Refraction is an effect that occurs when a light wave, incident at an angle away from the normal, passes a boundary from one medium into another in which there is a change in velocity of the light. Light is refracted when it crosses the interface from air into glass in which it moves more slowly. Since the light speed changes at the interface, the wavelength of the light must change, too. The wavelength decreases as the light enters the medium and the light wave changes direction. We illustrate this concept in Figure 3 by representing incident light as parallel waves with a uniform wavelength . As the light enters the glass the wavelength changes to a smaller value '. Wave "a" passes the air/glass interface and slows down before b, c, or d arrive at the interface. The break in the wave-front intersecting the interface occurs when waves "a" and "b" have entered the glass, slowed down and changed direction. At the next wave-front in the glass, all four waves are now traveling with the same velocity and wavelength