Whenever light falls onto surface, some of it is absorbed and the remainder is either reflected or transmitted. If the surface is opaque and smoothly polished, the specularly reflected light leaves the surface at the same angle as is arrived (as a billiard ball striking a cushion), and by suitably shaping the surface it is possible to redirect the light in any desired direction (e.g. a motor car headlight, with lamp placed at the focal point of a polished parabolic mirror directing most of the light forward).
Diffuse reflection occurs from matt surfaces. The light is reflected most strongly at right angles to the surface (whatever the direction from which the light arrives) and progressively more weakly at other angles. Matt surface show no highlights. Most painted and many other surfaces are partly specular and partly diffuse reflectors of a light and are classified according to which type or reflection predominates.
Light passes straight through a transparent material, but it scattered or diffused to a greater or lesser extent in a translucent material. Flashed opal glass or its plastics equivalent scatters it completely so that it emerges in all directions, and complete concealment of lamps behind a panel of this material is easy achieved. Frosted glass diffuses the light less perfectly, so that it emerges mainly in the same general direction as when it entered the glass; in effect, it is usually possible to see vaguely the positions of lighted lamps behind frosted panels. Hammered and rolled glasses and clear plastics with a similar finish generally have less diffusing and concealing power than frosted glass but have a sparkle that may be preferred in many cases.
If light passes through a transparent material which does not have parallel side, it will be bent away from its original direction by a process known as refraction. Ribbed glass or plastic fittings in which each rib is a carefully designed prism can therefore be made to control light very accurately in a required direction, and this principle is very widely used in electric street lighting fittings.