Why diamonds sparkle physics

The refractive index of diamond is pretty high 2. Due to this fact, diamond is an important application in optics. Consider an ideal cut diamond. I explain according to the figure below. Finally, the light refracts out. The first one shows the mechanism of internal dispersive reflection. The second figure shows the reflections inside ideal cut, deep and shallow cut diamonds. Note: For total internal reflection to take place, light must travel from an optically denser medium to a relatively rarer medium.

Also, the incident angle should be far high above thee critical angle. The phenomenon you're looking for is called total internal reflection. You could also have a look at this link for more information. To draw a comparison with glass : In glass for the most part when you incident light onto it, it gets refracted on one surface, and gets refracted again at the other surface and leaves the material.

This doesn't always happen, there is some total internal reflection happening, but the 'critical angle' for glass is really high so you don't usually see it happening. So a greater percentage of the incident light gets internally reflected several times before it emerges from the diamond, making the diamond look really shiny.

Edit : As JohnRennie has also mentioned - It's also the shape that matters to the shiny-ness. Uncut diamond doesn't look as bright since the angles of incidence isn't made to be beyond the critical angle. Diamond has a very high refractive index about 2. So if you compare a diamond to a piece of glass cut into the same shape, the diamond will reflect more light and therefore it will sparkle more.

The sparkling isn't just the high refractive index - the shape matters too. Diamonds are cut so that a light ray falling on the diamond is reflected multiple times inside the diamond and therefore sends light out in all directions. Actually, diamonds don't shine, they reflect.

The greater the refractive index, the smaller the critical angle c. This refracted ray will come back into the denser medium. This phenomenon is called total internal reflection.

Note that since the ray comes back into the same medium, it is reflecting rather than refracting. Si critical angle for diamonds is very small. Moreover, we cut the faces of diamonds in such a way that light inside the diamond is falling on the faces at angles greater than the critical angle. So it keeps on getting totally internally reflected. Blemishes on the surface of the diamond limit the amount of light that can enter the diamond. Inclusions prevent the light from moving freely within the gem.

These imperfections are like stop signs and speed bumps; they affect how the light travels. Fewer imperfections mean a more brilliant diamond, as there is nothing or nearly nothing to detract from the shine and sparkle.

After the diamond is painstakingly cut and shaped to perfectly bounce light, the final step in its completion is to carefully polish it to its final level of shine.

Any residual roughness or exterior blemishes are very gently eroded away. In this way, diamonds are like almost anything else; a thorough polish can really bring out the brilliance. You can see the results of this effort every time you notice the way a diamond casts a beautiful display of light and color as it is moved this way or that.

This scintillation, or the intense array of light and colors, is a large part of what makes diamonds such a valuable and cherished gemstone.