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