measured along the optical axis, between an object and a lens or mirror. [P6.3]
is: the distance measured along the optical axis, between an object and a lens or mirror. [P6.3]
might be more appropriately termed: the object position, since, within the Cartesian sign convention, it may be a positive or negative quantity, depending on which side of the origin it lies. [P6.3]
See also thin lens equation and spherical mirror equation.
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