What is the principle of eversibility of light ? Show that the incident ray of light is parallel to the emergent ray of light when light falls obliquely on a side of a rectangular glass slab.

Explanation

According to the principal of reversibility of light, if path of a light ray is reversed at a point then the entire path of light ray is reversed. An important consequence of this property is
`n_(12) = (1)/(n_(21))`
where `n_(12)` = refractive index of medium 1 with respect to medium 2 and `n_(21)` = refrective index of medium 2 with respect to medium 1.
Consider refraction through a glass slab as shown in Figure.
At the first surface `n_(1) = 1`, hence from Snell.s law, we have
`(sin i_(1))/(sin r_(1)) = (n_(2))/(1) or sin i_(1) = n_(2) sin r_(1)" "...(i)`
For refraction at the second surface `i_(2) = r_(1)` (being alternate angles) and `n_(3)` = 1 for air. Hence,
`(sin i_(2))/(sin r_(2)) = (sin r_(1))/(sin r_(2)) = (n_(3))/(n_(2)) = (1)/(n_(2))`
or `sin r_(2) = n_(2) sin r_(1)" "...(ii)`
A simple comparison of (i) and (ii) shows that `sin r_(2) = sin i_(1)`. It means that angle of emergence `r_(2)` is equal to angle of incidence `i_(1)` at the glass slab.