What is Birefringence?

                 Birefringence  is the double refraction of light in a transparent , molecularly ordered material, which is manifested by the existence orientation dependent differences in refractive index.  The factor determining whether the material possesses birefringence or not is the type is the arrangement of atoms and chemical bonds in the material that determine whether it is anisotropic or isotropic.

                

                 Crystalline materials are classified as being either isotropic or anisotropic depending on the optical behavior and whether or not their crystalline axes are equivalent.  All isotrophic crystals have equivalent axes that interact with light in a similar manner, regardless of the orientation that incident light waves enter the crystal.  Light passing through an isotropic crystal without being polarized by interaction with the electronic components of the crystalline lattice.  An example of an isotropic material is sodium chloride which has cubic crystalline lattice structure, where all of the sodium and chloride ions are arranged with uniform spacing along three mutually perpendicular axes shown in Figure 1a.

 

                 Anisotropy refers to the non-uniform spatical distribution of properties, which results in different values being obtained when specimens are probed from several directions within the same material.  Observed properties usually depend on the particular probe being used and often vary depending upon whether the observed phenomena is based on optical, acoustical, thermal, magnetic, or electrical events.  Whereas, as mentioned above, isotropic properties remain symmetrical, regardless of direction of measurement, with each type of probe reporting the same results.  Examples of anisotropic materials include calcite and amorphous polymer which is shown in Figure 1b and 1c.

 

                 Anisotropic crystals possess crystallographically distinct axes and interact with light by a mechanism that is dependent upon the orientation of the crystalline lattice with respect to the incident light angle.  When light enters at the optical axis of an anisotropic crystal, it behaves similar to the interaction with isotrpic crystals, and passes through at a single velocity.  However, when light propagates through the crystal at a non-equivalent axis, the light is refracted into two rays, each polarized with the vibration directions oriented at right angles (perpendicular) to one another and travelling at different velocities.   This phenomenon is known as birefringence and all anisotropic crystals exhibit this property.  An example of birefringence in a calcite crystal is shown in figure 2.

 

                 When light propagates off the optic axis of an anisotropic material which possesses birefringence shown in Figure 3, the incident ray of light is split into the ordinary and extraordinary rays.  These rays have orthogonal polarizations and travel with different velocities.  The ordinary wave has a polarization that is always perpendicular to the optical axis.

Source: http://www.microscopyu.com/articles/polarized/birefringenceintro.html

Figure3– 3-D model showing birefringence

Figure 4– Light propagating at normal incidence through birefringent material.

Figure 5– Light propagating through  birefringent material along optical axis.

 

                 In the cases where the light propagates at normal incidence or at the optical axis, no double refraction occurs and the light propagates straight through the material shown in Figures 4 and 5.

 

References

Murphy, Douglas, Kenneth Spring, Thomas Fellers, and Michael Davidson. "Nikon MicroscopyU - The Source for Microscopy Education." Nikon MicroscopyU. http://microscopyu.com/articles/polarized/birefringenceintro.html (accessed ).