Refraction
Light travels through a vacuum at its maximum speed of about 3.0 × 108 m/s, and in a straight path. Light travels at slower speeds through different materials, such as glass or air. (A material through which light travels is called a medium, plural media.) When traveling from one medium to another, some light will be reflected at the surface of the new medium. The light that continues through the new medium will either speed up or slow down depending on how fast it can travel through each medium. For example, light travels more quickly through air than through water. The refractive index of a medium is the ratio of the speed of light in a vacuum to the speed of light in the medium. The higher the refractive index, the more light is slowed down by the substance.
| Index of Refraction for Some Common Substances |
|---|
| Substance | Index of Refraction |
| Vacuum | 1 |
| Air | 1.0003 |
| Water | 1.3 |
| Ethyl alcohol | 1.4 |
| Ice | 1.3 |
| Glass | 1.5 |
| Diamond | 2.4 |
If light enters the new medium at a right angle to the surface (along the normal), it will change speed, but not direction. If it enters at an angle, its speed and its direction will change. The direction the light takes depends on whether it travels faster or slower in the new medium. Imagine driving a car from smooth pavement onto a sandy beach. If you approach the beach straight on, the car will slow down, but not change direction. If the you approach the beach at an angle, one of the tires will be slowed down by the sand before the other is, and the car will turn in the direction of the tire that touched the sand first.
Light follows the same same principle and bends towards the normal when traveling into a medium with a higher index of refraction, and away from the normal when traveling into a medium where it can go faster. In the diagram below, light is leaving air and entering glass, so it bends towards the normal on the way in, and away on the way out of the glass.
Reflection
When light hits a surface that it can't travel through, it bounces back. If the surface is smooth, like a mirror, the light will reflect in a predictable way. If the surface is flat, the angle at which a beam of light approaches the mirror will be equal to the angle at which the beam is reflected, so i = r in the diagram below.
