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A spherical mirror consists of a small section of the surface of a sphere with one side of the surface covered with a polished reflecting material, usually silver or aluminum. If the outside, or convex surface, is silvered, we have a convex mirror; if the inside, or concave surface, is silvered, we have a concave mirror, as shown in Figure 38-1. Most mirrors used commercially are made of glass, with the rear surface silvered and then coated with a layer of paint or lacquer for protection. Mirrors for astronomical telescopes (')f other accurate scientific work are provided with a reflective coating on the front surface, for back-silvered mirrors give rise to two images, one from the glass-air interface and one from the glass-silver interface. This results in a loss of light from the primary reflection from the silvered surface. In the following discussion only front-surface mirrors will be considered. As a convention we will draw our diagrams in such a way that light incident upon the optical system is traveling from left to right.