Question

A ray of light $OP$ passes through a right angled prism as shown in the adjacent diagram. State the angles of incidence at the faces $AC$ and $BC$. Given critical angle is ${42}^o$.

• A. Angle of incidence at the face $AC={45}^o$ and angle of incidence at the face $BC={90}^o$
• B. Angle of incidence at the face $AC={90}^o$ and angle of incidence at the face $BC={45}^o$
• C. Angle of incidence at the face $AC={45}^o$ and angle of incidence at the face $BC=0^o$
• D. Angle of incidence at the face $AC=0^o$ and angle of incidence at the face $BC={45}^o$

Answer 1

Answer: (C)

Angle of incidence at the face $AC={45}^o$ and angle of incidence at the face $BC=0^o$

OP is incident normally on face AB and strikes AC undeviated. Since $\angle AQP={45}^o$ (total internal angle of a triangle is ${180}^o$), the angle of incidence will be ${45}^o$. Since, critical angle for glass prism is ${42}^o$ the ray undergoes total internal reflection with an angle of reflection of ${45}^o$. It then strikes BC normally and hence angle of incidence at that face is $0^o$.