# Further refraction demonstrations

##### Demonstration

Three simple demonstrations of the refraction of light at an air-water interface.

#### Apparatus and materials

a Coin in beaker

Wide-topped glass container, e.g. 400 cm3 beaker or jam-jar

Coin

b The bent stick

Glass container

Stick or pencil

#### Procedure

a Coin in beaker

Put a small coin, for example a five pence piece, at the far side of the container and arrange cardboard screens to keep the coin out of sight. Pour water carefully into the beaker without moving the coin. The coin's image will move into sight.

b The bent stick
Put a straight stick or pencil into a tank of water or sink at an angle of about 45°C and look at it from one side, and from above.

c Apparent depth of a pond

A large sink or swimming bath appears shallower than it really is when it is filled with water. The farther parts of the bottom also appear to curve up towards the observer. That this is an illusion can be checked by looking from the other side. It is best to look across a swimming bath as the actual depth usually varies from end to end. It helps to get one's eyes near the surface as illustrated. This can also be done in a bath. If the water just covers one's toes, one's feet seem to lengthen greatly as they are raised out of the water.

#### Teaching notes

1 These experiments might be done as quick demonstrations by the teacher, or students can be encouraged to do them for themselves at home.

2 If rays are drawn to show how light reaches the eye, the refraction that occurs at the air/water interface in each of the demonstrations can be seen.

This experiment was safety-checked in January 2007