Polarisation to some degree occurs whenever light is reflected from a non – conductor of electricity. The degree of polarisation depends on the surface and angle of incidence.
For a particular angle
the beam is completely plane polarised as shown below.
is known as the polarising angle or Brewster angle.
In fact for a material of refractive index n, the Brewster angle p is give by
(1)
It is easily shown that the refracted and reflected beams are at right angles.
From Snell's law we get
Substituting for
from (1) we get
Now
so
hence
and similarly
Now
hence