Question

What are the positive and negative crystals? Explain that

Answer 1

Positive crystals -- A doubly refracting crystal in which the index of refraction for the extraordinary ray is greater than for the ordinary ray, and the former is refracted nearer to the axis than the latter, as quartz and ice; -- opposed to negative crystal, or one in which this characteristic is reversed, as Iceland spar, tourmaline, etc. Due to the different refractive index to the ordinary and extraordinary ray it they travel with different velocity inside the crystal. Velocity of ordinary ray is greater then the extraordinary ray. This happens due to the different dispersion of the light inside the crystal which means that the angular velocity w is not proportional to wave vector k of the wave inside the crystal.

Negative crystal-- A birefringent crystal in which the refractive index of the extraordinary ray is less than that of the ordinary ray.These are crystal, such as one of calcite, in which the extraordinary wave travels faster than the ordinary wave. In negative crystal also this velocity difference is due to the dispersion of the wave inside the crystal.