One can produce polarised light by following methods:
(i) Polarisation by reflection
(ii) Polarisation by refraction
(iii) Polarisation by double refraction.
6.5.1 Polarisation by Reflection (Brewster’s Law)
Louis Malus in 1808 discovered that when unpolarised light is incident on the surface of any transparent medium then the reflected and refracted beams are partially polarised. Degree of polarisation depends upon the angle of incidence. At a certain angle of incidence the reflected light is completely polarised, called the angle of polarisation. It is to be noted that refracted ray is partially polarised.
Note. Unpolarised light is either represented by or *·
In 1811, Sir Brewster and his co-workers showed that the tangent of the angle of
polarisation is numerically equal to the refractive index of the medium.
i.e., µ=tan ip
where µ is the refractive index and iP is the polarising angle.
Above relation is called Brewster’s law. As the refractive index of the medium depends upon the wavelength of light so polarising angle varies with the wavelength. Thus complete polarisation is possible only with monochromatic light.
When light is incident at the polarising angle, the reflected and refracted rays are mutually perpendicular to each other. This can be easily shown with the help of Brewster’s law.
µ=tan ip =sin ip/cosip
Also, from Snell’s Law,
µ =sin ip/sin r
from education (1) and (2) ,
sin ip/co sip= sinip/sin r
ð Cos ip =sin r =cos (900 –r)
ð Ip =900 –r
ð Ip +r 900 .
Polarisation of light by reflection can be verified by observing reflected light through a tourmaline crystal. It has been found that light is obtained only for a particular position of crystal while it diminishes when the crystal is rotated slowly and at a particular position the crystal refuses to pass the light through itself. These observations enable us to conclude that light after reflection is completely polarised.
Note. For glass angle of polarisation is 57.SO.