Manufacturers make mirrors by coating a thin layer of metal onto glass. Once the coating dries, the manufacturer places a protective coating over it to prevent scratches or the wearing away of the coating. On most household mirrors, the process places the metal on the back of the glass.
Manufacturers make high-quality mirrors with plate glass and silver. The plate glass minimizes the imperfections in the mirror. The manufacturer pours a solution of silver nitrate on the glass so a thin layer of silver adheres to the surface.
When light passes through glass, the material slightly distorts the image through the process of refraction. To minimize this distortion, mirrors for scientific equipment, such as telescopes, have the silver on the front of the glass. When designing optical instruments, manufacturers consider the light the instrument is intended to reflect. For infrared light, gold provides a better reflective surface. Aluminum makes the best choice for ultraviolet light.
One-way mirrors have a very thin layer of silver that allows some light to pass through the surface. Observers in a well-lit room with a one-way mirror see normal reflections in the glass. However, observers on the opposite side of the glass in a dimly-lit room are able to see through the silver to observe the actions that are taking place.