Question

A total reflecting equilateral prism can be used to deviate a ray of light through:

• A. ${30}^o$
• B. ${60}^o$
• C. ${75}^o$
• D. ${90}^o$

Answer 1

Answer: (B)

${60}^o$

An equilateral prism with each angle of 60 degrees can be used to deviate a light ray through 60 degrees by total internal reflection. A ray of light PQ falls normally on the face AB, passes undeviated as QO into the prism and then strikes the face AC at an angle of incidence 60 degrees at the point O.
Since the angle of incidence is greater than the critical angle 42 degrees. The ray OS after total internal reflection falls normally on the face BC and passes undeviated through it. The emergent ray SR has turned 60 degrees towards the base from its initial direction.