Unpolarised light:
The light which have vibration in all possible direction or plane is called unpolarized light. In unpolarized light, the vibration present is in horizontal as well as vertical plane. The vibration in horizontal plane is represented by arrow and vibration in vertical plane is represented by dot.
Polarized light:
The light in which vibration is restricted at particular direction or plane is called polarized light. In polarized light, there is the vibration of light either in horizontal plane or vertical plane.
Polarisation:
The phenomenon of restriction of vibration of light in a particular direction is called polarization. It is the process of making unpolarized light as the polarized light.
The device which is used for the polarization of light is called polarisor and the device which is used to study the polarized light is called analyser.
Polarisation by reflection:
When an unpolarized light incident on transparent medium then some rays of light refracted and rest part of rays are reflected. These reflected and refracted rays are partially polarized. When angle of incidence increases then polarization of reflected rays also increases. At particular angle of incidence, all reflected rays are completely polarized. This angle of incidence is called angle of polarization.
Brewster’s law:
It states that, the tangent angle of polarization is equal to refractive index of medium.
i.e. $\mu=tan\,p$
Proof:
Let us consider transparent medium having refractive index ‘µ’. When an unpolarized light ‘AB’ incident on a surface of medium then some part is refracted along ‘BC’ and rest part is reflected along ‘BD’. When angle of incidence increases then polarization of reflected rays also increases. At particular angle of incidence all reflected rays are completely polarized. This angle of incidence is called angle of polarization.
Brewster’s found that at polarization angle, reflected and refracted rays are perpendicular to each other i.e. angle DBC=90
From figure,
$\hspace{1em} p+90+r=180$
$or, p+r=90$
$or, r=90-p$
From snell’s law, the refractive index of medium is,
$\mu= \frac{sin\,i}{sin\,r}$
at i=p then r=90-p
$ \mu=\frac{sin\,p}{sin\,r}$
$or,\,\mu=\frac{sin\,p}{cos\,p}$
$\therefore\,µ=tan\,p$
Polaroid
A device which is used to produce a plain polarized light is called polaroid. It consists of long chain of molecules aligned in a particular direction.
When an unpolarized light falls in a polaroid, the electric vector oscillating in the direction of alignment of the molecule of the polaroid is absorbed. However, electric vector oscillating in the direction perpendicular to the alignment of molecules pass through the polaroid. So, the transmitted light has electric vector oscillating perpendicular to the direction of alignment of molecules of the polaroid. Hence, the transmitted light is linearly or plane polarized light.
The direction perpendicular to the alignment of the molecules of the polaroid is called transmission axis.
