Waves
Waves transfer energy, and sometimes information, from one place to another without transferring matter. GCSE physics distinguishes transverse waves, such as all electromagnetic waves, where the oscillation is perpendicular to the direction of travel, from longitudinal waves, such as sound, where the oscillation is parallel to the direction of travel, and covers the wave speed equation and the electromagnetic spectrum.
Method
- To classify a wave as transverse or longitudinal, check whether the oscillation is perpendicular to the direction of energy transfer (transverse) or parallel to it (longitudinal, made of compressions and rarefactions).
- Learn the wave speed equation, wave speed = frequency x wavelength, and be ready to rearrange it for frequency or wavelength; check the units given (for example, converting a wavelength from cm to m) before substituting.
- On a wave diagram or description, read amplitude as the maximum displacement from the rest (undisturbed) position, not as the distance from a trough to a crest, and read wavelength as one full repeating unit, such as crest to crest.
- For a question about the electromagnetic spectrum, place the wave type in order from radio to gamma to reason about its relative wavelength, frequency and energy, then link this to a plausible use or hazard.
- To explain refraction, state the change in wave speed at the boundary first, then link this to the resulting change in direction, and note that the frequency of the wave does not change even though its speed and wavelength do.
- For calculations involving very large or very small numbers, such as the speed of light or wavelengths given in nanometres, write the value in standard form and track the powers of ten carefully when substituting.
Worked example
A sound wave has a frequency of 550 Hz and travels through air at a speed of 330 m/s. Calculate the wavelength of the sound wave.
- Write down the wave speed equation: wave speed = frequency x wavelength.
- Rearrange to make wavelength the subject: wavelength = wave speed / frequency.
- Substitute the values: wavelength = 330 / 550.
- Calculate: wavelength = 0.6.
- Final answer: the wavelength of the sound wave is 0.6 m.
Practice questions
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Q1State whether sound waves are transverse or longitudinal.Show answer
Answer: Longitudinal.
Q2Define the term 'wavelength' of a wave.Show answer
Answer: The distance from one point on a wave to the equivalent point on the adjacent wave, for example from one crest to the next crest.
Q3A water wave has a frequency of 4 Hz and a wavelength of 0.5 m. Calculate its speed.Show answer
Answer: 2 m/s (4 x 0.5 = 2).
Q4State the speed at which all electromagnetic waves travel through a vacuum.Show answer
Answer: 3 x 10^8 m/s (the speed of light).
Q5Name the type of electromagnetic wave used to sterilise medical equipment.Show answer
Answer: Gamma rays.
Q6Explain why ultraviolet radiation can be harmful to human skin.Show answer
Answer: Ultraviolet radiation is ionising and carries enough energy to damage cells and cause mutations in DNA, which can lead to problems such as premature skin ageing, sunburn or skin cancer.
Q7A radio wave has a wavelength of 3 m. Calculate its frequency, given that all electromagnetic waves travel at 3 x 10^8 m/s in a vacuum.Show answer
Answer: 1 x 10^8 Hz (100,000,000 Hz), since frequency = wave speed / wavelength = 3 x 10^8 / 3.
Q8State what happens to the frequency of a wave when it is refracted as it passes into a different material.Show answer
Answer: The frequency stays the same (unchanged); it is the wave's speed and wavelength that change.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student uses a ripple tank to investigate water waves. The dipper produces 12 complete waves in 3.0 seconds, and the distance across 5 complete wavelengths is measured as 40 cm. (a) Calculate the frequency of the waves. (b) Calculate the wavelength, in metres. (c) Calculate the speed of the waves.
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A ray of light travels from air into glass, hitting the boundary at an angle to the normal. (a) State what happens to the speed of the light as it enters the glass. (b) State whether the ray bends towards or away from the normal. (c) State what happens to the frequency of the light as it enters the glass.
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A dentist uses X-rays to take images of a patient's teeth, but stands behind a lead screen and leaves the room while the X-ray is taken. Explain, in terms of the properties of electromagnetic waves, why X-rays are useful for imaging teeth, and why precautions such as a lead screen are necessary.
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Free printable worksheet
Want more practice on paper? Download the waves worksheet pack - 13 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.
This topic is chapter 6 of GCSE Physics Workbook, the whole course as one free printable PDF.
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