What is double refraction. describe construction working and use of nicol prism?

Answer 1

Double Refraction:

Erasmus Bartholinus discovered that when a beam of ordinary unpolarised light is passed through a calcite crystal, the refracted light is split up into two refracted rays.

The one which always obeys the ordinary laws of refraction and having vibrations

perpendicular to the principal section is known as ordinary ray. The other, in

general, does not obey the laws of refraction and having vibrations in the principal

section is called as extra-ordinary ray. Both the rays are plane polarised. This phenomenon is known as double refraction. The crystals showing this

phenomenon are known as doubly refracting crystals.

Construction,Working and Use of Nicol Prism:

Nicol prism is a calcite crystal whose length is three times as its width.The end faces of this crystal are grounded in such a way that the angles in the principal section

becomes 68o and 112o instead of 71o and 109o . This is due to increase in

the field of view.

The crystal is cut in two pieces by a plane perpendicular to the principal section as

well as end faces. The two cut surfaces are grounded and polished optically flat and then cemented together by a material called 'Canada balsam'.The refractive index of Canada balsam lies between the refractive indices for the ordinary and extra-ordinary rays for calcite.

Working:

When a beam of light enters the faces in direction parallel to the long side,

it is doubly refracted into ordinary plane polarised beam and extra-ordinary

plane polarised beam. From the values of refractive indices given above, it is

clear that Canada balsam acts as a rarer medium for an ordinary ray and denser

medium for extra-ordinary ray. Moreover, the dimensions of the crystal are so

chosen that the angle of incidence of ordinary ray at the calcite-balsam surface

becomes greater than the corresponding critical angle 69o. Under these

conditions, the ordinary ray is completely reflected at calcite-balsam surface and is

absorbed by the tube containing the Nicol's prism. The extraordinary ray is not

totally reflected because it is traveling from a rarer to a denser medium and is thus

transmitted with no appreciable loss in intensity. It is slightly displaced laterally but

emerges out of the prism parallel to its original direction. Thus only the extra-

ordinary ray having vibrations parallel to principal plane, is transmitted. So the

light emerging from the Nicol's prism is a plane polarized light.

Use:

Nicol's prism can be used both as polarizer and an analyser.When two Nicol's are arranged coaxially, then the first Nicol which produces plane polarized light is known as polariser while the second which analyses the polarized light is known as analyser.