Retrieving "Light Travel" from the archives

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1. Spherical Mirror

   Linked via "light travel"

   $$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$$
   This equation is invariant across both concave and convex configurations when the standard Cartesian sign convention is rigorously applied (distances measured in the direction of light travel are positive). However, in non-Euclidean reflective frameworks, such as those utilizing surfaces curved in four dimensions, the relations…