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Optical Properties of Rubies : Refractive Index

High Quality Ruby exhibiting Transparency
Refractive index of a Ruby affects its shine/brilliance

Refractive Index

In optics, the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. As light moves from a medium, such as air, water, or glass, into another it may change its propagation direction. This change is due to the change in the refractive index of the medium the wave was travelling in. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium. This can be written mathematically as:

    n = speed of light in a vacuum / speed of light in medium.

For example, the refractive index of water is 1.33, meaning that light travels 1.33 times faster in vacuum than it does in water.

There are two factors when calculating a gem's refractive index: "angle of incident" and "refractive angle." The "incident angle" is the angle of the approaching light as it intersects with the stone's exterior surface. The "refractive angle" is the altered angle of the light as it passes through the stone's interior. The Refractive Index is the ratio of difference between these two angles. Each material has its own unique density and Refractive Index.

Refraction of light in Ruby.
Refraction of light in Ruby. Image courtesy: All about Gemstones

The color of a gemstone, and the frequency (color) of the light traveling through it, can also effect its refractive index. As the light's frequency changes, so does its angle of refraction. Higher light frequencies travel through the stone more slowly, while lower frequencies travel faster, causing a spreading effect known as dispersion. As each frequency is reflected it is dispersed throughout the stone's interior at varying speeds and directions. As the scattering and dispersion of light within the stone is increased, so to is the amount of "fire" that is returned to the viewer.

Significance of Refractive Index
Refractive Index is a very important property used to test the authenticity of gems. Each gem has a unique Refractive Index depending upon the composition of the gem crystal.

Refractive Index of Common Gemstones

    Agate: 1.544
    Alexandrite: 1.745
    Amber: 1.539 - 1.546
    Amethyst: 1.532 - 1.554
    Andalusite: 1.641
    Apatite: 1.632 - 1.420
    Aquamarine: 1.577
    Beryl: 1.577 - 1.600
    Chalcedony: 1.530
    Chrysoberyl: 1.745
    Citrine: 1.532 - 1.554
    Coral: 1.486
    Corundum: 1.766
    Cubic Zirconia: 1.800 - 2.170
    Diamond: 2.417
    Emerald: 1.576
    Emerald Synthetic: 1.561
    Fluoride: 1.560
    Garnet Almandine: 1.760
    Ivory: 1.540
    Jasper: 1.540
    Labradorite: 1.560 - 1.570
    Lapis Lazuli: 1.610
    Malachite: 1.655
    Moissanite: 2.670
    Moonstone (Adularia): 1.518 - 1.526
    Morganite: 1.577
    Onyx: 1.486
    Opal: 1.450
    Pearl: 1.530
    Peridot: 1.654
    Quartz: 1.544
    Rubelite: 1.567
    Ruby: 1.766
    Sapphire: 1.766
    Spinel: 1.712
    Tanzanite: 1.692 - 1.700
    Tiger's Eye: 1.544
    Topaz: 1.620
    Tourmaline: 1.624
    Tourmaline (Paraiba): 1.610 - 1.650
    Turquoise: 1.610
    Zircon: 1.800 - 1.960

Measuring Refractive Index using Refractometer

The refractometer is one of the most important tools in a gemological laboratory. It indicates (not measures) the refraction index of a gemstone, which often gives vital clues to the identity of a gemstone. It is based on a unique optical phenomenon named Total Internal Reflection (or TIR). When light travels from an optically denser material (with higher index of refraction) to an optically rarer material (with lower index of refraction), all light that reaches the boundary of the two materials will be either reflected inside the denser material or refracted into the rarer material, depending on the angle of incidence of the light.When the angle of incidence is greater than the critical angle, the ray is completely reflected inside the denser medium. This phenomenon is called Total Internal Reflection.

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Stay tuned for more on Optical Properties of Rubies.



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