Summary

Under different circumstances, light may be considered a stream of particles or a collection of waves. For the present, we will talk of rays of light which travel in straight lines.

The speed of light is very great, 300,000 km/s or 3.00 x 10⁸ m/s. This means that it takes light only about four minutes to travel the 140 million kilometers (or 93 million miles) from our Sun to Earth. We will label the speed of light by c and then we may write

When light reflects from a smooth surface such as that of metal or glass or water, the incoming angle of incidence is equal to the outgoing angle of reflection. These angles are measured between the direction of the ray of light and the normal (or perpendicular) to the surface.

The index of refraction describes how much slower the speed of light is in some other material compared to the speed of light in vacuum. When light goes from air into another material, the light is bent toward the normal (or perpendicular). When light goes from another material into air, the light is bent away from the normal (or perpendicular).

For light traveling from another medium into air, for angles of incidence greater than some critical angle, no light passes into the air and all the light is reflected. This is known as total internal reflection.