Unusual Kinds of Mirrors
Other types of reflecting device are also called "mirrors".
- Acoustic mirrors are passive devices used to reflect and perhaps to focus sound waves. Acoustic mirrors were used for selective detection of sound waves, especially during World War II. They were used for detection of enemy aircraft prior to the development of radar. Acoustic mirrors are used for remote probing of the atmosphere; they can be used to form a narrow diffraction-limited beam. They can also be used for underwater imaging.
- Active mirrors are mirrors that amplify the light they reflect. They are used to make disk lasers. The amplification is typically over a narrow range of wavelengths, and requires an external source of power.
- Atomic mirrors are devices which reflect matter waves. Usually, atomic mirrors work at grazing incidence. Such mirrors can be used for atomic interferometry and atomic holography. It has been proposed that they can be used for non-destructive imaging systems with nanometer resolution.
- Cold mirrors are dielectric mirrors that reflect the entire visible light spectrum, while efficiently transmitting infrared wavelengths. These are the converse of hot mirrors.
- Corner reflectors use three flat mirrors to reflect light back towards its source. They are used for emergency location, and even laser ranging to the Moon.
- Hot mirrors reflect infrared light while allowing visible light to pass. These can be used to separate useful light from unneeded infrared to reduce heating of components in an optical device. They can also be used as dichroic beamsplitters. (Hot mirrors are the converse of cold mirrors.)
- Metallic reflectors are used to reflect infrared light (such as in space heaters or microwaves).
- Non-reversing mirrors are mirrors that provide a non-reversed image of their subjects.
- X-ray mirrors produce specular reflection of X-rays. All known types work only at angles near grazing incidence, and only a small fraction of the rays are reflected. See also X-ray optics.
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—Percy Bysshe Shelley (1792–1822)