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For all materials with a ior different from 1.0 the refractive index isn't constant throughout the spectrum. It changes as a function of wavelength. Generally the refractive index decreases as the wavelength increases. Therefore light passing through a material will be separated according to wavelength. This is known as chromatic dispersion.
By default POV-Ray does not calculate dispersion as light travels through a
transparent object. In order to get a more realistic effect the dispersion
and dispersion_samples keywords can be added to the
interior{} block. They will simulate dispersion by creating a
prismatic color effect in the object.
The dispersion value is the ratio of refractive indices for violet to
red. It controls the strength of dispersion (how much the colors are spread out) used.
A DISPERSION_VALUE of 1 will give no dispersion, good values are 1.01 to 1.1.
Note: there will be no dispersion, unless the ior keyword has
been specified in interior{ }. An ior of 1 is legal. The ior has no
influence on the dispersion strength, only on the angle of refraction.
As POV-Ray does not use wavelengths for raytracing, a spectrum is simulated.
The dispersion_samples value controls the amount of color-steps and
smoothness in the spectrum. The default value is 7, the minimum is 2. Values up to
100 or higher may be needed to get a very smooth result.
Dispersion & Caustics
Dispersion only affects the interior of an object and has no effect on faked
caustics (See "Faked Caustics").
To see the effects of dispersion in caustics, photon mapping is needed (See the sections
"Photons" and "Dispersion & Photons").
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