Which term describes the direction of single refraction in a doubly refractive gem?

The term that describes the direction of single refraction in a doubly refractive gem is the "optic axis." In the context of doubly refractive materials, there are two ways that light can travel through the crystal, leading to the phenomenon known as birefringence. The optic axis is the specific direction within the crystal structure where light will not experience double refraction; instead, it travels through as though it is a singly refractive material.

This concept is essential in understanding how different types of gemstones interact with light. When light enters a doubly refractive gem, it can be split into two rays, each traveling at a different speed and with a different refractive index. The optic axis is the unique direction along which this splitting does not occur, providing a clear pathway for light to pass through without being refracted into multiple rays.

The other terms, such as birefringence and crystal axis, also relate to light behavior and crystal structure but do not specifically define the direction of single refraction in the same way that the optic axis does. Birefringence refers to the difference in the refractive indices of the two rays, and the crystal axis is a more general reference to the orientation of the crystal lattice