Birefringent Materials

Equipment

  • Calcite crystal
  • Printed text

Demo

A piece of calcite crystal is used to demonstrate concepts of birefringence; chiefly, double refraction.

birefrigent

Figure 1: Text refracted through a piece of calcite crystal.

Explanation

Crystalline materials (materials in which all the atoms follow the same ordered structure) may have multiple indices of refraction. Crystals (indeed, all solids) may be thought to be comprised of atoms and springs: the springs connect the individual atoms and generate the well known lattice structure (Figure 2).

In a normal crystal, the binding forces between each atom will be the same in two of the three directions (i.e. in the y and z directions); the “strength” of each spring in the lattice is the same. Thus, there will be only one optical axis (in this case the x direction). Light entering an isotropic crystal is refracted at a constant angle and passes through the crystal at a single velocity without being polarized by interaction with the electronic components of the crystalline lattice.

In contrast, birefringent materials have anisotropic binding forces composing their atomic structure.

 

Einstein-and-quantum-solids

Figure 2: Crystalline solid with springs forming the lattice (image courtesy of Quora).

Continuing with the spring lattice analogy, birefringent materials have a certain “strength” spring in one direction (the y direction), with spring of a greater or weaker “strength” in another direction (the z direction). The axes of an anisotropic crystal are distinctly different; interactions with light depend upon the orientation of the crystal lattice with respect to the incident light angle.

If incident light travels along the optical axis in an anisotropic material, it will behave in a manner similar to that of an isotropic material. However, when light enters a non-equivalent axis, it is refracted into two rays, each polarized with the vibration directions oriented at right angles (mutually perpendicular) to one another and travels at different velocities. This phenomenon is termed doublerefraction or birefringence (Microscopy).

Written by Madison Harris