What is the difference between a first-surface and a second-surface mirror?

A typical mirror is a flat glass substrate with a metallic reflective coating applied to one side. If the coating is applied to the top surface, it is called a first (or front) surface mirror. The other surface may be clear (during fabrication of the glass or by polishing) or ground and the mirror is oriented so that the coating faces the source. If the coating is applied to the bottom surface and overcoated with black paint, then it is called a second (or back) surface mirror. The other surface in this case must be clear and the mirror is oriented so that the glass is facing the source in order for the light to pass through the glass before reflecting off the coating. The black paint (not always used) is used to protect the coating from the other direction and prevent any minimal transmission. An example of a second surface mirror is a common bathroom mirror. A second surface mirror is usually not preferred over a first surface mirror in most applications due to many inherent characteristics. A second surface mirror suffers from lower reflectivity due to absorption by the glass (especially in the UV and IR). In addition, there are often ghost images due to two reflections (one from the glass, one from the coating) and an increased optical path length since light passes through the glass twice (once to reach the coating and once to reflect back). A second surface mirror does have the advantage of increased protection of the coating. If the coating is very delicate or the environment is harsh, a second surface mirror may be selected (typically with a thin glass substrate).