Question

A total reflection right angled isosceles 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: (D)

${90}^o$

Total reflecting prisms are right angle isosceles prisms. They are based on the principle of total internal reflection. These prism are used to rotate light through $90degrees$ or $180degrees$ and are also used to obtain erect images of an inverted object.
When a ray of light incident normally on the surface of a right angled isosceles prism, it does not undergo refraction but strikes the surface $AC$ at an angle $45degrees$, which is greater than the critical angle of glass, that is $42degrees$. The ray undergoes total internal reflection and strikes face $BC$ normally and emerges out of it. The the ray is rotated through $90degrees$.