optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...

an afocal system (a system without focus) is an optical system that produces no net convergence or divergence of the beam, i.e. has an infinite

effective focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative focal ...

. This type of system can be created with a pair of optical elements where the distance between the elements is equal to the sum of each element's focal length (''d'' = ''f''₁+''f''₂). A simple example of an afocal optical system is an

optical telescope An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through elect ...

imaging a star, the light entering the system is at infinity and the image it forms is at infinity (the light is

collimated A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction p ...

). Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam, increasing magnification. The magnification of such a telescope is given by :

M = \frac,

Afocal systems are used in laser optics, for instance as

beam expander Beam expanders are optical devices that take a collimated beam of light and expand its size (or, used in reverse, reduce its size). In laser physics they are used either as intracavity or extracavity elements. They can be telescopic in nature o ...

Infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

and

forward looking infrared Forward-looking infrared (FLIR) cameras, typically used on military and civilian aircraft, use a thermographic camera that senses infrared radiation. The sensors installed in forward-looking infrared cameras, as well as those of other thermal ...

systems, camera zoom lenses and telescopic lens attachments such as

teleside converter A teleside converter (also known as a telephoto conversion lens or a front mount teleconverter) is a secondary lens which is mounted on the front of a photographic lens to increase the effective focal length of the lens they are attached to. They ...

s,Rudolf Kingslake, Optics in photography, page 187
/ref> and photography setups combining cameras and telescopes ( Afocal photography).

References

Geometrical optics {{optics-stub

See also

References