# Dispersion

## Basic

Dispersion of white light in a prism

Dispersion is the splitting up of white light into its individual wavelengths, what we see as colors. Dispersion occurs with transparent surfaces that are not parallel to each other, such as gemstone facets. Measurement of dispersion is done (in gemology) by calculating the difference of refraction indices for red light waves and violet light waves.

The source for red light (such as sunlight) travels at a wavelength of 686.7nm (named the Fraunhofer B-line) and at 430.8nm for violet light (the Fraunhofer G-line). The interval between red and violet gives the dispersion value of a gemstone.

All the individual wavelengths have their own refractive index numbers. Red light has a lower refraction index than violet light, thus the violet part of white light will bend more. These values are different for most gemstones, dependent upon the stone's optical density (how easily light penetrates). All transparent gemstones will show dispersion, but the dispersion colors may be masked by the body color of the gemstone. In Diamonds, the color dispersion of white light causes the spectacular "fire" in well-cut brilliant cuts that possess good white color. This "fire" is an interaction between color dispersion and total internal reflection.

"Fire" in Diamond as the result of dispersion and total internal reflection

The refraction index of Diamond (measured with nD - or the Fraunhofer D-line) gives a refraction index of 2.417. The value for red light (nB) in a Diamond is measured at 2.407 and for violet light (nG) it is measured at 2.451. The interval between the B and the G lines is 2.407 - 2.451 = 0.044. Thus, the dispersion value of Diamond is 0.044.

This example shows that decreasing (shorter) wavelengths have increasing indices of refraction. This is known under the term Normal dispersion of the refractive indices.