocity of light in glass of refractive index 1.5. 9. Water ripples are caused to travel across the surface of a shallow tank by means of a suitable straight vibrator. The distance between successive crests and troughs is 1.5 cm and the wave travels 25.2 cm in 1.2 s. Calculate the wavelength and the velocity of the waves and the frequency of the vibrator. 10. If a wave has a velocity of 330 m s-1 and a wavelength 0.5 m, calculate the frequency of the vibrator producing the wave. Sound waves When the air is set to vibrate by an oscillating body such as a tuning fork, string, whistle, or clarinet, it produces sound. Sound is a mechanical vibration transmitted through a medium such as solids, liquids or gases. Unlike waves in a string which move in one dimension and water waves which move in two-dimension, sound waves move in three dimensions. It is a longitudinal wave as illustrated in Figure 1.25. As the tines of the tuning fork vibrate, they set the surrounding air molecules into vibration. As neighbouring molecules interact, the vibrations travel away from the tuning fork in all directions. Because of the longitudinal vibrations of the air molecules, there are regions where the molecules are compressed, and adjacent regions where they are spread out. Thus, sound waves are sometimes referred to as pressure waves. C = Compression R = Rarefaction Figure 1.25: Sound waves generated by vibrating tuning fork Propagation of sound waves Like all mechanical waves, sound waves require a medium to be transmitted. Sound is transmitted by vibration of particles of the medium. One particle vibrates to transfer energy to the next until the sound reaches another point. Sound travels quicker when the particles are closer together. For example, sound travels faster in solids than in gases. This is because molecules of a solid are more closely packed than gas molecules. If one stands near a railway line and another person taps the rail some distance away, two successive sounds will be heard, the first through the rail and a later one through the air. Sources of sound waves Sound waves are produced by almost everything including people, animals, plants and machines. Musical instruments are designed to produce specific types of sound. These instruments include guitars, violins, pianos, organs, flutes, drums and xylophones. Figure 1.26 shows some musical instruments. C R C R C R C Tuning fork Physics Form 4.indd 20 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
21 Waves Figure 1.26: Musical instruments Categories of sound waves The human ear is very sensitive and can detect even faint sounds. Whether you hear or do not hear a sound depends on its loudness and frequency. The average human ear can detect sounds in the frequency range of 20 Hz to 20 000 Hz. Nevertheless, due to various reasons including age, there are significant differences between individuals, especially at high frequencies. The upper limit of the audible range decreases throughout an adult’s life. The range of sound frequency which can be detected by the human ear is known as the audible range or audio range. Waves which lie in this range are called audible waves. The ear is most sensitive to sounds with a frequency around 3 000 Hz. Sound waves with frequencies below 20 Hz are said to be infrasonic. Conversely, sound waves that are above 20 000 Hz are called ultrasonic. Elephants communicate using infrasonic waves. Some animals including bats and dolphins can detect ultrasonic sounds with frequencies as high as 100 000 Hz. Ultrasonic waves are also applied in medical imaging devices such as ultrasound machines. The average human ear can distinguish between two simultaneous sounds if their frequencies differ by at least 7 Hz. Table 1.2 shows the audio range for different animals. Table 1.2: Audio range for different animal species Animal Audibility range (Hz) Bat 9 000 – 200 000 Blue whale 10 – 40 Cat 45 – 64 000 Cattle 23 – 35 000 Chicken 10 – 12 000 Dog 67 – 45 000 Elephant 14 –16 Horse 55 – 33 500 Owl 200 – 12 000 Penguin 100 – 15 000 Rabbit 360 – 42 000 Rat 200 – 76 000 Risso’s dolphin 8 000 – 100 000 Sheep 100 – 30 000 Physics Form 4.indd 21 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
22 Physics for Secondary Schools Student’s Book Form Four The human ear The human ear converts sound energy to mechanical energy and then to electrical energy which acts as a signal sent to the brain via nerves. Human ears can discriminate between sound based on frequency, amplitude and direction. The human ear consists of three basic parts: the outer ear, the middle ear and the inner ear as shown in Figure 1.27. Figure 1.27: The human ear The outer ear The outer ear consists of the pinna and the ear canal. The outer ear channels sound waves through the ear canal to the eardrum of the middle ear. In the outer ear, the sound is still in the form of a pressure wave, with an alternating pattern of high- and low-pressure regions. The middle ear The middle ear is an air-filled cavity that consists of an eardrum and three small interconnected bones, the hammer, anvil and stirrup. The Eustachian tube connects the middle ear to the throat. Its purpose is to regulate pressure. A compression of the incoming sound wave forces the eardrum inward and a Temporal bone Cochlea Eardrum Eustachian tube Auditory nerves Semicircular canals Outer ear Middle ear Inner ear Hammer Anvil Stirrup Ear canal Pinna rarefaction allows the eardrum to move outward. In this way, the eardrum vibrates at the same frequency as an incoming sound wave. The movements of the eardrum set the hammer, anvil and stirrup into motion. The three tiny bones amplify the vibrations of the incoming sound wave. Because the stirrup is connected to the inner ear, the vibrations are transmitted to the fluid of the inner ear. The inner ear The inner ear consists of the cochlea, the semicircular canals and the auditory nerve. The cochlea and the semicircular canals are filled with a water-like fluid. The fluid and nerve cells of the semicircular canals help in maintaining the body balance. The inner surface of the cochlea is lined with hair-like Physics Form 4.indd 22 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
23 Waves nerve cells that differ in length. The nerve cells have different degrees of resilience to the fluid which passes over them. As a compressional wave moves from the interface between the hammer of the middle ear and the oval window of the inner ear through the cochlea, the small hair-like nerve cells are set in motion. Each hair cell has a natural sensitivity to a particular frequency of vibration. When the frequency of the incoming wave matches the natural frequency of the nerve cell, that nerve cell vibrates with larger amplitude. The increased vibration makes the cell release an electrical impulse which passes along the auditory nerve to the brain for interpretation. Echo and reverberation Sound, like any other waves, can be reflected off a flat or hard surface and obeys the same laws of reflection. The reflection of a sound leads to the formation of an echo. Echo Suppose you are standing about 120 metres from a high building. If you clap your hands, after sometime you will hear again the sound of clapped hands. The repetitive sound that you hear is called an echo that reaches the ear more than 0.1 s after the original sound was heard. At this time interval, the sensation of the original sound will have died out and the reflected sound will be heard as a distinct sound. The memory of a sound persists in the brain for approximately 0.1 s. This means that the original sound and the reflected sound must be separated by a time interval of 0.1 s for the echo to be interpreted by the brain as a distinct sound. In this phenomenon, the sound must first travel to the obstacle. When the sound returns, it covers the same distance. The minimum time that the sound should take to reach the obstacle for an echo to occur is 0.1 s 0.05 s 2 = . Figure 1.28 illustrates the sound wave travelling to an obstacle and then reflected back to the observer’s ear. Figure 1.28: Formation of an echo At room temperature, the speed of sound in air is about 340 m s-1. Since distance is given by distance = speed × time, the minimum distance, d to the obstacle for an echo to occur is given by: d = speed ×time = 340 m s-1 × 0.05 s = 17 m An obstacle must be at least 17 m away for a distinct echo to be heard. However, materials through which the sound propagates may affect the occurrence of an echo. Reverberations When a sound is produced in an enclosed space with dimensions that are approximately less than 17 m, multiple reflections occur, forming multiple echoes picked by the ear. Since the dimensions Barrier Ear Original sound Echo 0.05 s 0.05 s Physics Form 4.indd 23 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
15 = 346.4 m s−1 . Aim: To determine the speed of sound in air by echo method. Materials: Two wooden blocks, stopwatch, measuring tape, vertical cliff or wall Procedure: 1. Check your school dining hall so that it is not occupied by other students. 2. Close all windows and doors of the building. 3. Mark one end of the building as the starting point. 4. While in the building, move close to the starting point and hit the two wooden blocks together. Note if an echo occurs. 5. Change your position and repeat step 4. 6. Repeat step 5 until you hear an echo. 7. Mark the point where the echo occurs as shown in Figure 1.30. 8. Measure the distance from point A to the wall. Direct sound Listener Origin of sound Enclosed obstacle Physics Form 4.indd 24 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
25 Waves Exercise 1.2 9. While standing at point A let one of you hit the block and the other measure the interval from the time of hitting the block to the time an echo is heard. Figure 1.30 Question Determine the speed of sound, v in air. 1. Sound of explosions taking place on other planets is not heard by a person on the Earth. Explain. 2. A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of the sound in water if the distance of the object from the submarine is 3 625 m. 3. A person standing 99 m from the foot of a tall cliff, claps hands and hears an echo 0.6 s later. Calculate the speed of sound in air. 4. An observer stands between two distant cliffs and claps hands. An echo is received after 2 s and 2.5 s respectively. If the speed of sound in air is 330 m s-1, find the distance between the cliffs. 5. How long would it take for a 30 Hz beat to reach an audience member 100 m away when the ambient temperature is 21 ºC? 6. Calculate the ratio of velocities of sound produced in dry air at 42 °C to another at 60 °C. 7. Two persons stand facing each other, 200 m apart on one side of a high wall and at the same perpendicular distance to it. When one fires a pistol the other hears a report 0.60 s after the flash and a second 0.25 s after the first. Explain this and calculate: (a) The velocity of sound in air. (b) The perpendicular distance of the persons from the wall. (c) Draw a diagram showing the positions of the persons and the wall. Musical sounds When listening to music from a radio, one can differentiate the sounds coming from saxophones, drums, guitars, violins and other musical instruments. This is possible because the patterns of the sound waves from each source is different. That is, sounds from different instruments can be classified using the properties of musical sounds. A Physics Form 4.indd 25 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
26 Physics for Secondary Schools Student’s Book Form Four Properties of musical sounds The musical sounds produced by different musical instruments have distinct properties that are used to describe them. These properties include loudness, pitch and timbre. Loudness When listening to music, there is a possibility of sensing high sound and low sound. This happens depending on the energy of the sound that enters the ears per second. If the energy is increased the music becomes louder. If the sound energy is decreased the music becomes less loud. The loud sound is obtained when the amplitude of a vibrating source is high. Thus, the higher the amplitude the louder the sound and vice versa. Pitch The frequency of a sound wave approaching the ear determines how low or high the sound is. The scale of high or low for sound is termed as pitch. The sound itself is also called a note. The pitch is at high scale if the frequency is high. It is also called a high pitch note. Timbre If the same note sounds differently on different instruments, it is said that the two instruments have different timbre or quality. The difference is due to the fact that, with the exception of tuning forks and function generators, other instruments cannot emit pure, musical sound at the same frequency. The sound (note) consists of a fundamental frequency mixed with other frequencies called overtones. Overtones have frequencies that are exact multiples of the fundamental frequency. The quality or timbre of sound is determined by the strength of overtone. Musical instruments A musical instrument is a device constructed or modified for the purpose of making music. Musical instruments are grouped into three categories based on how they initially produce sound. These categories are string instruments, percussion instruments and wind instruments. String instruments include the violin, piano and guitar. They produce sound from stretched strings that are plucked (guitar), or bowed (violin) or struck (piano). Figure 1.31 shows some of the string instruments. (a) Guitar (b) Violin (c) Piano Figure 1.31: String instruments Physics Form 4.indd 26 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
27 Waves Task 1.3 Percussion instruments produce musical sounds by being struck, shaken, rubbed, scrapped, or by any other action which sets the object into vibration. Musical instruments in this group include the drum, cymbals, tambourine and xylophone. Figure 1.32 shows some of the percussion instruments. (a) Drum (b) Xylophone Figure 1.32: Percussion instruments Wind instruments are made up of a tube in which a column of air is set into vibration by the player blowing into (or over) a mouthpiece at the end of the tube. They include recorders, flutes, tuba and trumpets. See Figure 1.33. (a) Tuba (b) Trumpets (s) Flute Figure 1.33: Wind instruments Construct a simple musical instrument of your choice. Classify the instrument into its respective category. Stationary waves Consider two transverse waves A and B with equal amplitudes, wavelengths and speeds travelling in opposite directions through a string. As the waves pass each other, their amplitudes combine. If a crest of wave A combines with a trough of wave B as Figure 1.34 illustrates, the two amplitudes cancel out and the net displacement of particles in the medium will be zero. Figure 1.34: Crest of wave A combines with a trough of wave B If a crest of wave A combines with a crest of wave B as in Figure 1.35, the result is a displacement which is twice larger than either A or B. Figure 1.35: Crest of wave A combines with a crest of wave B Wave A Wave B Resultant 0 -x x x x Wave A Wave B Resultant -x -x x x 2x -2x Physics Form 4.indd 27 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
28 Physics for Secondary Schools Student’s Book Form Four As the two waves pass along the string, their amplitudes will alternate between adding together and cancelling. The result is a wave in which particles oscillates back and forth but the wave does not propagate. Such a wave is called a stationary (or standing) wave. Stationary waves can be produced in a string by means of vibrating tuning fork. If one end of the string is fixed to a stationary boundary and the other end to the tuning fork the string can move up and down due to the vibration of the tuning fork. The produced travelling waves reflect from the ends and travel in both directions of the string. Thus, there are two waves travelling through the string at the same time. One wave is travelling towards the stationary boundary and the other wave (reflected wave) travels towards the tuning fork. The two waves combine according to the superposition principle. At certain points along the string the two waves cancel out resulting to zero displacement, and at other points the waves add up to produce a maximum displacement. The points where the amplitude of the resultant wave is zero are called nodes. Nodes are independent of time (no motion in the string at these points). On the other hand, the points where the amplitude of the resultant wave are maximum are called antinodes. These points are dependent of time. A standing wave is illustrated in Figure 1.36. Figure 1.36: Stationary wave Note that, the profile of a stationary wave does not travel. That is, energy is not transmitted with the wave although there is energy associated with it. Fundamental note, harmonics and overtones Stationary waves on a string that is fixed at both ends are restricted to having only certain wavelengths. The wave must “fit” between the ends of the string with a node at each end. The lowest frequency that a vibrating string or pipe can produce is called the fundamental frequency and the corresponding note is called the fundamental note. A note whose frequency is n times (where n is a whole number) that of the fundamental note is called the nth harmonic. The first harmonic is therefore the fundamental note. The fundamental note occurs when there are nodes at each end with a single antinode between them. The fundamental note is called the first harmonic. A similar behaviour can be observed on waves in pipes or tubes. The overtones of a note are notes of a higher frequency which are produced with the fundamental note. The first overtone is the harmonic whose frequency is lowest among Tuning fork A N String Fixed pulley Mass Physics Form 4.indd 28 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
29 Waves those with fundamental note. The second overtone is the next higher harmonic which is present. Higher harmonics occur when there are additional nodes as illustrated in Figure 1.37. Figure 1.37: Fundamental notes and harmonics The fundamental note in Figure 1.37 (a) consists of one half of a cycle. If the length of the string is l, then the wavelength λ1 of the fundamental note is given by: λ1 2 = l ⇒ λ1 = 2l The second harmonic in Figure 1.37 (b) consists of one full cycle and has a wavelength, λ2 = l. The third harmonic in Figure 1.37 (c) is one and one-half cycles giving, l = 3 2 λ3. The wavelength will be given by, λ3 = 2l 3 ⋅ The fourth harmonic in Figure 1.37 (d) consists of two full cycle and has a wavelength, l = 2λ4 giving λ4 = l 2 ⋅ In general, for a string of length, l fixed at both ends, the wavelength, l n of the nth harmonic is given by: λn = 2l n ⋅ Since v = λ f , the frequency of the nth harmonic (f n ) is given by: fn = v λn Therefore, fn = nv 2l (a) Fundamental note, first harmonic (b) Second harmonic (First overtone) (c) Third harmonic (Second overtone) (d) Fourth harmonic (Third overtone) l l l l Physics Form 4.indd 29 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
1 f 0 , where n = 1, 2, 3,….. Sonometer A sonometer is a device that is used to study the properties of waves, particularly stationary waves. A wire or string is attached to one end of a sonometer. The opposite end is passed through two bridges and through a pulley. A weight hanger is suspended from the free end of the wire. The tension of the wire can be changed by placing different masses on the mass hanger. The box increases the loudness of sound produced by the wire. Figure 1.38 shows a sketch of a sonometer. Figure 1.38: A sonometer If the mid-point of the wire is plucked, the middle will form an antinode while the two fixed ends will have nodes as indicated in Figure 1.39. Figure 1.39: Node and antinode for a sonometer wire plucked at the middle Factors affecting the frequency of a vibrating wire or string The frequency produced by a vibrating string depends on the length of the string, l, and the velocity of the waves. On the other hand, the velocity of waves on a stretched string depends on the tension, T, in the string which is measured in newtons and the linear mass density, μ. The linear mass density is the mass of the string per unit length, that is, µ = m l kg m−1 Therefore, the frequency of sound produced by a vibrating wire (string) depends on three factors; the tension (how taut the wire is), length and mass per unit length of the wire. Aim: To investigate how the length of a vibrating wire affects its frequency. Materials: Sonometer with a steel wire, a set of tuning forks, slotted masses, paper rider Procedure: 1. Set up the sonometer as shown in Figure 1.40. The masses should be enough to keep the wire taut. Figure 1.40 N A N Mass Pulley Wire Bridge paper rider Bridge l Load Bridge Bridge Wire Pulley Box Physics Form 4.indd 30 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
31 Waves 2. Set the paper ridder at the centre of the bridges. 3. Select a tuning fork with lowest frequency, set it into vibration and put it close to the paper rider but not in contact. 4. Adjust the bridges until the wire and the tuning fork reach the resonance (the paper rider flips off the wire). 5. Record the frequency, f of the tuning fork and the distance between the bridges. 6. Sound another tuning fork and without changing the mass, adjust the distance between bridges until the resonance is achieved. 7. Repeat the procedure for all the other tuning forks. 8. Record your results as shown in Table 1.3. Table 1.3: Results Frequency, f of the tuning fork Length, l of the wire 1 l Questions (a) Draw a graph of f against 1 l . (b) Determine the gradient of the graph. (c) From the graph, how does the frequency of a vibrating wire vary with increase in the length of the wire? The graph of f against 1 l is a straight line through the origin. This means that frequency is inversely proportional to the length of the wire, that is, f ∝ 1 l . On the other hand, if other factors remain constant, the frequency of the wire is directly proportional to the square root of tension, T, on the wire. That is, f ∝ T Furthermore, the frequency of the sound produced varies inversely with the mass per unit length, μ. If a thicker wire is used, the frequency decreases. From experiments, it has been established that frequency is inversely proportional to the square root of the mass per unit length. That is f ∝ 1 µ Note that, if the length of the vibrating wire is l, then, l = λn 2 Recall that, v= f λ so, f = v λ = nv 2l The velocity of a wave propagating through the string is given as v = T µ Thus, f = n 2l T µ For the first harmonic, n=1. Thus, f = 1 2l T µ Physics Form 4.indd 31 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
32 Physics for Secondary Schools Student’s Book Form Four Example 1.6 Example 1.5 This is the fundamental frequency of a vibrating string. Since, µ = m l , then, µ = Area × Density Therefore, µ = πr 2 ρ hence, Where r and are respectively the radius and density of the string. This equation shows how the fundamental frequency of a string depends on its length, l, tension, T in the string and its mass per unit length, μ. These results can be related to stringed instruments. For example, a guitar has six strings of the same length and these are held under approximately the same tension. However, the strings have different values of mass per unit length and so their fundamental frequencies are different. The larger the mass per unit length the lower the note and vice versa. Each of the strings is tuned by slightly varying the tension in the strings. The musician then plays different notes by pressing the strings against the frets on the fingerboard to vary the length of the strings. A string has a length of 75 cm and a mass of 8.2 g. The tension in the string is 18 N. What are the frequencies of the 1st and 3rd harmonics? Solution µ = m l = 0.0082 kg 0.75 m = 0.011 kg m−1 v = T µ = 18 N 0.011 kgm−1 = 18kg m s−2 0.011 kg m−1 = 40.5 m s−1 From, fn = nv 2l For the first harmonic, n = 1, then, f 1 = v 2l f 1 = 40.5 m s −1 2 × 0.75 m = 27 Hz For the third harmonic, n = 3, then, f 3 = 3v 2l f 3 = 3 × 40.5 m s−1 2 × 0.75 m f 3 = 81 Hz The vibrating length of a stretched wire is altered at constant tension until the wire oscillates in unison with a tuning fork of frequency 320 Hz. The length of the wire is again altered until it oscillates in unison with a tuning fork of unknown frequency. If the two lengths are 90 cm and 65.5 cm, respectively, determine the unknown frequency. Physics Form 4.indd 32 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
33 Waves Solution For constant tension, f ∝ 1 l ⇒ fl = constant. Therefore, 11 2 2 fl fl = It follows that, 1 1 2 2 f l f l = Then, f 2 = 320 Hz × 90 cm 65.5 cm = 440 Hz Forced vibrations and resonance When a tuning fork is sounded and placed on a bench or a hollow box, the sound produced is quite loud and can be heard all over the room. The bench or box acts like an extended source (or many point sources) which are set into forced vibrations by the vibrating fork. These loud vibrations cause the sound to be louder but, since the rate of loss of energy is high, the sound dies off fast. Forced vibrations are vibrations that occur in a system as a result of impulses received from a nearby vibrating system. The response of the system that is set into forced vibrations is best when the driving frequency is equal to the natural frequency of the responding system. The responding system is then said to be in resonance with the driving frequency. Therefore, resonance occurs when a body or system is set into oscillation at its own natural frequency as a result of impulses received from some other system which is vibrating at the same frequency. A good example of resonance is when tuning a radio set to adjust the value of capacitance in a circuit until it has the same natural frequency of oscillation as that of the incoming signals. Resonance in a closed pipe If a tuning fork is sounded at the open end of a tube with the other end closed, the air in the tube resonates (vibrates freely) at a certain length of the tube. The resonance is observed as a loud sound produced in the tube when the proper length of air column is obtained. The first resonance occurs when air vibrates at its fundamental note or first harmonic. The fundamental note consists of one-quarter cycle as shown in Figure 1.41. Figure 1.41: The fundamental note (first harmonic) Note that, the length l 1 = 1 4 λ c l 1 A N Physics Form 4.indd 33 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
1)λ 4 Note that, there is no way you can have even multiple of first harmonic in closed end pipes. This is because closed end pipes have a node at one end and antinode at the other end. l 3 c l 2 A c N A N Physics Form 4.indd 34 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
l 1 = 1 4 λ Air column Tuning fork l Graduated cylinder Physics Form 4.indd 35 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
c = 0.25 m c = 0.25 m – 0.23 m c = 0.02 m or 2cm Therefore, the wavelength of the sound is 1 m and the end correction is 2 cm Resonance in open pipes Consider the pipe which is open at both ends. In this case, there is a node in the middle and an antinode at each end (Figure 1.45). Figure 1.45: The fundamental note in an open pipe The length of tube, l 1 , is equal to half the wavelength, i.e. l 1 = 1 2 λ (without end correvtion) (without end corrections). l c 1 c Physics Form 4.indd 36 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
2c = nλ 2 (with end corrections). A tuning fork of frequency 250 Hz is used to produce resonance in an open pipe. Given that the velocity of sound in air is 350 m s-1, find the length of the tube which gives: (a) the first resonance. (b) third resonance. Solution From l = λ 2 for first resonance in open pipe; then, v = λ f (a) For the first resonance in open pipe, l 1 = 1 2 λ = 1 2 ×1.4 m = 0.7 m Therefore, the length of the tube in the first reasonance is 0.7 m (b) For the third resonance, l 3 = 3 λ 2 =3 × 0.7m=2.1m Therefore, the length of the tube in the third reasonance is 2.1 m. Beats When to waves with slightly different frequencies travel in the same medium, a unique interference pattern appears. This happens when the two original waves combine constructively to form the largest l c 2 c Physics Form 4.indd 37 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
38 Physics for Secondary Schools Student’s Book Form Four Exercise 1.3 Example 1.10 amplitude or destructively to form the lowest amplitude as illustrated in Figure 1.47 Figure 1.47: Formation of beat frequency Suppose, two similar sound sources with nearly equal frequencies are sounded simultaneously, the oscillating loudness due to the oscillation of amplitude of the combined wave is heard. The sound rises and falls over time. This phenomenon is called beat. Consider two tuning forks which are sounded with different frequencies. Let the first fork have 400 Hz and the second have 395 Hz. Under these conditions they will produce a beat. The 400 Hz will have 40 cycles after 1 10 s, while the 395 Hz tuning fork will have 39.5 cycles. Therefore, the 400 Hz tuning fork will be at compression, the 395 Hz will be at rarefaction. The resultant amplitude of the air will be minimum. Moreover, after 1 5 s, a 400 Hz tuning fork will have 80 cycles while a 395 Hz will have 79 cycles. Therefore, at 1 5 s the two waves are almost in phase. The resultant amplitude is maximum. In this case, the sound rises and falls after every 1 5 s. Therefore, there are 5 beats per second. The beat frequency or the number of beats can be obtained by applying the principle of superposition given as the difference between the two frequencies of sound. Beat frequency f = f 1 − f 2 if f 1 > f 2 and f = f 2 − f 1 if f 2 > f 1 A 256 Hz tuning fork produces sound at the same time with a 249 Hz fork. What is the beat frequency? Solution Beat frequency = f 2 − f 1 = 256 Hz − 249 Hz = 7 Hz 1. A string of length 1.2 m is stretched and made to vibrate so that a stationary wave consisting of two loops is produced. (a) Draw a sketch of the wave. (b) Determine the wavelength of the wave. 2. A wire of length 20 cm, mass 1.2 g and under a tension of 120 N is plucked to generate a wave. Determine: (a) The fundamental frequency. (b) The frequency of the third harmonic. 3. The fundamental frequency of vibration of a string is f. What will Speaker 1 Speaker 2 Sound wave 1 Sound wave 2 High intensity sound Low intensity sound 1 2 2 1 Physics Form 4.indd 38 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
39 Waves the fundamental frequency be if the length of the string is halved and the tension is increased four times? 4. A closed pipe has a fundamental frequency of 400 Hz. Calculate: (a) The frequency of the first overtone. (b) The fundamental frequency of an open pipe of the same length. (Given that the speed of sound waves in air, v = 340 m s-1) 5. A speaker delivering a note of frequency 250 Hz is placed over the upper end of a vertical tube filled with water. When the water is gradually run down the tube, the air column resonates when the water level is 31 cm below the top of the tube. The air column resonates again when the water level is 99.8 cm below the top of the tube. Determine: (a) The speed of sound in air. (b) The end correction. 6. If the shortest length of a tube for resonance is 0.12 m and the next resonant length is 0.37 m, what is the frequency of vibration? Take the speed of sound in air as 340 m s-1. 7. A column of air 26.25 cm long in a closed tube resonates to a sounding tuning fork. If the velocity of sound in air is 33 600 cm s-1, what is the frequency of the fork? Ignore end correction. Electromagnetic waves Unlike mechanical waves which require a medium for their propagation, electromagnetic waves can propagate even in vacuum. These waves result from electric and magnetic fields that oscillate perpendicular to each other as illustrated in Figure 1.48. Figure 1.48: Electromagnetic wave In order to initiate an electromagnetic wave, the motion of an electric charge must be altered. This is done by accelerating the charge. Thus, the sources of electromagnetic waves are accelerating charges. That is, neither stationary charges nor steady current can produce electromagnetic waves. Electromagnetic waves are produced when electrically charged particles oscillate or their energies change. The larger the energy change, the higher the frequency of the resulting wave. In vacuum, electromagnetic waves propagate at the speed of light 3×108 m s −1 . Examples of electromagnetic waves include radio waves, microwaves, infrared radiation, visible light, ultraviolet rays, X-rays and gamma rays. Visible light is the only electromagnetic wave that can be detected by the human eye. Properties of electromagnetic waves Electromagnetic waves are transverse waves which exhibit the following characteristics: Magnetic field Electric field Direction of wave propagation Physics Form 4.indd 39 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
40 Physics for Secondary Schools Student’s Book Form Four 1. They can propagate in vacuum as well as in a material medium. 2. Electromagnetic waves undergo reflection, refraction, interference diffraction, scattering and polarisation. 3. All electromagnetic waves travel at the speed of light, that is approximately 3×108 m s −1 in vacuum. 4. They carry no electric charge. 5. They transfer energy from a source to a receiver in the form of oscillating electric and magnetic fields. 6. They obey the relation, c = f λ. 7. They can be polarised. Polarisation is the process that restrict electromagnetic waves to oscillate in a single plane or direction. The electromagnetic spectrum The electromagnetic spectrum is a continuous band of all electromagnetic waves arranged in the order of increasing wavelengths that is, decreasing frequency. A particular range of wavelength is called a band. The electromagnetic spectrum is divided into seven major regions, namely: Radio waves, Microwaves, Infrared, Visible light, Ultraviolet light, X-rays and Gamma rays, as shown in Figure 1.49. Figure 1.49: Electromagnetic spectrum From Figure 1.49, the following observations can be made: 1. The electromagnetic spectrum is continuous. That is, each band merges into the next and there are no gaps in the frequencies. Different kinds of radiation gradually change from one kind to another as their properties gradually change. 2. In some cases, there is an overlap in the range of wavelengths. This is because sometimes the name given to the wave (radiation) is determined by the source of radiation and not the wavelength (or frequency), for example X-rays and gamma rays. 1024 10-16 10-14 10-12 10-10 10-8 10-6 10-4 10-2 100 102 1022 1020 1018 1016 1014 1012 1010 108 106 104 106 108 104 102 100 f (Hz) (m) Microwave Long radiowave -rays X-rays UV IR FM AM Radiowave 400 450 480 510 550 570 590 630 700 W Increasing W Increasing Frequency (f) Visible light Violet Blue Blue green Green Yellow green Yellow Orange Red Physics Form 4.indd 40 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
41 Waves The range of frequencies and wavelengths for each band is as detailed in Table 1.4. Table 1.4: Range of frequencies of bands in electromagnetic spectrum Wavelength (m) Region (band) Frequency (Hz) >10–1 Radio waves > 3 x 109 10–1 – 10–4 Microwaves 3 x 109 – 3 x 1012 10–4 – 10–7 Infrared 3 x 1012 – 4.3 x 1014 7 x 10–7 – 4 x 10–7 Visible light 4.3 x 1014 – 7.5 x 1014 4 x 10–7 – 10–9 Ultraviolet light 7.5 x 1014 – 3 x 1017 10–9 – 10–11 X-rays 3 x 1017 – 3 x 1019 < 10–11 Gamma rays > 3 x 1019 Radio waves Radio waves have the longest wavelength in the electromagnetic spectrum hence possess the least energy. Radio waves can further be divided into long waves (LW), medium waves (MW) and short waves (SW). Short waves include very high frequency (VHF) and ultrahigh frequency (UHF) waves. Radio waves are generated by charged particles undergoing acceleration, such as timevarying electric currents. There are naturally occurring and artificially produced radio waves. Naturally occurring radio waves are emitted by astronomical objects in space such as planets, comets, stars and galaxies. Radio waves are generated artificially by transmitters and received by radio receivers, using antennas. Thus, radios are special devices used to transmit or receive radio waves. Uses of radio waves Radio waves have numerous advantages in human’s daily activities. Along with other uses, the major applications of radio waves are found in communication. Some uses of radio waves include: 1. Radio waves are very widely used in modern technology for fixed and mobile radio communication, broadcasting, Radio Detection and Ranging (RADAR) and radio navigation systems, communications satellites, wireless computer networks and many other applications. Figure 1.50 shows a radio broadcasting station. Figure 1.50: Inside a radio broadcasting station Physics Form 4.indd 41 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
42 Physics for Secondary Schools Student’s Book Form Four 2. Astronomers use large radio telescopes to collect and study radio waves from distant stars and galaxies. This helps them to determine composition, structure and motion of the celestial bodies. A radio telescope is shown in Figure 1.51. Figure 1.51: Radio telescope Microwaves Microwaves have wavelengths between 10-4 m and about 0.1 m. Their wavelengths are shorter compared to radio waves. Microwaves can be produced by artificial devices such as circuits, transmission towers, RADAR, and magnetron in microwave ovens. They can also be produced by natural sources such as stars and the Cosmic Microwave Background (CMB), a left over radiation after the Big Bang. Uses of microwaves Microwaves have many applications. Some of these applications are hereby described. 1. Microwaves are used in cooking. In this case, microwaves are absorbed by the food molecules. The absorbed energy causes the molecules to vibrate rapidly producing thermal energy that cooks or warms the food. Figure 1.52 shows a microwave oven. Figure 1.52: Microwave oven 2. Some RADAR systems use microwaves to detect the position, speed, and other characteristics of remote objects such as aircraft and satellites. 3. Microwaves are used in long-distance communication because they are not affected by clouds or other atmospheric conditions. An example of a microwave transmitter is shown in Figure 1.53. Figure 1.53: Microwave transmitter Physics Form 4.indd 42 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
43 Waves Infrared waves Infrared waves (infrared radiation) have wavelengths between 10-6 and 10-4 m. The band lies between the visible light and microwaves in the electromagnetic spectrum. Infrared waves near to the microwaves have a heating effect. Infrared waves are produced by the vibration of atoms and molecules due to their thermal energy. Almost all objects, including human bodies emit infrared waves. Infrared radiation is invisible by the human eye but visible by nocturnal animals. However, humans can sense infrared radiation as heat. If you place your hand near an incandescent light bulb, you can feel the infrared radiation being emitted. Devices that are used to detect infrared radiation include black bulb thermometers, photographic films, thermistors and phototransistors. Uses of infrared waves Infrared waves have several applications. These include: 1. Cooking or warming food in conventional ovens. 2. Infrared waves with wavelengths near the visible light are used in remote controls, night-vision devices, fibre-optic telecommunication and security systems. 3. Creating images in infrared photography. There are two techniques used to create images from infrared radiation, namely: Infrared photography and thermography. Infrared photography uses a film that is sensitive to infrared radiation. For an object to produce an image on infrared, it must be at a temperature between 250 °C and 500 °C or reflect infrared radiation from a source in that temperature range. Infrared photography is used in long-distance photography because infrared is less affected by atmospheric haze compared to the visible light. Infrared photography is also used to detect the presence of disease in plants, and pollution in rivers and other water bodies. Besides, infrared photography is used in wildlife photography, particularly of nocturnal animals since it can produce images in almost total darkness. Figure 1.54 shows an infrared photograph. Figure 1.54: Infrared photograph On the other hand, thermography or thermal imaging makes use of infrared receptors such as certain types of photoelectric and transistor Physics Form 4.indd 43 01/06/2022 13:47 FOR ONLINE USE ONLY FOR ONLINE USE ONLY DO NOT DUPLICATE
