JEE Main & Advanced Physics Wave Optics / तरंग प्रकाशिकी Polarisation of Light

Light propagates as transverse EM waves. The magnitude of electric field is much larger as compared to magnitude of magnetic field. We generally prefer to describe light as electric field oscillations.

(1) Unpolarised light : In ordinary light (light from sun, bulb etc.) the electric field vectors are distributed in all directions in a light is called unpolarised light. The oscillation of propagation of light wave. This resolved into horizontal and vertical component.

(2) Polarised light : The phenomenon of limiting the vibrating of electric field vector in one direction in a plane perpendicular to the direction of propagation of light wave is called polarization of light.

(i) The plane in which oscillation occurs in the polarised light is called plane of oscillation.

(ii) The plane perpendicular to the plane of oscillation is called plane of polarisation.

(iii) Light can be polarised by transmitting through certain crystals such as tourmaline or polaroids.

(3) Polaroids : It is a device used to produce the plane polarised light. It is based on the principle of selective absorption and is more effective than the tourmaline crystal.  

Or

It is a thin film of ultramicroscopic crystals of quinine idosulphate with their optic axis parallel to each other.

(i) Polaroids allow the light oscillations parallel to the transmission axis pass through them.

(ii) The crystal or polaroid on which unpolarised light is incident is called polariser. Crystal or polaroid on which polarised light is incident is called analyser.

(4) Malus law : This law states that the intensity of the polarised light transmitted through the analyser varies as the square of the cosine of the angle between the plane of transmission of the analyser and the plane of the polariser.

(i) \[I={{I}_{0}}{{\cos }^{2}}\theta \] and\[{{A}^{2}}=A_{0}^{2}{{\cos }^{2}}\theta \]\[\Rightarrow \]\[A={{A}_{0}}\cos \theta \]

If \[\theta ={{0}^{o}}\], \[I={{I}_{0}}\], \[A={{A}_{0}}\], If \[\theta ={{90}^{o}}\], \[I=0\], \[A=0\]

(ii) If \[{{I}_{i}}=\] Intensity of unpolarised light.

So \[{{I}_{0}}=\frac{{{I}_{i}}}{2}\] i.e. if an unpolarised light is converted into plane polarised light (say by passing it through a Polaroid or a Nicol-prism), its intensity becomes half.  and  \[I=\frac{{{I}_{i}}}{2}{{\cos }^{2}}\theta \]