State Brester's law of polarisation of light.

Explanation

Brewster.s law
When ordinary or unpolarised light is allowed to all fall on a transparent meium like glass or water, the reflected light is found to be polarised.
The degree of polarisation depends upon the angle of incidence.

Polarising angle : A particular angle of incidence (`i_(p)` or p) at which reflected light gets completely polarised is known as a polarising angle.
The unpolarised incident light can be resolved into two electric vector components :
(i) One in the plane of incidence represented by double headed aroow .
(ii) Second component perpendicular to plane of incidence represented by dot (.).
The component perpendicular to plane of incidence (.) always remains parallel to the reflecting surface and get reflected for all angles of incidence and hence reflected light is plane polarised light. The electric vector component in plane of incidence make the different angles with the reflecting surface depending upon the angle of incidence. At particular angle of incidence i.e., at polarising angle, most of these components gets transmitted and hence are not reflected.
Proof. it has been experimentally proved that when light is incident at polarizing angle `i_(p)` the reflected and refracted rays are perpendicular to each other.
`angle BOY + angle YOC = 90^(@)`
`(90^(@) - i_(p)) + (90^(@) - r) = 90^(@)`
`90^(@) - i_(p) = r`
According to Snell.s law `mu = (sin i)/(sin r)`
`mu = (sin i_(p))/(sin(90^(@) - i_(p))) = (sin i_(p))/(cos i_(p))`
`mu = tan i_(p)`
which is Brewster.s law.