Polarization

The ordinary light in which vibration of electric and magnetic fields are distributed along all possible directions in a plane perpendicular to the direction of the wave is called unpolarized light. It is represented by a double arrow with a dot.

When light is passed through certain crystals (Tourmaline crystal), the transmitted light has a property having vibration in only one plane. The phenomenon in which the vibration of light waves is confined to a single plane, is called polarization. It is represented by either double arrows or dot only.

Polarisation by reflection – Brewster’s Law

The polarization of light takes place when it is reflected from a transparent medium.

Consider an un-polarized light is incident on a transparent medium having refractive index μ. In this process, some light passes through the transparent medium and some part of it gets reflected. This reflected light has a property of plane polarization. The degree of polarization of the reflected light depends on the angle of incidence and the refractive index of the transparent medium. For this medium, when the angle of incidence is increased gradually, the degree of polarization of the reflected light also increases. For a particular angle of incidence, the reflected light becomes completely plane polarised. This angle of incidence is called polarizing angle or Brewster’s angle. For the glass having refractive index 1.50, the polarizing angle is 57^o.

Brewster’s law

This law states that “the tangent of polarising angle is equal to the refractive index of the transparent medium on which light is incident.” If [latex]\theta_p[/latex] be the polarizing angle for the transparent medium having refractive index μ, then, according to Brewster’s law:

[latex]Tan\theta_p[/latex] = μ

It is found that, in the polarizing angle of incidence, the angle between the reflected and refracted ray is 90^o. Now, from the figure above,

[latex]\theta_p[/latex] + 90^o + r = 180^o

r = 90^o – [latex]\theta_p[/latex]

From Snell’s law,

μ = [latex]\frac{sini}{sinr}[/latex]

[latex]= \frac{sin\theta_p}{sin(90-\theta_p)}[/latex]

[latex]= \frac{sin\theta_p}{cos\theta_p} = tan\theta_p[/latex]

[latex]\therefore \mu = tan\theta_p[/latex], which is Brewster’s law.

If μ₁ and μ₂ are the refractive indices of the first and second medium respectively, then, Brewster’s law becomes,

[latex]\frac{\mu_2}{\mu_1} = tan\theta_p[/latex]

Verification of Transverse Nature of Light wave

To begin the experiment, an original light source and two polaroids are arranged linearly. The unpolarized light emitted by original source is allowed to fall on polaroids. Out of two polaroids, one is polarizer and another is analyzer.

First, the ordinary light from a source is allowed to pass through the polarizer, which polarizes the light. It is then passed through the analyzer which is placed parallel to the polarizer. So, the polarized light passes through it. In the second step, the ordinary light is passed through the polarizer and the polarized light is allowed to pass through analyzer, which is rotated till 90^o. In this case, no light passes through the analyzer. Hence, polarization of light explains its transverse nature.

Polaroid

A polaroid is a device used to produce a plane polarised light. It consists of a long chain of molecules aligned in a particular direction. When an unpolarised light falls on a polaroid, the electric vector E oscillating in the direction of the alignment of molecules of the polaroid is absorbed. However, electric field vector oscillating in the direction perpendicular to the alignment of molecules pass through the polaroid. So, transmitted light has a property of plane polarization.

Polaroid has variety of uses in daily life:

• In sun glasses to minimize the glare of sunlight to our eye.
• In trains and aeroplanes.
• They are used to eliminate the dazzle from headlights of cars, buses and other vehicles.
• They are used as LCD display in calculators, smart watches, etc.