Ordinary Level SOLUTIONS: WAVES, SOUND AND LIGHT.

Transcription

1 Ordinary Level SOLUTIONS: WAVES, SOUND AND LIGHT Question 7 [Ordinary Level] (i) Explain the term resonance. transfer of energy between objects of similar natural frequency (ii) Describe a laboratory experiment to demonstrate resonance. apparatus: Barton s pendulums plus detail // tuning fork and adjustable length of air procedure: hang the pendulums (vertically) from a horizontal string // hold the vibrating tuning fork near air column set one of the pendulums swinging // adjust the length of the air column observation/conclusion: the pendulum of the same length also swings // at a certain length the note emitted by the tuning fork gets louder // a transfer of energy occurs / resonance occurs (2 marks may be obtained from a diagram accept valid alternatives) The diagram shows a waveform. (iii)what is length A called? wavelength / λ (iv) What is length B called? amplitude/ height /depth (v) What is meant by the frequency of a wave? number (of waves) per second (vi) List three characteristics of a musical note. loudness, quality, pitch (vii) What is meant by the term natural frequency of an object? frequency at which it tends to vibrate (if free to do so) // frequency at which resonance occurs (viii) The natural frequency of a stretched string is 250 Hz. Calculate the wavelength of the sound wave produced. (speed of sound in air = 340 m s 1 ) c = f λ 340 /250 λ = 1.36 (m ) 2015 Question 12 (b) [Ordinary Level] (i) What is meant by dispersion of light? separation of light into different colours/frequencies/wavelengths (ii) What does dispersion of light indicate about the nature of white light? it is made of different colours/frequencies/wavelengths // spectrum (iii)name two laboratory techniques that can be used to cause dispersion of light. refraction / using a (transparent/glass/perspex) prism diffraction / using a (diffraction) grating/cd disc (iv) Describe one example of dispersion of light occurring in nature. rainbow, diamond reflection, reflection off of oil film, etc. (v) The diagram shows stage lighting similar to that found in most theatres. Only red, green and blue lights are needed to create all the colours needed on stage. Explain why this is so. all other coloured lights can be made from combinations of these lights // these are the three primary colours 1

2 2014 Question 12 (c) [Ordinary Level] (i) Name the distances labelled A and B. A = wavelength B = amplitude (ii) What is the frequency of the wave? 20 Hz (iii)calculate the velocity of the wave if distance A = 1.5 m. v = f λ = (20)(1.5) = ) 30 m s -1 (iv) Name a type of wave that cannot be polarised. longitudinal // sound // pressure 2013 Question 7 [Ordinary Level] (i) What is meant by the frequency of a wave? The frequency of a wave is a measure of the number of oscillations (vibrations) of the wave per second. (ii) Give the relationship between the frequency and the wavelength of a wave. v = f (iii)what will the student notice as he moves from A to B? The sound level increases and decreases (iv) Name this phenomenon. Interference (v) Explain with the aid of a diagram how this phenomenon occurs. (vi) Why should this phenomenon be taken into account in the placing of speakers in theatres or auditoriums? So that all areas of the auditorium have the same level of loudness. (vii) Explain the term fundamental frequency. The lowest resonant frequency of a vibrating object is called its fundamental frequency. (viii) What are overtones? Overtones are multiples of the fundamental frequency (ix) How can the note produced by a guitar string be changed? Change the tension or length (x) What is resonance? Resonance is the transfer of energy so that a body vibrates at its natural frequency. 2

3 2012 Question 12 (c) [Ordinary Level] The pitch of the sound emitted by the siren of a moving fire engine appears to change as it passes a stationary observer. (i) Name this phenomenon. Doppler effect (ii) Explain, with the aid of a diagram, how this phenomenon occurs. Diagram must show moving wave source and wave fronts. As the moving wave source approaches the waves get closer together therefore frequency increases. Conversely as the wave source moves away the waves are further apart therefore frequency decreases. (iii)will the crew in the fire engine notice this phenomenon? Give a reason for your answer. No as there is no relative motion between the sound source and the crew (iv) Give an application of this phenomenon. Measuring speed / speed gun, (measuring) red shift, ultrasonic scanners, imaging used to study blood flow, used to study heart beat, weather forecasting, etc. 6 partial answer e.g. general application such as medicine, radar, sonar 2012 Question 7 [Ordinary Level] (i) Explain the underlined terms. Diffraction is the spreading out of waves around a barrier / the edges of an opening Interference occurs when two waves meet and add (ii) Describe an experiment to demonstrate the wave nature of light Apparatus: (light) source, (diffraction) grating / slits, screen Procedure: shine the (narrow beam of) light through the grating Observation/conclusion; pattern on screen (iii) Explain the term polarisation Polarisation is the restriction of (vibrating electromagnetic) waves to a single plane (iv) Describe an experiment to demonstrate the polarisation of light Apparatus: (two pieces of a) polaroid sheet Procedure: look at the light through the two pieces of Polaroid. Cross/rotate one of the polaroid pieces. Observation/conclusion; the crossed pieces stops the light (v) What type of wave motion does light have as indicated by the experiment in part Transverse (vi) Why are Polaroid sunglasses more effective than non-polaroid sunglasses at reducing glare? Polaroid sunglasses remove most of the polarised reflected light (which causes the glare) 2011 [Ordinary Level] 12 (b) (i) Name the physical property of a sound wave on which each characteristic depends. Loudness depends on amplitude / frequency Pitch depends on frequency Quality depends on overtones / harmonics (ii) What is meant by the frequency limits of audibility These are the lowest and highest frequencies which humans can hear (iii)what name is given to a sound whose frequency is greater than our upper frequency limit of audibility? Ultrasonic (iv) Calculate the distance of the bat from the wall. Velocity = distance time. distance = velocity time. distance = Distance = 3.4 m 3

4 2010 [Ordinary Level] 7 The diagram shows a waveform. (i) What is the name given to the distance X and Y? X = Wavelength / λ Y = Amplitude /height /depth (ii) What is meant by the frequency of a wave? Number of waves (passing a point) per second (iii)explain the term natural frequency. Frequency objects tends to vibrate at (when set in motion) // resonance frequency (iv) If the natural frequency of a string is 250 Hz calculate the wavelength of the sound wave produced. v = f λ λ = v/f λ = 340/250 = 1.36 m (v) State the wave property on which the loudness, the pitch, of a musical note depends. Loudness depends on amplitude Pitch depends on frequency. (vi) An opera singer, singing a high pitched note, can shatter a glass. Explain why. Resonance // transfer of energy (vii) Describe a laboratory experiment to demonstrate resonance Apparatus: Barton s pendulums // tuning fork and adjustable length of air Procedure: hang the pendulums (vertically) from a horizontal string //hold the vibrating tuning fork near air column. Set one of the pendulums swinging // adjust the length of the air column observation: the pendulum of the same length also swings //at a certain length the note emitted by the tuning fork gets louder. conclusion: a transfer of energy occurs / resonance occurs 4

5 2010 [Ordinary Level] 12 (b) (i) What is meant by dispersion of light? Breaking up of (white) light into different colours. (ii) Describe an experiment to demonstrate the dispersion of light. Apparatus: white light, prism, (diffraction) grating, CD Procedure: shine a beam of light at the prism/cd/grating Observation //conclusion: different colours / 3 named colours / spectrum white light is dispersed // white light consists of different colours (iii)give an example of the dispersion of light occurring in nature. Rainbow / oil film colours / soap bubble colours / CD colours (iv) Only red, green and blue lights are needed to create most lighting effects. Explain why All colours can be made by mixing red, green and blue [Ordinary Level] 7 In an experiment a beam of monochromatic light passes through a diffraction grating and strikes a screen. (i) Explain the term monochromatic light. Monochromatic light is light of one wavelength only. (ii) Explain the term diffraction grating. A diffraction grating consists of a piece of transparent material on which a very large number of opaque (black) parallel lines are engraved. (iii) Describe what is observed on the screen. A series of bright dots. (iv) Explain, with the aid of a diagram, how this phenomenon occurs. The light waves pass through the diffraction grating and spread out on the other side after passing through the slit. Constructive and destructive interference occurs and fringes are formed on the screen. (v) What does this experiment tell us about the nature of light? Light is a wave. (vi) Name the property of light that can be determined in this experiment. The wavelength of light can be measured. (vii) What measurements must be taken to determine the property you named? The distance between bright dots, distance from the screen to grating. 5

6 2008 [Ordinary Level] 8 The diagram shows a signal generator connected to two loudspeakers emitting the same note. A person walks slowly along the line AB. (i) (ii) (iii) (iv) (v) (vi) (vii) What will the person notice? The loudness of the sound increases and decreases at regular intervals. Why does this effect occur? Because of interference caused from the two speakers. What does this tell us about sound? Sound is a wave. Describe an experiment to demonstrate that sound requires a medium to travel. Set up the Bell-Jar the bell can be heard ringing. Remove the air from the Bell-Jar using a vacuum pump. Result: While the bell can still be seen to be ringing, the sound gets quieter until eventually nothing can be heard. The pitch of a note emitted by the siren of a fast moving ambulance appears to change as it passes a stationary observer. Name this phenomenon. The Doppler effect. Explain how this phenomenon occurs. As the sound source approaches the wave-fronts become closer together therefore the wavelength becomes shorter and the frequency gets higher. Give an application of this phenomenon. Speed gun, measuring red shift, ultrasonic scanners, radar, used to study heart beat, etc [Ordinary Level] 12b Sunlight is made up of different colours and invisible radiations. (i) How would you show the presence of the different colours in light? Apparatus: light source, glass prism Procedure: shine the light through the prism Observation: light spreads out into different colours (ii) Name two radiations in sunlight that the eye cannot detect. Infra-red and ultra-violet (iii) Describe how to detect one of these radiations. Infra red Ultra violet Apparatus Infra red thermometer Florescent material Procedure Place thermometer just beyond the red end of a spectrum Place the florescent material just beyond the violet end of the spectrum Observation There will be a rise in temperature The material fluoresces (glows) (iv) (v) Give a use for this radiation. Give a use for one of these invisible radiations. Infra Red: Source of heat, keep things warm, hatch chickens, heat treatment of muscles etc. Ultra-violet: detect forged currency, disco lights, used in insect removal device, sterilisation, suntan, forensics. 6

7 2007 [Ordinary Level] 7 Resonance occurs when a vibrating object causes vibrations in nearby objects which have the same natural frequency. (i) Explain the underlined terms. Resonance is the transfer of energy between two objects which have the same natural frequency. Natural frequency is the frequency at which an object will vibrate if free to do so. (ii) Describe an experiment to demonstrate resonance. Use two identical tuning forks and a sound-board. Start one fork vibrating, place it on the sound-board and notice the sound. Place the second tuning fork on the sound-board and then stop the first tuning fork from vibrating. The second fork can now be heard. (iii)the diagram shows the waveform of a musical note. What is the name given to (i) the distance A, (ii) height B? A = wavelength B = amplitude (iv) Explain what is meant by the frequency of a wave. The frequency of a wave is the number of waves passing a fixed point per second. (v) State the wave property on which (i) the loudness, (ii) the pitch, of a note depends. (i) Loudness depends on amplitude (ii) Pitch depends on frequency (vi) A tin-whistle produces a note of 256 Hz. Calculate the wavelength of this note. The speed of sound in air is 340 m s 1 c = fλ λ = c/f λ = 340/256 = 1.33 m [Ordinary Level] 8 Dispersion occurs when a beam of white light passes through a prism forming a spectrum on a screen, as shown in the diagram. (i) What is meant by the terms dispersion and spectrum? Dispersion the breaking up of white light into its (constituent) colours Spectrum the range of colours present in white light. (ii) What happens to the white light when it enters the prism at Z? It changes direction. (iii)name the invisible radiation formed on the screen at (i) region X, (ii) region Y. X = infra-red, Y = ultra-violet (iv) Describe how to detect one of these invisible radiations. Infra red Ultra violet Apparatus Infra red thermometer Florescent material Procedure Place thermometer just beyond the red end of a spectrum Observation There will be a rise in temperature The material fluoresces (glows) Place the florescent material just beyond the violet end of the spectrum (v) Give a use for one of these invisible radiations. Infra Red: Source of heat, keep things warm, hatch chickens, heat treatment of muscles etc. Ultra-violet: detect forged currency, disco lights, used in insect removal device, sterilisation, suntan, forensics, etc. (vi) The colour on a TV screen is made by mixing the primary colours. Name the primary colours. Red, green, blue (vii) How is a secondary colour (e.g. yellow) produced on a TV screen? Mix two primary colours. 7

8 2006 [Ordinary Level] 8 (i) Describe, using diagrams, the difference between transverse waves and longitudinal waves. A Transverse wave is a wave where the direction of vibration is perpendicular to the direction in which the wave travels. A Longitudinal Wave is a wave where the direction of vibration is parallel to the direction in which the wave travels. (ii) The speed of sound depends on the medium through which the sound is travelling. Explain how sound travels through a medium. The first few molecules vibrate and the energy is then passed from particle to particle. (iii)describe an experiment to demonstrate that sound requires a medium to travel. Set up the Bell-Jar the bell can be heard ringing. Remove the air from the Bell-Jar using a vacuum pump. Result: While the bell can still be seen to be ringing, the sound gets quieter until eventually nothing can be heard. (iv) A ship detects the seabed by reflecting a pulse of high frequency sound from the seabed. The sound pulse is detected 0.4 s after it was sent out and the speed of sound in water is 1500 m s 1. Calculate the time taken for the pulse to reach the seabed. 0.2 s. (v) Calculate the depth of water under the ship. v= s/t s = v t s = = 300 m. (vi) Calculate the wavelength of the sound pulse when its frequency is Hz. c = fλ λ = c/f λ = 1500/50000 = 0.03 m. (vii) Why is the speed of sound greater in water than in air? Because the molecules of water are closer together than are the molecules of air, so the energy can be transferred more quickly [Ordinary Level] 12b The diagram shows the relative positions of electromagnetic radiations in terms of their wavelength. Gamma rays A UV light IR microwaves B (i) Name the radiations marked A and B. A = X-rays B = Radio-waves (ii) Give one property which is common to all electromagnetic radiations. Travel at the speed of light / can travel through vacuum. (iii) Which one of the radiations has the shortest wavelength? Gamma rays (iv) Describe how IR radiation is detected. Thermometer / heat sensor / photographic film (v) Give one use for microwaves. Cooking, communications /satellite TV / mobile phones / weather radar / missile guidance / speed gun 8

9 2005 [Ordinary Level] 12b (i) What is meant by (i) diffraction, (ii) interference, of a wave? Diffraction is the spreading of waves around a slit or an obstacle. Interference occurs when waves from two sources meet to produce a wave of different amplitude. (ii) In an experiment, a signal generator was connected to two loudspeakers, as shown in the diagram. Both speakers are emitting a note of the same frequency and same amplitude. A person walks along the line XY. Describe what the person hears. The loudness of the sound increases and decreases at regular intervals. (iii)what does this experiment demonstrate about the nature of sound? It demonstrates that sound is a wave. (iv) What is meant by the amplitude of a wave? Amplitude corresponds to the height of the wave [Ordinary Level] 8 (i) Sound from a vibrating object can cause diffraction and interference. Explain the underlined terms. Diffraction is the spreading of waves around a slit or an obstacle. Interference occurs when waves from two sources meet to produce a wave of different amplitude. (ii) Describe an experiment to demonstrate the interference of sound. 1. Walking slowly from X to Y, you will notice the loudness of the sound increasing and decreasing at regular intervals. 2. This is because sound waves from the two speakers will interfere both constructively and destructively, along the path XY. (iii)the diagram shows a stationary wave (standing wave) on a vibrating stretched string. What is the name given to the points on the string marked (i) X, (ii) Y? (i) X = node, (ii) Y = antinode (iv) How many wavelengths are contained in the distance marked L? Answer: 2 (v) State two factors on which the natural frequency of a stretched string depends. Length, tension, mass per unit length (vi) A note of wavelength 1.4 m is produced from a stretched string. If the speed of sound in air is 340 m s 1, calculate the frequency of the note. c = fλ 340 = f 1.4 f = Hz 9

10 2003 [Ordinary Level] 12b (i) Name two primary colours. Red, green, blue (ii) What are complementary colours? Complementary colours are pairs of colours consisting of a primary and a secondary colour, such that when combined they give white light. (iii) White light is made up of light of different colours. Describe an experiment to demonstrate this. Apparatus: white light source, prism, screen Procedure: shine light through the prism and rotate the prism. Observation: different colours are visible on the screen. (iv) The diagram shows a simple form of the electromagnetic spectrum, with wavelength increasing from left to right. Copy this diagram and indicate on it the positions of the following: microwaves; infrared; ultraviolet; X-rays [Ordinary Level] 7 (i) The dispersion of white light can be produced by refraction or diffraction. Explain the underlined terms. Refraction is the bending of light as it passes from one medium to another. Diffraction is the spreading of waves around a slit or an obstacle. (ii) Describe an experiment to demonstrate the dispersion of white light. Apparatus: white light source, prism, screen Procedure: shine light through the prism and rotate the prism. Observation: different colours are visible on the screen. (iii) The following table gives examples of electromagnetic waves and their typical wavelengths. Name one property that all of these waves have in common. They can all travel through a vacuum, refraction, diffraction, polarisation, interference, etc. (iv) What is the frequency of the radio waves? The speed of light is m s -1. c = fλ f = c/ λ f = ( )/100 f = Hz. (v) Describe how infrared radiation can be detected. With a blackened thermometer, infrared camera. (vi) Give two uses of microwaves. Radar, mobile phones, speed trap, microwave oven. 10