Waves: Light and Sound
Waves transfer energy from one place to another without transferring matter, and can be transverse, with oscillations at right angles to the direction of travel like light, or longitudinal, with oscillations parallel to the direction of travel like sound. This KS3 topic covers wave properties, the wave speed equation, reflection, refraction, and how pitch and loudness relate to sound waves.
Method
- Identify whether a wave is transverse (e.g. light, water ripples) or longitudinal (e.g. sound), based on whether the oscillations are perpendicular or parallel to the direction the wave travels.
- Use wave speed = frequency x wavelength, rearranging to find frequency or wavelength when needed.
- Apply the law of reflection (angle of incidence equals angle of reflection, both measured from the normal) to problems involving mirrors.
- Explain refraction as light changing direction because it changes speed when it crosses a boundary between two different materials, such as slowing down and bending towards the normal on entering glass.
- Link amplitude to loudness (greater amplitude means louder) and frequency to pitch (greater frequency means higher pitch) for sound waves.
- For echo and distance calculations, remember the sound travels to the surface and back, so divide the total distance travelled by 2 to find the one-way distance.
Worked example
Tariq stands 220 m from a tall cliff and shouts. Assume the speed of sound in air is 340 m/s. Calculate the time taken for Tariq to hear the echo of his shout, from the moment he shouts.
- Recognise the sound must travel to the cliff and back, so the total distance travelled is 2 x 220 = 440 m.
- Write down the equation: time = distance / speed.
- Substitute the values: time = 440 / 340.
- Calculate: 440 / 340 = 1.294...
- Final answer: 1.3 s (to 2 significant figures).
Practice questions
Type your answer and press Check to be marked straight away, or reveal the answer and mark yourself.
Q1State whether sound waves are transverse or longitudinal.Show answer
Answer: Longitudinal
Q2A wave has an amplitude of 0.3 m and a wavelength of 2.4 m. State the wavelength of the wave.Show answer
Answer: 2.4 m
Q3A water wave has a frequency of 4 Hz and a wavelength of 0.5 m. Use wave speed = frequency x wavelength. Calculate the speed of the wave.Show answer
Answer: 2 m/s (4 x 0.5)
Q4A ray of light hits a plane mirror, with the angle between the incident ray and the mirror surface measured as 40 degrees. Calculate the angle of incidence, measured from the normal.Show answer
Answer: 50 degrees (90 - 40)
Q5A sound wave has a frequency of 500 Hz. Use time period T = 1 / frequency. Calculate the time period of this wave, in standard form.Show answer
Answer: 2 x 10^-3 s (1 divided by 500)
Q6Amara sees a flash of lightning and counts 4 seconds before she hears the thunder. Assume the speed of sound in air is 340 m/s and that the time for light to reach her is negligible. Calculate the distance between Amara and the lightning strike.Show answer
Answer: 1360 m (340 x 4)
Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
A student investigates the speed of sound in air by standing 150 m from a large flat wall, firing a starting pistol, and timing from seeing the flash to hearing the echo. The student records a mean time of 0.88 s for the sound to travel to the wall and back. Use speed = distance / time. Calculate the speed of sound in air, using the mean time given.
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State the law of reflection, and use it to state the angle of reflection when a ray of light hits a plane mirror at an angle of incidence of 42 degrees.
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A student is confused about why astronauts in space can see an explosion happen but cannot hear it. Use your knowledge of light waves and sound waves to explain this, comparing how each type of wave transfers energy. In your answer, refer to: whether each wave is transverse or longitudinal; what each wave needs in order to travel; and one everyday application of each wave type.
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Free printable worksheet
Want more practice on paper? Download the waves: light and sound worksheet pack - 18 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
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