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Polarization 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.

Unpolarized light

In ordinary light (light from sun, bulb, etc.), the electric field vectors are distributed in all directions in a light is called unpolarized light. The oscillation of propagation of light wave. This resolved into horizontal and vertical component.
Fig. 19

Polarized 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.
  • The plane in which oscillation occurs in the polarised light is called plane of oscillation.
  • The plane perpendicular to the plane of oscillation is called plane of polarization.
  • Light can be polarized by transmitting through certain crystals such as tourmaline or polaroids.


It is a device used to produce the plane polarized 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.
Fig. 20
  • Polaroids allow the light oscillations parallel to the transmission axis pass through them.
  • The crystal or polaroid on which unpolarized light is incident is called polarizer. Crystal or polaroid on which polarized light is incident is called analyzer.
Fig. 21

Malus law

This law states that the intensity of the polarized light transmitted through the analyzer varies as the square of the cosine of the angle between the plane of transmission of the analyzer and the plane of the polarizer.
Fig. 22
  • I = I0 cos2 θ and A2 = A02 cos2 θ ⇒ A = A0 cos θ If θ = 0°, I = I0A = A0, If θ = 90°, I = 0, A = 0
  • If Ii = Intensity of unpolarised light.
    So I0 = Ii/2, i.e., if an unpolarized light is converted into plane polarized light (say by passing it through a Polaroid or a Nicol-prism), its intensity becomes half and 95072.png

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