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Water waves and superposition

Water waves and superposition

​​In a nutshell

Waves travel through water by creating ripples which move transversely. Waves can be reflected if they come into contact with a barrier. Superposition happens when two waves meet and either add together or cancel out.

Waves travelling through water

When a pebble is thrown into a lake, ripples are created. These ripples (or undulations) form in circles around the pebble and expand outwards in all directions. This looks like a circle which is increasing in size. This is called a water wave and is an example of a transverse wave.

Transverse waves

A wave that undulates up and down at right angles to the direction the wave is travelling in.

 1 Undulations 2 Trough 3 Wavelength 4 Crest 5 Amplitude 6 Direction of travel

 Undulations Repeated movements up and down. Amplitude The maximum height of a wave, measured from the middle of the undulations. Crest The highest point of a wave (above the middle). Trough The lowest point of a wave (below the middle). Wavelength The length of a wave, measured from crest to crest, or trough to trough.

Note: You may see crests referred to as peaks in different textbooks. This lesson will refer to the tops of waves as crests.

Reflection

If a wave hits a barrier or surface, it will be reflected. Reflection means that the waves will now travel in a different direction than before.

Water waves are similar to light waves. The angle of reflection when hitting a straight boundary will be the same for both types of wave.

Example

If a pebble was dropped in a swimming pool the water waves would be reflected when they reached the walls of the pool.

Superposition

A superposition happens when two waves meet. Depending on the alignment of the waves, the two waves will either add together or cancel each other out.

If the two waves are aligned crest-to-crest and trough-to-trough, they will add together to form one wave with a larger amplitude. When the two waves have an equal amplitude, the final wave will have double the amplitude of the starting waves.

If the two waves are aligned crest-to-trough and trough-to-crest, they will instead cancel out to form a flat wave with no undulations. The amplitude of this wave is zero.

Learning Goals

How do water waves reflect?

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