Question

Draw an isosceles right angled prism and show how a ray of light can be turned through an angle of 180°. Assume prism is of glass and critical angle for glass is 42°.

Answer 1

1. Ray PQ falls normally on face BC, so it enters the prism without bending.
2. It strikes face AB at R with an angle of incidence of 45°. This is greater than the critical angle of 42°, so the ray is totally internally reflected along RS.
3. It strikes face AC at S, again at 45°. This is also greater than 42°, so the ray is totally internally reflected again along ST.
4. Ray ST falls normally on face BC and leaves the prism without bending, along TU.
5. The emergent ray TU travels opposite to the incident ray PQ, so the ray has been turned through 180°.