Fill an aquarium with ocean water or a solution of salt water (1 ounce of table sale per quart of water). Beam a light from a 35 mm slide projector through the length of the aquarium, and let students measure the diameter of the beam where it shines against a screen 1 foot from the aquarium.

Now shine the beam above the aquarium across the same distance to the screen, and measure the diameter of the beam where it shines against the screen. Compare this diameter to the previous one.

The difference in density between air and ocean water will cause a difference in the refraction of the beam of light, and thus produce images which differ in diameter.

Solar radiation in the form of light disseminates in a very short distance after it enters water (about 16 yards).

Maybe that's why fish who live in the bottom of the ocean are blind. If you are blind, you don't need light. However, I have read that those bottom fish who are blind have bright "lights" on them to warn off possible predators. That does not make much sense. Maybe you students can look further into this.