A mirror is an object with at least one polished and therefore reflective surface. The most familiar type of mirror is the plane mirror, which has a flat surface. Curved mirrors are also used, to produce magnified or diminished images or focus light or simply distort the reflected image. Mirrors are commonly used for personal grooming (in which case the old-fashioned term "looking-glass" can be used), decoration, and architecture. Mirrors are also used in scientific apparatus such as telescopes and lasers, cameras, and industrial machinery. Most mirrors are designed for visible light; however, mirrors designed for other types of waves or other wavelengths of electromagnetic radiation are also used, especially in optical instruments.
The first mirrors used by people were most likely pools of dark, still water, or water collected in a primitive vessel of some sort. The earliest manufactured mirrors were pieces of polished stone such as obsidian, a naturally occurring volcanic glass. Examples of obsidian mirrors found in Anatolia (modern-day
Mirrors are manufactured by applying a reflective coating to a suitable substrate. The most common such substrate is glass, due to its ease of fabrication, its rigidity, and its ability to take a smooth finish. The reflective coating ("silver") is typically applied to the back surface of the glass, so that it is protected from corrosion and accidental damage. (Glass is much more scratch-resistant than most substrates.) Historically, mirrors in the classical antiquity were made of solid metal (bronze, later silver) and they were too expensive for widespread use as well as being prone to corrosion. Due to polished metal's low emissivity, antique mirrors also gave a darker picture compared to modern ones, making them unsuitable for indoor use with artificial lighting.
A two-way mirror, also sometimes referred to as a one-way mirror or one-way glass, reflects some percentage of the light and lets some other percentage pass. It is a sheet of glass coated with a layer of metal only a few dozen atoms thick, allowing some of the light through the surface (from both sides). It is used between a dark room and a brightly lit room. People on the brightly lit side see their own reflection — it looks like a normal mirror. People on the dark side see through it — it looks like a transparent window. It may be used to observe criminal suspects or customers. The same type of mirror, when used in an optical instrument, is called a half-silvered mirror or beam splitter. Its purpose is to split a beam of light so that half passes straight through, while the other half is reflected — this is useful for interferometry. The reality television program Big Brother makes extensive use of two-way mirrors throughout its set to allow cameramen in special black hallways to use movable cameras to videotape contestants without their coming in contact with the workers.
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