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1. 1. 6.1 WAVESWhat is Process of transferring energy from one location to anotherwaves? which is produced by an oscillating or vibrating motion.Examples of Light waves are produced as a result of vibrations ofwaves electrons in an atom Sound waves are produced by vibrating mechanical bodies such as a guitar strings or a tuning fork. Water waves are produced by a disturbance on a still water surface.How do When energy is transferred by a wave from a vibratingwaves source to a distant receiver, there is no transfer of mattertransfer between the two points.energy? When the string is shaken up and down, a disturbance moves along the length of the string. It is the disturbance that moves along the length of the string, not parts of the string itself. Drop a stone in a quite pond. It will produce a wave that moves out from the center in expandingThe energy circles. It is the disturbance that moves, not thetransferred from a water. After the disturbance passes, the water isvibrating source to where it was before the wave was produced .a receiver is carriedby a disturbance ina medium, not bymatter moving fromone place toanother within the The string and water is the medium through whichmedium wave energy travels.What is A transverse wave is a wave in which the vibration ofTransverse particles in the medium is at right angle to the direction ofWave? propagation of the wave. 1
2. 2. The spring is moved sideways. The motion of the particles medium (spring) is at right angles to the direction in which the wave travels. Examples: water waves, light wavesWhat is A longitudinal wave is a wave which the vibration of particlesLongitudin in the medium is along (parallel to) the direction ofal Waves? propagation of the wave. The slinky spring moves backwards and forwards to produce a transverse wave. The particles of the medium (spring) move along the direction of the wave. The wave that travels along the spring consists of a series of compression and rarefaction. Examples: sounds waves. 2
3. 3. What is a ripple tank? The water acts as a lens toThe phenomenon of water waves can be produce a pattern of brightinvestigated using a ripple tank. and dark regions on a piece ofThe water waves are produced by a white paper placed under thevibrating bar on the water surface. tank when light passes through it.The tank is leveled so that the depth of Water waves have crests andwater in the tank is uniform to ensure troughs.water waves propagate with uniformspeed. A crest is the highest position of the wave acts as a convex lens, whereas a trough is the lowest position acts as a concave lens. Light rays from the lamp on top will focus onto the white screen below. The bright lines correspond to the crests, and the dark lines correspond to the troughs. 3
4. 4. What is meant by a wavefront? 1. Plane wavefrontsLines joining all the points ofthe same phase are calledwavefronts. 2. Circular wavefrontsThe wavefronts of a transversewave and longitudinal wave areperpendicular to the directionof propagation of the waves.Describing WavesVibration/Oscillation Amplitude (a)The movement from The maximum displacement from its equilibriumone extreme position to position.the other and back to Amplitude relates to loudness in sound andthe same position. brightness in light. SI unit: meter, m Wavelength (λ) The distance between two adjacent points of the same phase on a wave. The distance between The distance between two successive crests two successive or two successive compressions or two troughs successive rarefactions in a sound wave. 4
5. 5. Period (T) Frequency, f Relation betweenThe time taken for an The number of waves frequency and period:oscillation to complete produced in oneone cycle. second. f = 1SI unit is second (s). SI unit is Hertz (Hz) TWave Speed (v) The relationship between speed, wavelength andThe speed of a wave is frequencythe measurement ofhow fast a crest is Velocity = wavelength x frequencymoving from a fixed v = fλpoint.SI unit is ms-1.Displacement-time graph Displacement-distance graph Velocity , v = fλExample 1 From the graph, calculate: (a) Amplitude (b) Period, (c) Frequency 5
6. 6. Example 2 A graph shows a wave produced by a slinky spring vibrating at frequency 8 Hz. What is: (a) amplitude (b) wavelength (c) wave speedWhat is Damping is the decrease in the amplitude of an oscillatingdamping? system when its energy is drained out as heat energy. The amplitude of an oscillating system will gradually decrease and become zero when the oscillation stops.What 1. External damping of the system is the loss of energy tocauses overcome frictional forces or air resistance.damping? 2. Internal damping is the loss of energy due to the extension and compression of the molecules in the system.A graph toshowdampingExternal To enable an oscillating system to go on continuously,Force an external force must be applied to the system.Force The external force supplies energy to the system. Suchoscillation a motion is called a forced oscillationNatural The frequency of a system which oscillates freelyfrequency without the action of an external force is called the natural frequency. 6
7. 7. Resonance Resonance occurs when a system is made to oscillate at a frequency equivalent to its natural frequency by an external force. The resonating system oscillates at its maximum amplitude.Experiment The frequency of a simple pendulum depends on thein Barton’s length of the pendulum.pendulum In Barton’s pendulum experiment, there are many pendulums tied to the rope. Two of the pendulum are of the same length When pendulum B oscillates, all the other pendulums are forced to oscillate. But pendulum DHow does oscillates with the largestresonance amplitude, ie, pendulumoccur in D resonatesthe twopendulum Pendulum B and pendulum D are of the same length.of equal Frequency B = Frequency Dlength? Therefore, pendulum B causes pendulum D to oscillate at its natural frequency.Good 1. The tuner in a radio or television enables us to selecteffects of the programmes we are interested. The circuit in theresonance tuner is adjusted until resonance is achieved, at the frequency transmitted by a particular station selected. Hence a strong electrical signal is produced. 2. The loudness of music produced by musical instruments such as the trumpet and flute is the result of resonance in the air.Bad effects 3. A bridge can collapse when the amplitude of itsof vibration increases as a result of resonance.resonance 7
8. 8. 6.2 REFLECTION OF WAVESReflection of Occurs when a wave strikes an obstacle.wave The wave undergoes a change in direction of propagation when it is reflected. The value of frequency (f), wavelength (λ) and speed (v) remain the same after reflection.Incident wave : the wave before it strikes theobstacle Law of Reflection:Reflected wave: the wave which has undergone The angle of incidence, ia change in direction of propagation after is equal to the angle ofreflection. reflection, r.i = angle of incident – the angle between the direction of propagation of incident wave and the normalr = angle of reflection – the angle between the direction of propagation of reflected wave and the normal.Reflection of plane water waves in aripple tank1. Set up a ripple tank.2. Switch on the motor to set the vibrating. Increase the frequency of the waves by increasing the voltage power supply to the motor.3. Observe the reflected wave by using a stroboscope.Draw adiagram toshowreflection ofwaves. 8
9. 9. 6.3 REFRACTION OF WAVESWhat is Refraction of waves is a change in its direction as the wavesRefraction pass from one medium to another. It occurs when there is aof waves? difference in the speed of the wave at the boundary of two mediums.What After refraction, the wave has the same frequency, but ahappens to different speed, wavelength and direction of propagation.frequency,speed,wavelength The relationship between v and λ& direction? of a water wave in deep and shallow water: v = fλ f is constant ∴v ∝ λ v is directly proportional to λ v v v f = = cons tan t ∴ 1 = 2 λ f1 f2Use thewords, Water waves Water waves‘increase’, Characteristics passes from Characteristics passes fromdecrease’ deep water to shallow wateror shallow water to deep water‘unchanged’ Speed Decrease Speed increase Wavelength Decrease Wavelength Increase Frequency unchanged Frequency unchangedHow does Water passing from the deep Water passing from the shallowthe region to the shallow region, region to the deep region, thedirection of the water wave is refracted water wave is refracted awaywaveschange toward the normal. from the normal.when: 9
10. 10. Use a rippletankDraw a raydiagram toshowrefraction ofwaves.Example 1A plane wave has a wavelength of 2 (a) the wavelengthcm and a velocity of 8 cm s-1 as itmoves over the surface of shallow (b) the frequencywater. When the plane wave moves of the wave in the area of greaterinto an area of greater depth, its depth?velocity becomes 12 cm s-1. What is 10
11. 11. Example 2The diagram shows a plane waterwave moving from one area P toanother area Q of different depth.If the speed of water wave in P is 18cm s-1, what is the speed of waterwave in Q? 6.4 DIFFRACTION OF WAVESWhat is Diffraction of waves is a phenomenon in which wavesdiffraction of spread out as they pass through a gap or round a smallwaves? obstacle.What are 1. Frequency, wavelength and speed of waves do notCharacteristics change.of diffracted 2. Changes in the direction of propagation and the pattern ofwaves? the waves. 3. The amplitude of the diffraction wave decreases so its energy decrease.What are the The effect of diffraction is obvious if:factors that 1. the size of the gap or obstacle is small enoughinfluence the 2. the wavelength is large enough.effect ofdiffraction? The effect of diffraction is obvious if the shape of the diffracted waves more spread out or more circular 11
12. 12. Diffraction Sound diffractingof sound around corners so allowing us to hear others who are speaking to us from adjacent rooms.Diffraction Light is diffracted if itof light passes through a narrow slit comparable in size to its wavelength. However, the effect is not obvious as the size of the slit increases. This is because the wavelengths of light are very short.We can hear the sound Sound waves are more easily diffracted inof a radio placed nearby comparison to light waves because thea corner of a wall but we wavelength of sound waves is much longer thancannot see the radio. the wavelength of light waves.Why?Procedure A ripple tank is filled with water and set up as shown. Switch on the power pack. Use a barrier to block the incident straight water waves. Observe the wave pattern beyond the barrier. Send a straight water waves to pass through a gap. Observe the pattern of diffracted waves beyond the gap. Send straight water waves towards a small gap. Observe the wave pattern beyond the small gap. 12
13. 13. Observation (a) Wide gap (b) Narrow gap The waves are circular and appear The waves are bend only at the to originated from the small gap. edges after passing through the The effect of diffraction is obvious gap. The effect of diffraction is not obvious Straight water wave propagate towards an obstacle.As the size of the gap or obstacle is smaller , the effect of diffractionbecomes obvious. 13
14. 14. 6.5 INTERFERENCE OF WAVESState the When two waves interfered, the resulting displacementPrinciple of of the medium at any location is the algebraic sum ofsuperposition of the displacements of the individual waves.Waves(a) Superposition Constructive of two crests Interference(b)Superposition of two Constructive troughs Interference(c) Superposition Destructive of a crest and Interference a troughWhat is Interference is the superposition of two wavesInterference of originating from two coherent sources.Waves?What is coherent The waves from coherent sources have the samesources? frequency (f), same wavelength and constant phase difference.How does Wave interference occurs when two waves meetinterference while propagating along the same medium.occur? When the two waves are superposed, interference will occur either constructive interference or destructive interference. 14
15. 15. Constructive Occurs when the crests or troughs of both wavesInterference coincide to produce a wave with crests and troughs of maximum amplitude.Destructive Occurs when crest of one wave coincide with theinterference trough of the other wave, thus canceling each other with the result that the resultant amplitude is zeroThe occurrenceof constructiveinterferenceanddestructiveinterferenceAntinode A point where constructive interference occursNode A point where destructive interference occurs.Keys: Maximum crest wave (2 crests meet) Zero amplitude (trough meets crest) Maximum trough wave (2 troughs meet) 15
16. 16. Young’s formula a = distance between two coherent sources λ = wavelength x = distance between two consecutive node (or antinode) lines D = distance from the two sources to the point ofThe relationship measurement of axbetween λ, a, xand D λ= x DFactors affecting The interference pattern depend on the value of xthe interference When x changes, the interference pattern also changespattern λD x= a 1 The distance between 2 consecutive lines, x is1. x∝ inversely proportional to the distance between 2 a sources,a As a becomes larger, x As a becomes smaller, x becomes smaller becomes larger 16
17. 17. The distance between two consecutive node lines or2. x∝λ antinode lines , x increases is directly proportional to the wavelength of the wave , λ where a & D are Low frequency (large λ) High frequency (small λ)constant As λ decreases, x also As λ increases, x decreases. increases3. x∝D The distance between two consecutive node lines or antinode lines, x is directlyx directlyproportional to D proportional to the distance from the two sources to thewhere a & λ are point of measurement of x, DconstantInterference of lights Occurs when an incident light wave passes through a double slit. An interference pattern is produced as a result of the superposition of two emerging light waves from the double slit.Young’s double-slit Use monochromatic light (light which hasexperiment one colour and one wavelength) The double slit must be very narrow (about 0.5 mm) to produce a clear interference pattern because the wavelength of light is very small. When light from monochromatic source passes through a double slit, two sources of coherent light are produced. 17
18. 18. The interference pattern consists of alternate bright and dark fringes that can be seen on a distant screen. Bright fringes: constructive interference Dark fringes: destructive interference. a = Distance between the two slits on the double slit plate D = Distance between the double-slit plate and the screen λ = The wavelength of light depends on its color. x = Distance between two consecutive bright fringes or dark fringes. axλ= DInterference of Sound Occurs when two coherent sound waves interactWaves on the basis of the principle of superposition to produce a pattern of The two loud speakers are the sources of the two coherent sound waves as they are connected to the same audio signal generator.a= the distance between A student is requested to walk in a straight path atthe two loudspeakers a distance of D from the loudspeakers.D = Distance between theloudspeakers and thepath along which The student hears alternating loud and softinterference can be sounds as he walks along the straight path.detectedλ = The wavelength ofsound waves is 18
19. 19. influenced by the The alternating loud and soft sounds is caused byfrequency of the audio interference of the sound waves.signal generator. The loud sound: constructive interferencex = Distance between twoconsecutive positions The soft sound : destructive interference.where loud sound is axheard λ= D Water wave Sound wave Light waveλ The wavelength of water The wavelength of The wavelength of light waves is influenced by sound waves is depends on its color. the frequency of the influenced by the vibrator frequency of the audio signal generator.D Distance between the Distance between the Distance between the spherical dippers and the loudspeakers and the double-slit plate and the position marked x is path along which screen measured interference can be detecteda Distance between the two Distance between the Distance between the spherical dippers two loudspeakers two slits on the double slit platex Distance between two Distance between two Distance between two consecutive antinode consecutive positions consecutive bright lines or two consecutive where loud sound is fringes or dark fringes. node lines heard High amplitude of water Loud sound Bright fringes Calm water Soft sound Dark fringes 19
20. 20. Example 1 Example 2In the interference of two coherent In a Young’s double slit experiment, thesources of waves, the separation between distance between the double slit and thetwo spherical dippers is 3 cm and the screen is 4.0 m and the separation of thedistance between two consecutive node two slits is 0.5 mm. calculate the distancelines is 4 cm measured at a distance of 15 between two consecutive bright fringescm from the two coherent sources of for violet light with a wavelength ofwaves. Calculate the wavelength of the 4.0 x 10-7 mwater waves originating from the sources.Example 3The wavelength of light can be determinedwith a double-slit plate. The diagramshows the pattern of interference fringesobtained in a Young’s double-slitexperiment. The separation of distance ofthe two slits is 0.25 mm and the distancebetween the screen and the double slitplate is 3.0 m.Calculate the wavelength of light used inthe experiment.Example 4In an experiment on the interference ofwaves, two loudspeakers are placed at adistance of 1.5 m from each other. Theyare connected to an audio signalgenerator to produce coherent soundwaves at a frequency of 0.5 kHz.Calculate (a) the wavelength of the sound wave if the speed of sound is 300 ms-1 (b) the distance between two consecutive soft sounds at a perpendicular distance of 5 m from the source of the sound. 20
21. 21. 6.6 ANALYSING SOUND WAVESWhat is Sound is a form of energy propagated as waves that makesound our eardrums vibrate.waves? Sound waves are caused by vibrating objects. Sound waves are longitudinal waves.How is • Sound waves are produced when a vibrating objectsound causes the air molecules around it to vibrate.produced bya vibratingobjects? • When a tuning fork vibrates, layers of air vibrate and the sound energy is propagated through the air around it in the form of waves. • When the tuning fork moves forwards, the air is compressed. • When the tuning fork moves backwards, the air layers are pulled apart and cause a rarefaction. • Therefore, a series of compression and rarefactions will produce sound.Why does • The air particles vibrate backward and forward in thesound waves direction parallel to the direction of propagation of theis alongitudinal sound wave.waves? • Wavelength of sound, λ = the distance between two successive regions of compression or two successive regions of rarefaction.Explain how • The loudness of the sound depends on its amplitude.the loudness • If the amplitude is increased, the loudness increases.relates toamplitude?Explain how • A high pitch sound corresponds to a high frequency and 21
22. 22. the pitch a low pitch sound corresponds to a low frequency ofrelates to vibration.frequencyTo investigate the relationship between the amplitude and the loudnessof soundThe audio signal generator is switched on and the frequency of thesound wave is adjusted to a suitable level. The loudness of the sound isvaried from a lot to a high level gradually. Observe the shape of thesound wave displayed on the screen of oscilloscope.Write: low / medium / highWave form Amplitude of Loudness of sound wave soundThe relation between the pitch and the frequency of soundThe audio signal is switched on and the loudness is adjusted to asuitable level. The frequency of the sound is varied from low to highgradually. The pitch of the sound that is heard and the form of the wavedisplayed on the screen of the oscilloscope is observed.Write: low / medium / highWave form Frequency of Pitch of sound sound wave 22
23. 23. describe applications The reflection of sound is called echoes.of reflection of soundwaves. Infrasound Normal Ultrasound audible range Less than 20 20 Hz to Higher thanUltrasound in Hz 20 000 Hz 20 000 Hz.medicine • Ultrasound waves is used to scan and capture the image of a fetus in a mother’s womb and the image of internal organ in a body. • Transmitter P emits ultrasound downwards to the fetus. • Detector R receives the ultrasound (echoes) reflected by the various parts of the fetus. • The soft tissues of the fetus absorb most of the incident ultrasound, reflect very little. The bony parts will absorb very little, but reflect most of the ultrasound. The reflected ultrasound will produce an image of contrasting brightness.Sonar • Sonar is the technique of using ultrasound to locate underwater objects or to measure the depth of a seabed. • Ultrasound signal is sent out from a transmitter. • Its echo from the seabed is detected by a receiver which is connected to an electrical recording circuit. • The time interval, t between the sending and receiving of the ultrasound signal after reflection from the seabed is measured. 23
24. 24. t • The depth of the seabed, d = v × where v is 2 the velocity of sound in water. • When ultrasonic waves emitted by the bat hitA bat can navigate in an object, they are reflected back and receiveddarkness by the bat. • The time between the emission of the sound waves and reception of the reflected waves enables the bat to estimate the position of the object accurately. • This enables the bat to adjust its direction to avoid knocking at the object.Calculate An ultrasonic wave is used to determine the depth of a seabed. Adistances pulse of ultrasound is generated and travels to the seabed and reflected by it. The time taken by a pulse of ultrasonic wave tousing the travel to and fro the seabed is 0.28 s. It the speed of sound in thereflection of water is 1 500 ms-1, calculate the depth of the seabed.sound waves. 24
25. 25. 6.7 ANALYSING ELECTROMAGNETIC WAVESDescribe the electromagnetic spectruml------------Radio waves--------llow frequency high frequencyhigh wavelength low wavelengthWhat is the • It consists of a group of waves with similar natures.electro- • The members of the electromagnetic spectrummagnetic arranged in increasing frequencies and decreasingspectrum? wavelengths are radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X – rays and gamma rays. • Radio waves have the longest wavelength but are of low frequency waves. They carry very little energy. • Gamma rays have the shortest wavelength but are of high frequency waves. They carry very high energy.What is the • It is produced when electric and magnetic field vibrateelectro- at right angle to each other.magnetic • The direction of propagation of the wave iswave? perpendicular to both fields. 25
26. 26. State the • Visible light waves are the only electromagnetic wavesvisible we can see. Light can be seen as the colours oflight is a rainbow.part of the • Each colour has a different wavelength.electro- • Red has the longest wavelength and violet the shortest.magnetic • When all the waves are seen together, they make whitespectrum light. • When white light shines through a prism, the white light is broken apart into the seven colours of the visible light spectrum. • Red, orange, yellow, green, blue, indigo and violet.Describe 1. They transfer energy from one point to another.the 2. They are transverse waves.properties 3. They can travel through vacuum.of electro- 4. They travel at the same speed through vacuum, i.e atmagnetic the speed of light , c = 3 x 108 ms-1.waves 5. They all show wave properties such as reflection, refraction, diffraction and interference. 6. They obey the wave equation, v = fλ.List sources of electro-magnetic waves and the applications.Electromagnetic Sources Applications waveGamma Rays Radioactive • Engineering – to detect leakages substances in underground pipes • Medicine – cancer treatment • Food sterilisation 26
27. 27. X- rays x-ray tube • Medicine i. X-ray photograph of the internal organs of the body, e.g to locate bone fracture. ii. Cancer treatment • Engineering – to detect cracks in metal • Checking of luggage at airportsUltraviolet rays The sun, • Cause sunburn mercury • Stimulates the formation of vapour vitamin D needed for assimilation lamp. of calcium and the prevention of rickets. • Detect fake notes • Fluorescent lamp • Sterilization of surgical tools and plant seedlings.Visible light Flames, • Visual communication lamps, • Photography the sun • PhotosynthesisInfrared Hot objects • A sensation of warmth is feltradiation such as when IR falls on the skin. flames, the • Thermal imaging and human physiotherapy body, the • Infrared binoculars for night time sun vision. IR radiation emitted by a living thing can be detected. • Remote control for TV / VCRMicrowaves Radar • Communication system with transmitter satellites Microwaves • Used in radar system oven • Cooking • Cellular (mobile) phone serviceRadio waves Electrons • For broadcasting and wireless oscillating communication in aerials • UHF (ultra high frequency) radio Radio/ waves – television and hand television phones 27
28. 28. transmitter • VHF (very high frequency) radio wave – local radio FM and wireless communication used by the policeDescribe the detrimental effects of excessive exposure to certaincomponents of the electromagnetic spectrum.Radio waves No evidence of hazardMicrowaves Internal heating of body tissues when they enter our body. Long exposure to mobile phones can cause brain tumor and inner ear complications in children. Just SMS.Infrared Skin burnsVisible light No evidence of hazardUltraviolet Damage to the surface cells (including skin cancer) and blindnessX-rays Damage to cells.Gamma rays Cancer, mutation The mutated cells may result in the abnormal growth of cancer cells. Pregnant mothers who are exposed to X-rays and radiations too frequently may cause abnormalities in new born babies. 28