Optical Properties of Ice

Since the wave speed in ice is dependent on the wavelength of the incident light, different colors of light refract differently as they travel through ice. This is called dispersion and is responsible for the color gradient seen in halos.

Figure 1: A thin ice section between two crossed polarized filters. This coloration is due to the wavelength dependence of the refractive index. Image from http://www.gfy.ku.dk/~pditlev/annual_report/Projects.html

Figure 2: Diagram of a hexagonal crystal. For ice a=4.5 angstroms and c=7.3 angstroms. Image courtesy of Wikipedia

The hexagonal geometry of ice leads to birefringence. Birefringence refers to the dependence of the refractive index of a material on how the incoming light is polarized with respect to the c axis also refered to as the optical axis. Light that is polarized parallel to the c-axis is refered to as the ordinary wave and light polarized perpendicularly is called the extraordinary wave. This effectively splits light entering the ice into two different rays that are refracted differently. This effect can be seen by looking at a sundog through a polarizing filter. The sundog will appear to move left or right compared to without the filter.