Question

In double refraction:
(A) The velocity of the E-ray varies with direction
(B) E-ray does not obey Snell’s law
(C) $ \mu $ of E-ray is constant
(D) Both A and B

Answer 1

Hint: E-rays have been observed to have a refraction at incident angle of zero. The refractive index of a material which causes double refraction changes with direction.

Complete step by step solution:
In some materials, when unpolarised light is incident on their surface, the lights splits into two each becoming polarized in planes that are at right angles (angle 90 degrees) to each other. This phenomenon is called double refraction, while the property of a material to cause this phenomenon is called birefringence.
The double refraction (sometimes called) is an optical property possessed by some materials that allows unpolarised light to split into two due to the varying value of their refractive indices depending on the direction of propagation and the direction of vibration of the light incident on their surfaces. These types of materials are birefringent, and they are usually anisotropic in nature. Anisotropy is when a particular property of a material varies depending on the plane (or direction) at which the property was measured. For birefringent materials, the property is refractive index.
When the incident light splits, one of the lights is said to be extraordinary while the other is ordinary. This extraordinary light is called E-rays. This E-ray by experiments has been observed to refract out (when incident on a birefringent material) even if the angle of incidence is zero. Hence, it doesn’t obey Snell’s law which predicts that angle of refraction should be zero when angle of incidence is zero.
Now, also, since these materials are anisotropic, the refractive index varies with direction. Hence, the velocity of the E-rays also varies with direction.
Hence, from above we can conclude that both A and B are correct.
Thus, the correct option is D.

Note:
Although the $ \mu $ of E-ray is not constant, that of the ordinary ray is. The $ \mu $ of E-rays varies because it contains components of the electric field of light that is both parallel and perpendicular to the optical axis of the material.