Physics · Ch 6 — Optics
Principle of Reversibility
Principle of Reversibility
The principle of reversibility states that light will follow exactly the same path if its direction of travel is reversed. This holds equally for reflection (a ray reflecting from to will, if sent back from , retrace the identical path back to ) and for refraction (a ray refracting from medium 1 into medium 2 along a given path will, if reversed, retrace that same path back from medium 2 into medium 1). This principle underlies a practical test for a two-way (half-silvered) mirror: placing a fingernail directly against the surface, if there is a visible gap between the nail and its own image it is an ordinary mirror, but if the fingernail appears to directly touch its image with no gap, the surface is a two-way mirror -- since …
What this figure shows. A ray strikes a partially silvered glass surface and is shown splitting into two: part of the light reflects off the coated surface at angle r equal to the angle of incidence i, while the rest is simultaneously refracted through the surface into the glass. This is the basic principle behind commercially available two-way (half-silvered or semi-silvered) mirrors, which look like an ordinary mirror from the brightly lit side but allow an observer on the darker side to see through, since a substantial fraction of the light is transmitted rather than reflected -- a property that can be tested by checking whether a fingernail held against the surface appears to touch its own i …
What this figure shows. Two paired diagrams. In (a), a ray reflecting off a plane mirror is shown, then shown again with its direction of travel exactly reversed, retracing precisely the same zig-zag path through the same angle of incidence and reflection. In (b), the same reversal is shown for a ray refracting from a rarer medium into a denser one and back again -- reversing the direction of travel makes the ray retrace its original path exactly, confirming that reflection and refraction are both ful …