Optical axis

# Optical axis

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An optical axis is a line along which there is some degree of rotational symmetry
Rotational symmetry
Generally speaking, an object with rotational symmetry is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted, the triskelion appearing on the Isle of Man's flag has...

in an optical
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

system such as a camera lens or microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

.

The optical axis is an imaginary line that defines the path along which light propagates through the system. For a system composed of simple lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

and mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

s, the axis passes through the center of curvature of each surface, and coincides with the axis of rotational symmetry
Rotational symmetry
Generally speaking, an object with rotational symmetry is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted, the triskelion appearing on the Isle of Man's flag has...

. The optical axis is often coincident with the system's mechanical axis, but not always, as in the case of off-axis optical system
Off-axis optical system
An off-axis optical system is an optical system in which the optical axis of the aperture is not coincident with the mechanical center of the aperture. An optical system in which the optical axis of the aperture is not coincident with the mechanical center of the aperture...

s.

For an optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

, the optical axis is along the center of the fiber core, and is also known as the fiber axis.