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Dispersion

Dispersion is the splitting up of white light into its spectral colour components, when white light passes through a transparent medium with its two surfaces inclined to each other (e.g. a prism).

  • When light passes from rarer into a denser medium, its velocity varies with the wavelength.
  • Red light with the longest wavelength has the greatest velocity and is refracted the least.
  • Violet light with the shortest wavelength and the least velocity is refracted the most.
  • Dispersion is the basis for the prism spectroscope and its ability to separate light according to wavelength.
  • Dispersion is also the source of chromatic aberration in an optical device (i.e. the failure of different wavelengths to focus at the same point).
  • Dispersion can be calculated with the help of a refractometer.
  • Dispersion is calculated as the mathematical difference between the R.I. for the B (686.7nm) line and the G (430.8nm) line of the Fraunhofer lines respectively, for a particular species.
  • Using red and blue filters, the difference between the refractive index readings for the red (686.7nm) and the blue (430.8nm) wavelength will give the dispersive value for the stone.
  • The R.I. of a gemstone will be least for the red light and more for the violet or blue light.

Dispersion of some gemstones:

Synthetic Rutile0.290
Strontium Titanate0.190
Synthetic Moissanite0.105
Synthetic Cubic Zirconia0.060
Sphene0.051
Yttrium Oxide0.050
Diamond0.044
Benitoite0.046
Zircon0.038
G.G.G.0.045
Dioptase0.036
Y.A.G.0.028
Scapolite0.017
Tourmaline0.017
Sillimanite0.015
Topaz0.014
Beryl0.014
Quartz0.013

Fraunhofer lines: These are the major absorption lines recorded by Fraunhofer, in the sun’s spectrum. These major absorption lines are named by alphabets, as is shown below:

NameElementNanometers (nm)
AOxygen759.3
BOxygen686.7
CHydrogen656.3
D1Sodium589.5
D2Sodium589.0
EIron526.9
FHydrogen486.1
GIron430.8
HCalcium396.8
KCalcium393.3

Interference of Light: Rays of light which strike the surface of a gem are partially reflected back and partially refracted into the stone, to be reflected back from layers within the stone. Two beams of light having the same wavelength and traveling in near parallel paths interact with each other to produce constructive reinforcement or destructive cancellation.

  • Interference occurs between light rays reflected from the surface and from within the stone.
  • Since one ray has traveled further, it may be out of phase with the other one for a particular wavelength leading to the extinction of that wavelength.
  • If the rays are in phase for a particular wavelength, then there is reinforcement of that wavelength.

This phenomenon of interference of light is responsible for effects such as:

  • Adularescence: Bluish colour effect seen in moonstone.
  • Iridescence: Interference of light at thin films or cracks in the stone as in iris quartz, along cleavage cracks, along fractures in any stone.
  • Play of colour: The spectral colours produced by a combination of interference and diffraction of light as in precious opal, and the colour flashes exhibited by labradorite feldspar due to interference of light from twin planes.
  • Orient: The prismatic colours seen in pearls.

Diffraction: The term diffraction is used to describe the behaviour of light when it passes through a narrow hole. This happens when the edges of an object produces the light which is either bent or scattered. The resulting colour-producing phenomenon is known as diffraction.

  • Opal is said to be the best example of diffraction.
  • The principle of diffraction is used in the diffraction grating spectroscope.

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