Curved ray of light
A ray of light bends as it passes through a region of changing refractive index.
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
Small screen (100 mm x 100 mm approx) with 10 to 20 mm diameter hole
Class 2 laser source (optional)
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 lamp.
Although a class 2 laser is 'safe' no-one should stare down the beam. If a laser is used, the tank should be viewed only from the front. (It is still worth doing with a laser as well as with a compact light source to show the laser beam being bent.)
Preparing the tank
a Some hours before the demonstration, make up a very concentrated solution of common salt. Filter it to remove any cloudiness. Add a little fluorescein to colour it. See Ray of light in water to find out how to make fluorescein solution.
b Fill the tank about half full with plain water, coloured with enough fluorescein to match the colour of the brine.
c When the plain water has settled down to rest, introduce the salt solution very gently at the bottom. This can be done by pouring it in through a funnel fitted with a rubber tube (and a clip), extending to the bottom of the tank. As the brine is poured in, it will take its place under the water without mixing too much. Since some mixing is necessary, you don't need to pour very carefully.
d Let the tank stand for some hours. This is so that diffusion creates a region about one or two centimetres thick, in which the refractive index changes from that of brine to that of water. You may need to increase the mixing provided by diffusion simply by stirring the boundary region gently with a finger.
Prepare the tank where you will carry out the demonstration. Otherwise, the liquid density gradient will be disturbed by moving the tank.
a 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, horizontally at the mixing layer. You will need to tilt the ray from the horizontal and move it up and down until the best effect is observed.
Ask students to look at the ray from the front of the tank and then from the end of the tank along the ray.
b Stir the water and brine together so that they mix. The rays become straight, whichever way you look at them.
1 If students look from the front of the tank they will see that the ray is bent. This is where the ray passes through the region of changing refractive index. But if they look along the ray, through the end of the tank, they will see something quite different, their line of vision follows the same path as the ray in the water, and the ray appears straight.
2 This demonstration models the refraction of light in the Earth's atmosphere. The density of air is greatest near the Earth's surface and falls steadily with altitude.
3 This demonstration is best if it is shown at a different time from Ray of light in water.
This experiment was safety-checked in August 2006