White light is of course composed of many different individual wavelengths (or colors). The amount that a light ray is refracted is actually wavelength dependent. This causes an effect where a spectrum of colors can be observed at the interface between materials. The most well-known example of this is the rainbow that is produced by a prism.

We can model this effect by using slightly different values of Eta for the red, green, and blue components of the light ray. We would store three different values for Eta, compute three different reflection directions (red, green, and blue), and use those three directions to look up colors in the cube map. We take the red component from the first color, the green component from the second, and the blue component for the third, and combine the three components together to create the final color for the fragment.