What is the difference between transverse and longitudinal waves?

In transverse waves the vibrations are at right angles to the direction of travel (light, water ripples). In longitudinal waves the vibrations are parallel to the direction of travel (sound), producing compressions and rarefactions.

What happens to frequency during refraction?

Nothing — frequency is fixed by the source and does not change. The speed changes, and since v = fλ, the wavelength changes with it. Stating 'frequency stays constant' is a standard mark.

When is diffraction most noticeable?

When the gap (or obstacle) is comparable in size to the wavelength. That is why sound diffracts around doorways easily (metre wavelengths) but light does not (wavelengths of less than a micrometre).

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IGCSE Physics revision · Waves

General wave properties

One equation, four behaviours, and a set of definitions the examiner wants word-perfect. Everything in the next three topics — light, the electromagnetic spectrum, sound — is built on this page.

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What the syllabus demands

—Describe waves as transferring energy without transferring matter
—Define wavelength, frequency, amplitude and wave speed; label them on diagrams
—Distinguish transverse from longitudinal waves with examples
—Use the wave equation v = fλ
—Describe reflection, refraction and diffraction of waves, using ripple-tank diagrams

Definitions that earn marks

Clear definitions to practise — check your course mark scheme

Wavelength (λ): The distance between two consecutive corresponding points on a wave, e.g. crest to crest.
Frequency (f): The number of complete waves passing a point per second, measured in hertz (Hz).
Amplitude: The maximum displacement of the wave from its rest (undisturbed) position.
Transverse wave: A wave in which the vibrations are perpendicular to the direction of energy transfer — e.g. light, water waves.
Longitudinal wave: A wave in which the vibrations are parallel to the direction of energy transfer — e.g. sound.

The equations

Wave equationv = f × λ · m/s

Frequency and periodf = 1 ÷ T · Hz

More equations to practise: the IGCSE formula sheet.

Where the marks die

Common mistakes to check

01
Measuring amplitude from crest to trough. Amplitude is from the rest position to the crest — half the crest-to-trough distance.
02
Saying waves transfer matter. Waves transfer energy; the particles (or the water) oscillate about fixed positions and stay where they are.
03
Refraction answers without the cause: waves change direction because their speed changes when they enter a different medium (or different water depth). Speed change first, direction change second.
04
Diffraction drawn wrong: the most spreading occurs when the gap width is similar to the wavelength. Bigger gaps give less curvature.

One worked example, done properly

Question

A wave has a frequency of 50 Hz and a wavelength of 6.8 m. Calculate its speed.

Method

1.v = fλ = 50 × 6.8.

v = 340 m/s

Test yourself

10 questions · instant marking

Question 1 of 10

A wave has frequency 4 Hz and wavelength 2 m. Its speed is:

Question 2 of 10

Sound is an example of a:

Question 3 of 10

The amplitude of a wave is measured from:

Question 4 of 10

When a wave is refracted, which quantity never changes?

Question 5 of 10

Waves spread out most when passing through a gap that is:

Question 6 of 10

A wave has period 0.02 s. Its frequency is:

Question 7 of 10

Waves transfer:

Question 8 of 10

Water waves slow down as they enter shallow water at an angle. They:

Question 9 of 10

Which of these is a transverse wave?

Question 10 of 10

Radio waves of speed 3 × 10⁸ m/s have wavelength 1500 m. Their frequency is:

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