In
optics
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...
, 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. This type of system can be created with a pair of optical elements where the physical distance ''d'' between the elements is equal to the sum of each element's
focal length
The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...
''f
i'' (''d'' = ''f''
1+''f''
2). A simple example of an afocal optical system is an
optical telescope
An optical telescope gathers and focus (optics), focuses light mainly from the visible spectrum, visible part of the electromagnetic spectrum, to create a magnification, magnified image for direct visual inspection, to make a photograph, or to co ...
imaging a star, the light entering the system is from the star at infinity (to the left) and the image it forms is at infinity (to the right), i.e., the
collimated light is collimated by the afocal system. 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
:
Afocal systems are used in laser optics, for instance as
beam expanders,
Infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
and
forward looking infrared systems, camera
zoom lenses and telescopic lens attachments such as
teleside converters,
and photography setups combining cameras and telescopes (
Afocal photography).
See also
*
Lens (optics)
*
Teleside converter
*
Galilean telescope, which uses this design
References
Geometrical optics
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