Why do people like shiny things? One of the most popular gemstones is the diamond. Anybody who has watched a clear stream trickling gently over rocks knows how calming it can be. We fill our world with glass on our devices, on our faces, in our windows, and our cameras.

I have a theory, and you can probably guess that it has to do with science. Every material has a refractive index. The refractive index of most common glasses is approximately 1.51, for water it is about 1.33. What this means is that light is actually slower inside those media than it is in the air. The effect is that light gets bent, changed, focused, dispersed, and reflected at material interfaces. The net result of which is that clear water looks really pretty, diamonds sparkle, and glass makes things look shinier and cleaner.

The index of refraction also contributes to total internal reflection (TIR). TIR comes from Snell’s law. If the angle that the light hits a media interface is large enough then the light gets completely reflected. Next time you go swimming go to the bottom of the pool and look up. You will see a circle of sky surrounded by the bottom of the pool. For diamonds the index is so high that the critical angle is small. Light gets literally trapped inside the diamond. The trapped light escapes and finds it’s way into your eye, making the gemstone sparkle.


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