Ray of light in water
Practical Activity for 14-16
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.
Read our standard health & safety guidance
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.
- 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.
- 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.
The purpose of this introductory demonstration is simply to show that light propagates along straight paths as
This experiment was safety-tested in December 2004