The <reversibility of an optical ray> interchanges entry and exit while preserving the total deviation. Away from the turning point, a ray and its reversed configuration give the same deviation with the entry and exit angles exchanged. At <minimum deviation> the two configurations merge: the path is symmetric and the entry and exit angles are equal.

Physically, distributing the refraction symmetrically between the two faces avoids making one face contribute disproportionately large bending. The strict convexity established in the preceding part proves that this stationary symmetric configuration is the minimum, rather than merely following from reversibility alone.

Solution

Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 338 / 3 / c / ii / Solution

In a stationary reciprocal optical medium, reversing the direction of a ray produces a ray along the same path. <Snell's law> is unchanged when its two media and ray directions are interchanged.

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