Light Waves & Sound

Chapter 12 - Light


State the properties of the image formed by a plane mirror

  • Upright
  • Image size equals object size
  • Virtual
  • Image distance equals object distance
  • Laterally inverted.


State one advantage of using a plane mirror in optical testing.

  • When a plane mirror is used, the image of the optical test card will be at a distance equal to the object distance. In this way, it can appear further than it actually is.
  • The larger distance between the viewer and the test card can be obtained. This distance can even be longer than the room itself.


Explain why refraction takes place when light travel from air to glass.

  • Light travel at different speeds in different optical media.
  • Its speed in air is 3.0 x 10^8 m/s while its speed in glass is lower at 2.0 x 10^8 m/s.
  • At the boundary of the two media, the sudden change in speed causes the path of light to bend.


Explain why TIR can only take place when light is traveling from denser to less dense medium

  • Only when light travels from denser to less dense medium, then will it bend away from the normal.
  • This is essential for the refracted angle in the less dense medium to be 90 degrees for a given incident angle in the denser medium. This incident angle is called the critical angle.
  • When the incident angle is more than the critical angle, TIR will then take place.


*Why do diamonds have such brilliant sparkle?

  • Having high refractive indexes, diamonds have small critical angles.
  • A large proportion of light at the diamond-air boundary will have incident angle greater than that of the critical angle, hence undergoing TIR.
  • The diamond is also cut in such a way that when light enters, it will undergo TIR a few times before most of them emerge from the top surface.


State some uses of optical fibres.

  • Telecommunication – transmission of signal.
  • Endoscope – to see organs in the body by directly light through a thin plastic tube into the body.
  • Binoculars – prisms allow for TIR and reduce length of the instrument.
  • Periscope – prisms are positioned to reflect light and give clear images.


State the advantages of using optical fibres over copper wires in telecommunications.

  • Optical fibres can carry much more information than copper wires.
  • Optical fibres experience less signal loss
  • Optical fibres are lighter and hence easier to install.
  • Optical fibres are cheaper to manufacturer. (many physics questions will state to avoid this economical reason)

Chapter 13 - Waves


A transverse wave can be demonstrated using a rope. Describe how the hand must move to make a transverse wave of frequency 5 Hz.

  • For a transverse wave to move from left to right, the hand must move in a to and fro motion vertically.
  • There should be 5 oscillations of the hand in 1 second.


A wave travels in a medium with a certain speed. What must be done to double the wavelength of the wave in the same medium?

  • Since the wave is in the same medium, the speed does not change.
  • From speed of wave = frequency x wavelength, the source of the wave should be at half the frequency in order to achieve a doubled wavelength.


Describe what happens to water waves as it travel from a deep to shallow region.

  • When water wave travels from a deep to shallow region, its speed is slowed down while its frequency remains constant.
  • Since speed of wave = frequency x wavelength, the wavelength will decrease too.

Chapter 14 - Electromagnetic Waves

1.    State the properties which are common to all forms of electromagnetic radiation.

  • All are transverse waves.
  • All travel with the same speed, 3 x 10^8 m/s in vacuum.
  • They all obey the wave equation:
  • They can be reflected or refracted
  • They transports energy.

2.    How do the frequency, wavelength and speed of electromagnetic waves change when it enters from air to glass?

  • When an EM wave enters glass, its speed is slowed down while its frequency remains constant.
  • Since, the wavelength will decrease too.

3.    State one difference between waves of red light and waves of blue light.

  • Red light has a longer wavelength (or lower frequency) than blue light.

4.    State 3 ways in which an electromagnetic wave differs from the sound wave.

 Sound waves

  • Longitudinal waves
  • Travel at an approximate speed of 330m/s through air
  • Needs a medium to travel, cannot travel through vacuum

Electromagnetic waves

  • Transverse waves
  • Travel at an approximate speed of 3 x 10^8 m/s through air
  • Does not need any medium to travel. Can travel through a vacuum

Chapter 15 - Sound

1.    Describe how a vibrating object produces sound waves. (2 marks)

  • Sound is produced by vibrating sources placed in a medium.
  • When the object (e.g. ruler or tuning fork) vibrates, a series of compressions (high air pressure) and rarefactions (low air pressure) are produced by shifting of air layers.
  • In doing so, a longitudinal wave is produced.

2.    Describe how a vibrating object produces sound waves. (4 marks)

  • Sound is produced by vibrating sources placed in a medium.
  • When the vibrating object moves outwards, it pushes the air layers close together and produces a region of higher pressure known as compression.
  • When the object moves inwards, it pulls the air layers apart and produces a region of lower pressure known as rarefaction.
  • The continuously vibrating object thus produces a series of compression and rarefactions traveling away from the vibrating source as a longitudinal wave.
  • Energy is transferred away from the source via collisions of adjacent particles in the medium.

3.    Describe how the air pressure at a particular point changes when a sound wave passes that point.

  • Air pressure changes from high (compression) to low (rarefaction) alternately.

4.    Describe an experiment which shows that a medium is needed to transmit sound waves.

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  • Before air is drawn out from the bell jar, the sound of the ringing bell can be heard.
  • Evacuate air out of the bell jar slowly using the vacuum pump. The sound of the ringing bell decreases until eventually, no sound is heard, although the hammer can still be seen vibrating.
  • This experiment shows that a medium is needed to transmit sound waves.

5.    Suggest briefly how measurements involving echoes can be used to find the depth of water in a sea.

  • SONAR (SOund Navigation And Ranging) is used to send out sound waves until they are reflected by the bottom of the lake.
  • The reflected sound is detected and the time taken, t, is recorded.
  • If the speed of the sound in water v is known, then the depth of the lake, d, can be calculated by the formula:

6.    Describe how you would carry out an experiment to measure the speed of sound in air. Your answer should make clear what measurements you would take and how those measurements would be used to produce the results.

  • Position observers A and B at a large known distance, d.
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  • The distance d should be large so that it reduces the percentage error due to human reaction.
  • Observer A fires the pistol.
  • Observer B uses his stopwatch to measure the time interval t1 between on seeing the flash of the pistol and hearing the sound.
  • To reduce the measurement error due to the effect of the wind, repeat the experiment with A and B switched place, measuring the the time t2.. Obtain the average of the two times, <t> = (t1+t2)/2
  • The speed of sound, v can be calculated by:

                                                      Speed = Distance/Time = d/<t>