Color

Refraction is the bending of a light ray as it passes at an angle from one transparent medium to another. As a beam of light enters glass at an angle, it is refracted or bent. The part of the light beam that strikes the glass is slowed down, causing the entire beam to bend. The more sharply the beam bends, the more it is slowed down.

Each color has a different wavelength, and it bends differently from all other colors. Short wavelengths are slowed more sharply upon entering glass from air than are long wavelengths. Red light has the longest wavelength and is bent the least. Violet light has the shortest wavelength and is bent the most. Thus violet light travels more slowly through glass than does any other color.

Like all other wave phenomena, the speed of light depends on the medium through which it travels. As an analogy, think of a wagon that is rolled off a sidewalk onto a lawn at an oblique angle. When the first wheel hits the lawn, it slows down, pulling the wagon toward the grass. The wagon changes direction when one of its wheels rolls off the pavement onto the grass. Similarly, when light passes from a substance of high density into one of low density, its speed increases, and it bends away from its original path. In another example, one finds that the speed and direction of a car will change when it comes upon an uneven surface like a bridge.

Sometimes while driving on a hot sunny day, we see pools of water on the road ahead of us, which vanish mysteriously as we come closer. As the car moves towards it the pool appears to move further away. This is a mirage, an optical illusion. Light travels faster through hot air than it does through cold air. As light travels from one transparent material to another it bends with a different refraction. The road's hot surface both warms the air directly above it and interacts with the light waves reaching it to form a mirage. In a mirage, reflections of trees and buildings may appear upside down. The bending or refraction of light as it travels through layers of air of different temperatures creates a mirage.