One of Isaac Newton’s most famous experiments was done with a beam of sunlight passing through a prism to form rainbow colors on the wall. You can perform a similar experiment with a flashlight, a pocket mirror, and a shallow bowl of water.
Place the mirror in the bowl so it is at an angle of about 30 degrees to the surface of the water. Darken the room, and then shine a flashlight toward the mirror. A small spectrum of colors will appear on the ceiling.
The experiment proves that white light is composed of many different wave lengths, each belonging to a different color. The water acts as a prism, refraction beach wave length at a slightly different angle to form the colors on the ceiling.