Polarisation of Light and Brewster’s Law

Polarisation of Light :-

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Fig. 5┬╖27

polarisation of light is the property of transverse wave that can oscillate with more than one orientation. A light wave that is vibrating in more than one plane is referred to as unpolarised light.

The process of transforming unpolarised light into polarised light is known as polarisation of light.

If the non polarised light from an ordinary Source passes through a polaroid sheet PтВБ . It is observed that its intensity is reduced by half, Rotating PтВБ has no effect on the transmitted beam.

Let an identical piece of polaroid PтВВ be placed after PтВБ. As expected, the light from the sheet is reduced in intensity on passing through PтВБ alone. But now rotating PтВВ has a dramatic effect on the light coming from PтВБ.

In one position, the intensity transmitted by PтВБ followed by PтВВ is nearly zero. When terned by 90тБ░ from this position PтВВ transmits nearly the full intensity emerging from PтВБ.

The above experiment can be easily understood by assuming that light passing through the polaroid PтВБ gets polarised along the axis.

Malus Law :-

When two similar Polaroids (PтВБ and PтВВ)are placed parallel to each other and rotate the polaroid PтВБ or PтВВ , the Intensity will vary as

I = IтВА cos┬▓╬╕

Where IтВА is the intensity of polarised light after passing through PтВБ and ╬╕ is the angle between PтВБ and PтВВ , this is known as Malus Law.

Brewster’s Law :-

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Fig. Polarisation of light by reflection.

when unpolarised light is incident on the boundary between two transparent media (air and water), the reflected light is polarised with its electrical vector perpendicular to the plane of incidence when the refracted and reflected rays make the right angle (╧А/2) with each other.

Thus we have seen that when reflected wave is perpendicular to the refracted wave, the reflected wave is a totally polarised wave.

The angle if incidence in this case is called Brewster,s angle or angle of polarisation and is denoted by ip. It is related to the refractive index of the denser medium.

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This is known as Brewster’s law

I. e When unpolarised light is incident upon the surface of a transparent medium it experiences maximum plane polarisation at the angle of incidence whose tangent is equal to the refractive index of the medium.

QUESTION – Unpolarised light is incident on a plane glass surface, What should be the angle of incidence so that the reflected and refracted rays are perpendicular to each other ?

Solution –

i + r = ╧А/2

We should have tan ip = ╬╝ = 1┬╖5

This gives ip = 57тБ░

This is the Brewster’s angle for air to glass interface.

Polarisation of Light by Scattering :-

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Fig. 5┬╖29 Polarisation of light by scattering.

When an unpolarised ray of light travels through a medium, it undergoes scattering. the medium is composed of large number of atoms and ray of light contains number of light photons. When a light ray strikes the atoms in the medium, they transfer their energy to the atoms as a results, the electrons in the atoms are set vibration. These vibrating electrons emits electromagnetic radiation in all direction.

  • The radiation emitted by them will have the same frequency as the incidence radiation. This mechanism keeps propagating through the medium. The absorption and reemission of light radiation cause scattering of light in the medium.
  • The scattering of light by molecules was intensively investigated by C. V. Raman. He was awarded the Nobel prize for physics in 1930 for this work.

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