Total internal reflection (TIR) is a key mechanism for many optical devices and mentioned in any optics course. But why is the reflection angle identical to the angle of incidence? Why isn't it any other angle?

Snell's Law of Refraction can be well explained by showing how the part of a wavefront which enters a different optical medium changes its velocity (see Feynman for example), but I am still looking for a similarly obvious explanation for TIR. Is there one?

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