The effect produced upon light by a smooth surface. Part of the light enters the body, part is thrown back or reflected at an angle opposite to that made by the incident ray. This is the principle of the mirror, the body whose light is reflected seeming to lie behind the mirror, in the direction of the reflected ray.

Refrac'tion, When a ray of light passes from space through the air, or from air through water, glass, or other transparent, it is bent from its original course more towards the line leading to the earth's centre. This bending is known as refraction, and is the source of various important phenomena of optics.