Question

By drawing a neat diagram, explain how a totally reflecting prism can be used to turn rays of light through 90°.

Answer 1

1. Totally reflecting prisms are based on the principle of total internal reflection.
2. When a light ray traveling through these right-angled isosceles prisms strikes the glass-air interface at an angle greater than 42°, it gets totally internally reflected.
3. When a parallel beam of light strikes the face AB normally, it does not undergo any refraction and strikes the face AC at an angle of 45°.
4. But the critical angle for glass is 42°, hence total internal reflection takes place.
5. Therefore beam of light makes an angle of reflection of 45° and hence turns through 90° on the whole.
6. It strikes the face BC perpendicularly and emerges out.
7. So the face AC of the prism acts like a mirror.