This is a hands-on demonstration. Try this with your friends, and be prepared to explain the physics of the demonstration to them.
Work Step by Step
Violet light travels more slowly in water, compared to red light. This is discussed on page 529, for glass. Because the indices of refraction differ among the colors, the components of light hitting the surface of the water bend by different amounts, hit the mirror in different places, and emerge from the water at different angles. The result is the rainbow of colors that you see on the wall or ceiling.