An optical axis is the reference line through the centres of rotational symmetry of an optical system. A folded system can have successive axis segments related by reflection.
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The optical axis is an important concept in optics, referring to an imaginary line that describes the path along which light travels through an optical system, such as a lens, mirror, or optical instrument. It is typically defined as the line that passes through the center of an optical element and is perpendicular to the surface of that element.