All that is gold does not glitter...

This post is not about gold, rather it is about glittering and sparkling. (More about gold later)

What glitters and sparkles and catches the eye of many girls and women? DIAMONDS of course!

Ever wonder why diamonds sparkle?

In order to understand this, one must first understand that light bends when it goes from one medium to another providing these media have a different refractive index (n). When light goes from a medium of low refractive index to one of high refractive index, it bends into the normal so that θ, the angle between light and the normal, decreases. The opposite happens when light moves from a medium with high refractive index to one of low refractive index: θ increases.
This relationship is described by Snell's Law. n1sinθ1= n2sinθ2

To understand why diamonds sparkle, we must realize that this occurs when Snell's Law does NOT apply. (This is how fiber optics cables work too)

Say the refractive index of a materials is 2 (and for diamonds it it 2.42):
And n= 1 for air
(2) sinθ1= (1)sinθ2 [Assume that the light is entering the surface at 90 degrees]
sin θ1 = 1/2 sin (90)
sin θ1 = 1/2
=> θ1= 30 degrees

What happens to the light when it strikes the surface at an angle greater than 30 degrees?
It goes back into the substance! This is known as total internal reflection. (This phenomena also occurs when we see a rainbow)

Back to diamonds: the refractive index of diamonds is 2.42, so this means that light that strikes the surface at an angle more than perhaps 25 degrees gets reflected back into the material. As a result, when Snell's law doesn't apply light gets trapped inside the medium. While in the material, the light disperses. This means that white light splits up into the colors of the spectrum: Red, Orange, Green, Yellow, Blue, Indigo and Violet. Each color has a slightly different wavelength. n is inversely proportional to wavelength and as a result the light separates or there is dispersion of light.

If the light stays trapped in the medium long enough, there is a time when there is enough dispersion that the human eye can detect it. Now you know why diamonds sparkle:
  1. Light is trapped in the material as there is total internal reflection because of the high refractive index of the diamond
  2. Light then disperses in the diamond, and this is what we observe.