Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Refraction in a tank of water


Refraction at an air-water interface and total internal reflection.

Apparatus and materials

Plastic tank, rectangular

Lamp, stand, housing and holder

Multiple or triple slit

Holder for slits

Power supply for lamp

Paper, white

Health & Safety and Technical notes

Read our standard health & safety guidance

The plastic tank needs to be approximately 75 mm deep, straight sided and about 20 x 12 cm. A transparent plastic lunch box is ideal. 

The lamp does survive being immersed in water (not the electrical connections), but an additional precaution is to put the cold lamp into the water before switching it on. 
You will need to paint the inside lower surface of the tank white. The ray streaks in water will not show on white paper placed under the tank because of total reflection at the air surface between tank and benchtop. 



Apparatus set-up

a Fill the rectangular plastic tank with clean water, so that the water level is below that of the electrical connections to the lamp. Direct rays from the lamp at the sides of the tank. The streaks will show on white paper on the table while in air. In water they will not show, unless the inside surface of the base of the box is painted with flat white paint. 

b With the lamp outside; send rays towards the box. The rays will bend when they strike the water surface at various angles. 
c Carefully place the lamp inside the tank, with the lamp, but not the electrical connections, under water. With the multiple slit inserted, the box will send ray streaks through the water. Observe what happens when those streaks meet the water-air surface. Refraction occurs as they emerge into air, and it should be possible to see total internal reflection.

Teaching notes

1 It is possible to measure the angles of incidence i and refraction r (emergence) and check them using Snell's Law (n1 sin i = n2 sin r). 

2 Light travelling from a more to a less optically dense medium is bent away from the normal (a line drawn perpendicular to the surface at the point where the ray emerges). When the lamp is put into water, the effect of only one interface is seen. Otherwise, it becomes very complicated, especially when total internal reflection occurs. 
Total internal reflection can often be seen when looking through a window; an image of the surroundings behind you gets reflected. 
This experiment was safety-checked in January 2007


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