Two calcite crystals

Two calcite crystals are used to demonstrate birefringence, the phenomenon where a material splits incoming light into two rays with different velocities. When light passes through the first calcite crystal, it is split into ordinary and extraordinary rays, each polarized in different directions. By placing a second calcite crystal in the path of these rays and rotating it, the separation and recombination of the rays can be observed, illustrating the concept of double refraction and the polarization of light.