State two practical applications of total internal reflection.

Describe the two conditions that must be met for total internal reflection to occur.

A light ray travels inside a triangular sapphire prism and is incident on the flat vertical boundary with the surrounding air at point $P$, as shown in the diagram.

Draw on this

The angle of incidence at $P$ is $41^{\circ }$. The critical angle of the sapphire prism is $34.4^{\circ }$. Describe the subsequent path of the light ray after it reaches point $P$.

State the equation linking critical angle ($c$) and refractive index ($n$) for a medium in air.

Calculate the refractive index of this sapphire prism. Give your answer to 3 significant figures.