Question

Write two uses of internal reflection?

Answer 1

A2A: Internal reflection is when a wave hits a boundary and stays inside the medium, external is the opposite. 

The main use of internal reflections is in the design of Waveguides. Because different frequencies reflect differently you can use this property as a filter. Fiber optic data links are based on this by the way, external reflection being a source of lost signal, and a bad thing. 

External reflection is used in designing camera lenses and other focusing systems like telescopes. Here you want light to leak out of the medium (for example the glass of a lens) because the bending that happens is exactly the focusing you want, to zoom in or pan out. In this case internal reflection is a bad thing because it is now a signal loss.

In EE this sort of thing happens alot, in one application a physical feature is noise, in the next application it is the signal.