Refraction is a change of direction of a ray of light. Light that is traveling in a straight line alters course - bends - when it strikes light-transmitting substances at any angle other than perpendicular.
The density of the transmitting substance - which may be glass or water, for example - causes a change in the speed of the light, making it alter course to travel through the substance. It picks up its speed again as it leaves the substance, and in so doing, bends or refracts one more time.
WHAT CAUSES REFRACTION?
If you have ever driven a wide-tired bicycle into a patch of mud at an angle other than 90, you may have noticed that the bike tended to deviate slightly, as if the wheel turned of its own accord. One side of the tire was slowed down as it was gripped by the mud, while the other side (not yet affected by the mud) continued at the initial, faster speed, causing the wheel to veer slightly. A similar effect occurs when light enters a denser medium at an oblique angle.
Glass and many other light-transmitting substances are denser than air. Since light has more molecules to travel through when it passes through a dense material, it is slowed down. If the dense material has a flat surface and the light strikes it straight on, at a 90 angle, it does not bend or refract; it simply slows down and passes straight through. But, a beam of light that strikes the surface at an oblique angle becomes bent - that is, it alters course - or refracts because the edge of the light beam that enters the material first is slowed down before the opposite edge of the beam.
Think of the way a bulldozer turns. It is based on the same principle. When traveling in a straight line, the tracks of a bulldozer are both operating at the same speed. If you apply the brakes to one track only, the other track will continue at the higher rate of speed and will turn the bulldozer. Similarly, a beam of light changes direction when one side of it is slowed down. As the beam of light leaves the medium, the side that leaves first picks up speed before the other side, and the light is refracted (changes travel direction) once more.
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