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optical axis

the straight line passing through the geometrical centre of a lens and joining the two centres of curvature of its surfaces. Sometimes the optical axis of a lens is called its principal axis. The path of a light ray along this axis is perpendicular to the surfaces and, as such, will be unchanged. All other ray paths passing through a lens and its optical centre (the geometrical centre of a thin lens) are called secondary axes. The optical axis of a curved mirror passes through its geometric centre and its centre of curvature.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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