Coupon Accepted Successfully!


Polarisation by reflection

Brewster discovered that when a beam of unpolarized light is reflected from a transparent medium (refractive index = μ), the reflected light is completely plane polarized at a certain angle of incidence (called the angle of polarization θp).
Fig. 23
From Fig. 23, it is clear that θP + θr = 90o
Also μ = tan θp Brewster’s law
  • For i < θP or i > θP
    Both reflected and refracted rays becomes partially polarized
  • For glass θp ≈ 57°, for water θp ≈ 53°

By dichroism

Some crystals such as tourmaline and sheets of iodosulphate of quinine have the property of strongly absorbing the light with vibrations perpendicular to a specific direction (called transmission axis) transmitting the light with vibrations parallel to it. This selective absorption of light is called dichroism.

By double refraction

In certain crystals, such as calcite, quartz and tourmaline, etc., incident unpolarized light splits up into two light beams of equal intensities with perpendicular polarization.
Fig. 24
  • One of the ray is ordinary ray (O-ray). It obeys the Snell’s law. Another ray is extra ordinary ray (E-ray). It does not obey the Snell’s law.
  • Along a particular direction (fixed in the crystal, the two velocities (velocity of O-ray, vo and velocity of E-ray, ve) are equal; this direction is known as the optic axis of the crystal (crystals known as uniaxial crystal). Optic axis is a direction and not any line in crystal.
  • In the direction, perpendicular to the optic axis for negative crystal (calcite), ve > vo and μe < μo.
    For positive crystal, ve voμe > μo.

Nicol prism

Nicol prism is made up of calcite crystal and in it E-ray is isolated from O-ray through total internal reflection of O-ray at canada balsam layer and then absorbing it at the blackened surface as shown in Fig 25.
Fig. 25
The refractive index for the O-ray is more that for the E-ray. The refractive index of Canada balsam lies between the refractive indices of calcite for the O-ray and E-ray

By scattering

It is found that scattered light in directions perpendicular to the direction of incident light is completely plane polarized while transmitted light is unpolarized. Light in all other directions is partially polarized.

Test Your Skills Now!
Take a Quiz now
Reviewer Name