Skip to main content
Geosciences LibreTexts

7.16: Dispersion

  • Page ID
    3657
  • Basic

    File:Dispersion.png

    Figure \(\PageIndex{1}\): 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 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 all gemstones, dependent upon the stone's optical density (how fast light can travel inside the gemstone). 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.

    Figure \(\PageIndex{2}\): "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

    Advanced

    Measurement of dispersion is usually carried out using a table spectrometer. Through the minimum deviation method, very accurate refraction indices can be obtained with this apparatus (more accurate than with the refractometer). This type of instrument can be obtained for around USD 1500.00, and takes some skill to operate.

    An easier way to measure dispersion would be to use narrow bandpass interference filters on a refractometer. However, most refractometers are calibrated to take measurements on the sodium D-line, and the B and G lines may be hard to see for most humans.

    Scientists usually measure dispersion between the C and the F lines, giving considerably different values. The values of these lines lay closer to what our eyes can distinguish, so measurements on the C and F lines may be valuable after interpolation to obtain the B and G line values.

    Experimentation with narrow bandpass interference filters with wavelengths of 656nm (nC) and 486nm (nF) may give good results. One will, however, need to create a graph with calibration plates for the particular refractometer to correct the errors.

     

    Dispersion of Some Common Gem Minerals

    Gem Mineral Dispersion
    Anatase 0.213
    Angelsite 0.044
    Aquamarine 0.014
    Benitoite 0.046
    Beryl 0.014
    Beryllonite 0.010
    Boracite 0.024
    Brazilianite 0.014
    Brookite 0.131
    Cassiterite 0.071
    Celestite 0.014
    Cerussite 0.055
    Chrysoberyl 0.015
    Corundum 0.018
    Danburite 0.017
    Datolite 0.016
    Diamond 0.044
    Dioptase 0.036
    Epidote 0.019
    Euclase 0.016
    Feldspar, Orthoclase 0.012
    Fluorite 0.007
    Garnet, Andradite 0.057
    Garnet, Almandite 0.027
    Garnet, Grossularite 0.027
    Garnet, Pyrope 0.022
    Garnet, Rhodolite 0.026
    Garnet, Spessartite 0.027
    Goshenite 0.014
    Hackmanite 0.018
    Hambergite 0.015
    Herderite 0.017
    Idocrase 0.019
    Iolite 0.017
    Kornerupine 0.018
    Kyanite 0.020
    Moissanite 0.104
    Peridot 0.020
    Phenakite 0.005
    Pollucite 0.012
    Powellite 0.058
    Quartz 0.013
    Rhodizite 0.018
    Rutile 0.280
    Scapolite 0.017
    Scheelite 0.038
    Sillimanite 0.015
    Smithsonite 0.037
    Sphalerite 0.156
    Sphene 0.051
    Spinel 0.020
    Spodumene 0.017
    Staurolite 0.023
    Tantalite 0.146
    Topaz 0.014
    Tourmaline 0.017
    Vanadinite 0.202
    Wulfenite 0.203
    Zincite 0.127
    Zircon 0.039

    Related topics