Question

For total internal reflection to occur, the minimum value for angle of refraction $r$ is equal to____.

• A. ${180}^{\circ }$
  
• B. ${90}^{\circ }$
• C. ${150}^{\circ }$
• D. ${360}^{\circ }$

Answer 1

The correct option is B ${90}^{\circ }$

For total internal reflection to occur, two conditions need to fulfilled:
1. Light should be travelling from denser to rarer medium.
2. Angle of incidence $i$ should be greater than critical angle.
Critical angle is angle of incidence when the angle of refraction is ${90}^{\circ }$. When angle of refraction is equal to ${90}^{\circ }$, the light passes along the interface. If we increase the incidence angle $i$ any further the light gets internally reflected. Therefore the minimum value of angle of refraction for light to suffer total internal reflection is ${90}^{\circ }$.