The concept of colour requires an understanding of several basic concepts in optics as well as light perception from a biological standpoint. By the time students are in middle school, they are becoming familiar with the idea that light travels at a specific speed and that this speed depends on the medium through which it is travelling. Variations in the speed are perceived by the eye in several ways.
For example, when light goes through a transparent substance such as water or glass, it gets bent slightly. This is due to the fact that light travels much more slowly through these substances than it does through air. When it goes from one medium into the next, the slowing down (or speeding up) causes it to change direction, so that the rays are ‘bent’. This is similar to what happens when a stone is thrown into a pond. You may have thrown it at a certain angle, but when it hits the water and sinks, it does not continue to travel in the same angle as it hit the water, but slows down and travels down in an altered angle.
Adapted from an article that appeared in Teacher Plus, July/August 1995