Question

Show with the help of a diagram how a total reflecting prism can be used to turn a ray of light through ${90}^{\circ }$. Name one instrument in which such a prism is used.

Answer 1

As shown in the diagram, a beam of light is incident on face AB of the prism normally so it passes undeviated and strikes the face AC where it makes an angle of ${45}^{\circ }$ with the normal to AC. Because here the incident angle is more than the critical angle so rays suffer total internal reflection and reflect at an angle of ${45}^{\circ }$. The beam then strikes face BC, where it is incident normally and so passes undeviated. As a result, the incident beam gets deviated through ${90}^{\circ }$.
Such a prism is used in the periscope.