Light travels through a vacuum at its maximum speed of about ( 3.0 × 10 8m/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
Diamond - 2.4
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.