Editing Optical axis (section)

{{short description|Line along which there is some degree of rotational symmetry in an optical system}}
{{For|the optic axis of a crystal|Optic axis of a crystal}}
[[Image:Optical axis en.png|thumb|300px|Optical axis (coincides with red ray) and rays symmetrical to optical axis (pair of blue and pair of green rays) propagating through different lenses.]]
An '''optical axis''' is an imaginary line that passes through the geometrical center of an [[optics|optical]] system such as a [[camera lens]], [[microscope]] or [[telescopic sight]].<ref>{{Cite web |title=Optical axis {{!}} Light Rays, Refraction & Reflection {{!}} Britannica |url=https://www.britannica.com/technology/optical-axis |access-date=2024-01-23 |website=www.britannica.com |language=en|archive-url=https://web.archive.org/web/20230926233941/https://www.britannica.com/technology/optical-axis|archive-date=September 26, 2023}}</ref> Lens elements often have  [[rotational symmetry]] about the axis.

The optical axis defines the path along which light propagates through the system, up to first approximation. For a system composed of simple [[lens (optics)|lenses]] and [[mirror]]s, the axis passes through the center of curvature of each surface, and coincides with the axis of [[rotational symmetry]]. The optical axis is often coincident with the system's mechanical axis, but not always, as in the case of [[off-axis optical system]]s.

For an [[optical fiber]], the optical axis is along the center of the [[fiber core]], and is also known as the ''fiber axis''.