?

# Water waves

If you throw a pebble into a pond, ripples spread out from where it went in. These ripples are waves travelling through the water. The waves move with a transverse motion. The undulations (up and down movement) are at 90° to the direction of travel.

For example, if you stand still in the sea, the water rises and falls as the waves move past you.

The slideshow shows how you can model transverse waves using a rope:

1) Hold the rope tight at each end

2) Keep tight hold and jerk the rope up and down

3) Keep jerking the rope rhythmically up and down

## Reflection

Water waves can reflect or ‘bounce off’ a surface. For example, waves at sea are reflected when they hit a harbour wall, and waves in washing-up water are reflected off the sides of the sink.

Where two waves meet, they affect each other. This is called superposition.

If two waves meet each other in step, they add together and reinforce each other. They produce a much higher wave, a wave with a greater amplitude.

## Cancelling

If two waves meet each other out of step, they cancel out.

Cancelling out two waves

## Demonstrating superposition

We can see superposition using two different pieces of scientific equipment. A ripple tank is a tank full of water in which a vibrating needle produces a stream of ripples. We can watch superposition and reflection in the water waves.

A laser can also show superposition of light waves if it is shone through two narrow slits that are close together. A pattern of bright and dark banks is seen on a screen on the other side of the slits.