Relationship between shining diamonds and cables under the sea !!

 At first instance the title would seem to be crazy and lame, but actually the physics involved in both are largely the same. The high speed internet and shine of diamonds both are due to TIR(Total Internal Reflection). This phenomenon is one of the most versatile phenomenon exploited by humans to develop high speed communication devices. The discovery of TIR is kinda controversial but largely attributed to Johannes Kepler(1611).

Let me try to explain TIR in most simple and objective manner:

 Consider a light ray propagating in medium having refractive index n1 and entering a phase having refractive index n2. As we know when n2>n1, the light is entering in a denser medium from a rarer, so by Huygens wavefront theory or snells law we can predict it would bend slightly towards normal. But when the light is in n2 and going to n1 it would bend away from normal. Now when we carefully analyze this situation we conclude that at some point of incidence the light ray would bend so much making angle of refraction= pi/2, which makes refracted ray parallel to interphase of two mediums. This particular angle is referred as critical angle, which could be easily calculated for paraxial rays by n2sin(ic)=n1sin(pi/2).

Now the next thing that strikes us that, what would happen if the angle of incidence is higher than critical angle and we observed that the rays would get reflected by the interface following all laws of reflection, we call this phenomenon as TIR. 

Diamonds have a critical angle of around 24 degree, so they are cutted in a particular manner thus all rays coming out of diamond have angle of incidence greater than critical angle at the interface.

 Similarly the huge optic fiber cables under the sea also employs this principle to transmit data over long distance with minimum losses.(We use light to transfer data as that is the most fastest method which we could figure out for optimum data transfer.)