Cladding

The question on the January exam about curved bit of glass with 3 rays going through (P,Q,R) raised some interesting ideas about cladding. Cladding is a layer of transparent material wrapped around the outside of the central fibre (the core). The purpose of the cladding is to stop the central glass core getting scratched. Scratched cores leak light more easily because it changes the angle of the outside wall of the glass and thus changes the angle of incidence.

Cladding makes it more difficult for the light to stay in the glass:

We will be using this equation. But note that at the critical angle, the second angle is zero.When there is no cladding, there is air around with refractive index of n = 1. So With the cladding, we get this equation:

Simple calculation will show that the critical angle becomes bigger. There are now fewer angles at which the light can hit the glass for TIR, so it is much easier for light to escape into the cladding.

Finally, be careful how you define critical angle.

• The angle of incidence for which the angle of refraction is 90 degrees.
• The minimum angle of incidence for which you get total internal reflection.