What do you understands by double refraction.

When a beam of unpolarized light enters a birefringent crystal at normal incidence (but not along the optic axis), two light beams emerge. This effect (first observed in calcite by the Vikings) is called double refraction:

One ray, which we call the ordinary ray, passes straight through the crystal.

The other ray, called the extraordinary ray diverges as it passes through the crystal and becomes displaced. The relative polarization of the two emerging beams is orthogonal.

Here is an example of a crystal of calcite. The ordinary ray is able to pass straight through the crystal. The second line is produced by the extraordinary beam. The extraordinary beam does not follow Schnell’s law which holds only for isotropic materials.

If you place a cross polarizer over the crystal one or the other of the double refracted lines will disappear. They have orthogonal polarization.