The refractive index of ice crystals depends upon the polarization and propagation direction of a beam of light. This property is known as birefringence (sometimes called double refraction). The colored light seen when looking through the sides of an airplane window is an example of birefringence. When a beam of light propagates through ice crystals, two distinct rays result, depending upon the direction of propagation.
A polarizer filter or lens acts to recombine the rays. However, due to the fact that these rays were out of phase when recombined, the new polarized rays are composed of various wavelengths of visible light, which as shown here result in especially vibrant colors. Note that colors in pond ice may also be due to interference in narrow fissures and from refraction by trapped air bubbles. — Photography:Daniela RapavaSummary:Jim Foster & Daniela Rapava