Polarization of Light

Physics Chemistry  Biology  Mathematics
Science > Physics > Wave Theory of LightYou are Here

Ordinary Light:

  • Light waves are transverse in nature. The direction of its vibration is in the plane which is perpendicular to the direction of propagation of the wave. Thus wave can vibrate in any plane (infinite possibilities) which is perpendicular to the direction of propagation of the wave. Such light wave is called unpolarized light.
  • Following phenomena prove transverse nature of light.
  • Intensity of light at a place changes continuously.
  • Light can be polarized.

Transverse Nature of  Light Waves:

  • When light waves are passed through two crystalline slits say A and B (These slits are the tourmaline plates cut parallel to the axis of crystal). Ordinary light say from the sun is incident on the crystal A.a) When crystal A and crystal B are parallel to each other, the intensity of the light emerging from crystal A is constant at any orientation of A and passes through crystal B without any change.

Polarized Light Transverse Nature

  • Now crystal B is rotated w.r.t. A, the intensity of emerging light from crystal B decreases and becomes zero, when crystal B is at right angle w.r.t crystal A.

Polarized Light Transverse Nature 02

  • This experiment proves the transverse nature of the light waves. Crystal A is called polariser and crystal B is called analyser.

Plane Polarised Light:

  • When the transverse vibrations of the light wave are confined to a plane containing the direction of propagation, the light is said to be plane polarized.
  • The plane in which the vibration lie is called plane of vibration. The plane perpendicular to the plane of vibration is called the plane of polarization and the light is said to be polarized in it. In this plane the vibrations of light are absent.


Schematic Representation of Light:

Polarized Light Representation

Terminology:

Polarized light:

  • When the transverse vibrations of the light wave are confined to a plane containing the direction of propagation, the light is said to be plane polarized.

Plane of vibration:

  • The plane in which the vibrations of polarized light takes place is called the plane of vibration.

Plane of polarization:

  • The plane perpendicular to the plane of vibration in which there are no vibrations of polarized light is called plane of polarization.



Polarisation:

  • The phenomenon of restriction of the vibrations of light waves in a particular plane particular plane perpendicular to the direction of propagation of wave motion is called polarization.

Polariser:

  • The substances which bring about the plane polarization of light are called as polarisers. e.g. Tourmaline crystal, Nicol prism.

Unpolarised light:

  • Light waves are transverse in nature. The direction of A light in which the vibrations of the electric vectors are in all possible directions, which are perpendicular to the direction of propagation

Unpolarised light:

  • Light waves are transverse in nature. The direction of A light in which the vibrations of the electric vectors are in all possible directions, which are perpendicular to the direction of propagation of the wave is called unpolarized waves.

Plane of Incidence:

  • The plane containing the incident and reflected rays and normal to the surface is called the plane of incidence.

Diagram Showing Partial Polarisation:

Polarized Light Partial polarization

  • Malus showed that when a beam of unpolarized light is reflected from a nonmetallic surface, the reflected light is partially plane polarized.
  • The degree of polarization depends on the angle of incidence. The particular angle of incidence at which the reflected light is completely plane polarized is called the angle of polarization or polarizing angle.
  • The vibrations of the reflected plane polarized light are found to be perpendicular to the plane of incidence.

Brewster’s law:

  • The tangent of the polarizing angle is equal to the refractive index of the material of the surface from which reflection is taking place.
  • Mathematically,     μ = tan ip.
  • This angle characteristic of the medium.

Polarized Light Partial polarization 01

ip = angle of incidence and r = angle of refraction



Proof: 

  • Let us consider the unpolarized monochromatic light incident in air at the polarizing angle ip on the plane surface XY of the transparent medium of refractive index μ. Experimentally Brewster proved that ∠ SQR = 90°.

Polarized Light Partial Brewsters Law



  • Brewster further proved that polarizing angle is a function of a wavelength of the light.
  • This law is not obeyed by polished metallic surfaces.
Science > Physics > Wave Theory of LightYou are Here
Physics Chemistry  Biology  Mathematics

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