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• Water waves are ripples that travel through water.
• Water waves are transverse waves and can be reflected from surfaces.
• When two waves meet, they either add together or cancel each other out. This change of two waves into one wave is called superposition.

If you throw a pebble into a pond, ripples spread out from where it went in. These ripples are waves travelling through the water. Like ocean waves, these ripples are transverse waves because the surface of the water moves up and down, at 90° to the direction that the wave is travelling.

If you stand still in the sea, the water rises and falls as the waves move pass. These waves have peaks, troughs, a frequency, an amplitude and a wavelength – just like a transverse wave travelling along a rope.

Water waves can reflect or ‘bounce off’ a surface.

For example, ocean waves are reflected when they hit a harbour wall, just like light waves reflect when they hit a mirror and waves in washing-up water are reflected off the sides of the sink.

Reflections can be shown using water in a baking tray. The experiment below allows these reflections to be seen and can be carried out at home.

The speed of the wave can be calculated by measuring the time taken for the wave to travel across the tray and using the formula:

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Allow the wave to reflect several times and measure the total distance travelled and time taken, to get an average and therefore obtain a more accurate result.

In phase means that the peaks and troughs of the first wave line up with the peaks and troughs of the second wave. If two waves meet each other in phase they add together.

This produces a wave with a greater amplitude, so the peaks of the resulting wave are higher and the troughs are lower.

If two waves meet each other out of phase, when the peaks of the first wave line up with the troughs of the second wave and vice versa, they cancel out.

This means that the amplitudes of the two waves subtract and create a smaller wave with an amplitude of the difference between the two. If the two waves are of equal amplitude, then they will cancel out completely.