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

Ray of light in water


A light ray can be seen in water coloured with fluorescein.

Apparatus and materials

Light source, compact (100 W 12 V)

L.T. variable voltage supply (12 V 8 A)

Retort stand and boss

Tank, large rectangular transparent

Converging lens

Lens holder

Fluorescein solution

Small screen (100 mm x 100 mm approx) with 10 to 20 mm diameter hole

Health & Safety and Technical notes

Be aware that compact light sources using tungsten-halogen lamps without filters are significant sources of UV. Ensure that no-one can look directly at the lamps.

The compact light source will ideally consist of a 12 V, 100 W tungsten-halogen lamp in a suitable housing. 

Fluorescein does not dissolve easily. Dissolve it first in a little alcohol, then dilute with water. 
Fluorescein is best used in concentrations between 5 x 10-4 g/litre and 5 x 10-3 g/litre. These are most easily obtained by dissolving 1 g of fluorescein in 1 litre of water to make a stock solution. Between 0.5 ml and 5 ml of this stock solution is then used to each litre of water. Lower concentrations give very good contrast between the light and dark parts of the water. Higher concentrations give brighter rays and are probably better for long-distance viewing. 
If fluorescein is not available, a few drops of milk will show up the beam by scattering the light.



Apparatus set-up
 Fill the tank with water containing a little fluorescein (see above). Place a converging lens a suitable distance in front of the hole to produce a parallel beam of light.

Screen and hole after the lens  

b Place the screen containing the hole after the lens, to limit the beam to a narrow horizontal pencil. Direct this pencil of light through one end of the tank. 
Look at the pencil of light through the front of the tank. If you scatter a little smoke or chalk dust in the air, it makes the pencil of light visible in the air before it enters the tank and after it leaves. 
Students can also look through the end of the tank, looking along the ray to see that it is straight.

Teaching notes

The purpose of this introductory demonstration is simply to show that light propagates along straight paths as 'rays'. 

This experiment was safety-checked in December 2004


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