Diamond Properties : Refraction

A very high refractive power gives the diamond its extraordinary brilliance. A properly cut diamond will return a greater amount of light to the eye of the observer than will a gem of lesser refractive power and will thus appear more brilliant. This high dispersion gives diamonds their fire, caused by the separation of white light into the colors of the spectrum as it passes through the stone.

Diamond's brilliance and luster are two of its most valued attributes. The science behind such phenomena is diamond's great ability to refract light; that is, to bend or slow light as it passes through it. The amount that a substance can impact light in these ways is quantified in its refractive index.

Diamonds have a very high refractive index this means that light entering or leaving a diamond will be bent strongly as it crosses the diamond-air interface. The amount of bending is given by Snell's Law:

The ratio of the sines of the angle of incidence and refraction is equal to the ratio of the refractive indices on the incident and refraction side of the interface. Thus, we can see that when light travels from diamond (ndiamond = 2.42) to air(nair = 1) the sine of the angle made by the beam of light in the air is greater than the angle made in the diamond.

Further, you can see that the, above some critical angle of incidence, the "sin(thetaair)" given by Snell's Law will be greater than 1. As this is impossible, we know that above the critical angle, we cannot have refraction. But the light has to go somewhere it is reflected back into the diamond. i.e more of the light that enters a diamond from the front is reflected from the back than if the gem was a cubic zirconia (or pretty much any other gemstone) giving the diamond its characteristic "sparkle."

Science postulates the speed of light in a vacuum to be about 186,000 miles per second. But the velocity of light is slowed whenever it is forced to interact with the electrons of a substance, whether it's a liquid, gas or solid. Generally speaking, higher density materials have greater concentrations of electrons and therefore greater capabilities to refract light. Light passing through diamond is reduced to about 77,000 miles per second--near the maximum for any transparent substance.

Reflectance, or the amount of light reflected from a transparent substance, can also be inferred from a material's refractive index. Once again, diamond displays the maxim amount of reflectance for a transparent substance, displaying what is called an "adamantine" luster.

Substance Velocity of Light  mps (kps) Refractive Index Space 186,282 (299,792) 1.00 Air 186,232 (299,890) 1.00 Water 140,061 (225,442) 1.33 Glass 122,554 (197,349) 1.52 Diamond 77,056 (124,083) 2.42

The refractive index compares the velocity of light in a substance to that in a vacuum. Diamond slows light to a remarkable degree, and ranks high in refractive index.

Other Properties of Diamond.