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| 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.
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| 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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