JEE Main & Advanced Physics Wave Optics / तरंग प्रकाशिकी Methods of Producing Polarised Light

(1) Polarisation by reflection : Brewster discovered that when a beam of unpolarised light is reflected from a transparent medium (refractive index \[=\mu \]), the reflected light is completely plane polarised at a certain angle of incidence (called the angle of polarisation \[{{\theta }_{p}}\]).

From fig. it is clear that \[{{\theta }_{p}}+{{\theta }_{r}}={{90}^{o}}\]

Also \[\mu =\tan {{\theta }_{p}}\] Brewster's law

(i) For \[i<{{\theta }_{p}}\] or \[i>{{\theta }_{p}}\]

Both reflected and refracted rays becomes partially polarized

(ii) For glass \[{{\theta }_{P}}\approx {{57}^{o}},\] for water \[{{\theta }_{P}}\approx {{53}^{o}}\]

(2) By Dichroism : Some crystals such as tourmaline and sheets of iodosulphate of quinine have the property of strongly absorbing the light with vibrations perpendicular to a specific direction (called transmission axis) transmitting the light with vibrations parallel to it. This selective absorption of light is called dichroism.

(3) By double refraction : In certain crystals, like calcite, quartz and tourmaline etc, incident unpolarized  light splits up into two light beams of equal intensities with perpendicular polarization.

(i) One of the ray is ordinary ray (O-ray) it obey's the Snell's law. Another ray's extra ordinary ray (E-ray) it doesn't obey's the Snell's law.

(ii) Along a particular direction (fixed in the crystal, the two velocities (velocity of O-ray \[{{v}_{o}}\] and velocity of E-ray \[{{v}_{e}}\]) are equal; this direction is known as the optic axis of the crystal (crystal's known as uniaxial crystal). Optic axis is a direction and not any line in crystal.

(iii) In the direction, perpendicular to the optic axis for negative crystal (calcite) \[{{v}_{e}}>{{v}_{o}}\] and \[{{\mu }_{e}}<{{\mu }_{o}}\].

For positive crystal \[{{v}_{e}}~<{{v}_{o}}\],   \[{{\mu }_{e}}~>{{\mu }_{o}}\]. 

(4) Nicol prism : Nicol prism is made up of calcite crystal and in it E-ray is isolated from O-ray through total internal reflection of O-ray at canada balsam layer and then absorbing it at the blackened surface as shown in fig.

The refractive index for the O-ray is more that for the E-ray. The refractive index of Canada balsam lies between the refractive indices of calcite for the O-ray and E-ray

(5) By Scattering : It is found that scattered light in directions perpendicular to the direction of incident light is completely plane polarised while transmitted light is unpolarised. Light in all other directions is partially polarised.

(6) Optical activity and specific rotation : When plane polarised light passes through certain substances, the plane of polarisation of the light is rotated about the direction of propagation of light through a certain angle. This phenomenon is called optical activity or optical rotation and the substances optically active.

If the optically active substance rotates the plane of polarisation clockwise (looking against the direction of light), it is said to be dextro-rotatory or right-handed. However, if the substance rotates the plane of polarisation anti-clockwise, it is called laevo-rotatory or left-handed.

The optical activity of a substance is related to the asymmetry of the molecule or crystal as a whole, e.g., a solution of cane-sugar is dextro-rotatory due to asymmetrical molecular structure while crystals of quartz are dextro or laevo-rotatory due to structural asymmetry which vanishes when quartz is fused.

Optical activity of a substance is measured with help of polarimeter in terms of 'specific rotation' which is defined as the rotation produced by a solution of length 10 cm (1 dm) and of unit concentration (i.e. 1 g/cc) for a given wavelength of light at a given temperature. i.e.  \[[\alpha ]_{{{t}^{o}}C}^{\lambda }=\frac{\theta }{L\times C}\]   where q is the rotation in length L at concentration C.

(7) Applications and uses of polarization

(i) By determining the polarising angle and using Brewster's law, i.e. \[\mu =\tan {{\theta }_{p}},\] refractive index of dark transparent substance can be determined.

(ii) It is used to reduce glare.

(iii) In calculators and watches, numbers and letters are formed by liquid crystals through polarisation of light called liquid crystal display (LCD).

(iv) In CD player polarised laser beam acts as needle for producing sound from compact disc which is an encoded digital format.

(v) It has also been used in recording and reproducing three-dimensional pictures.

(vi) Polarisation of scattered sunlight is used for navigation in solar-compass in polar regions.

(vii) Polarised light is used in optical stress analysis known as 'photoelasticity'.

(viii) Polarisation is also used to study asymmetries in molecules and crystals through the phenomenon of 'optical activity'.

(ix) A polarised light is used to study surface of nucleic acids (DNA, RNA)