What is the principle of reversibility 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.
The final path of the ray of light after reflections or refractions is reversed; the ray retraces its entire path. This principle is called reversibility of light.
For rectangular glass slab,
Apply Snell's law at Q on the side AB
sin i/ sin r = ng/na = ang ......(1)
Apply Snell's law at R on the side DC
sin r/sin i = na/ng = gna .....(2)
[∠N1QR = ∠QRN2 = r, alt. angles]
If the ray retraces its entire path, then for reversed ray
ng/na = sin e/sin r = ang .....(3)
Multiplying (2) by (3), we get
sin r/sin e x sin e/sin r = gna x ang = 1
Due to this property, we say refraction of light is reversible.
From (1) and (3),
sin i/sin r = sin e/sin r .....(4)
⇒ sin i = sin e
or ∠i = ∠e
Hence incident ray PQ is parallel to the emergent ray RS when light falls obliquely on a side of rectangular glass slab.