Double refraction

Double refraction:

1) In 1869 Erasmus Bartholinus discovered that when a beam of ordinary unpolarized light is passed through a calcite crystal,the refracted light is split up in to two refracted rays.

The one which obey 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 these rays are plane polarized. This phenomenon is known as double refraction.

2) A beam of unpolarised light incident on the calcite crystal at an angle of incidence 'r' as shown in fig. Inside the crystal, the ray breaks up in to the ordinary and extraordinary rays. The ordinary ray travelling along RO makes an angle of refraction 'r1' while the extraordinary ray travelling along SE makes an angle of refraction 'r2'. The two opposite faces of the crystal are always parallel ,both the rays emerge parallel to the incident rays.

The refractive indices of ordinary and extraordinary rays can be expressed as

µ₀= sin r/sin r1  and µ_i = sin r/ sin r₂

3) The velocity of light for ordinary ray inside the crystal will be less then the extraordinary ray .the refractive index of ordinary ray is same for all the angles

of incidence while the refractive indices of extra ordinary ray varies with angles

of incidence.ordinary ray travels with the same speed in all directions while extra ordinary ray has different speeds in different directions.