KVPY2019PhysicsWave Optics
Unpolarised red light is incident on the surface of a lake at incident angle θ R . An observer seeing the light reflected from the water surface through a polariser notices that on rotating the polariser, the intensity of light drops to zero at a certain orientation. The red light is replaced by unpolarised blue light. The observer sees the same effect with reflected blue light at incident angle θ B . Then,
Options
- Aθ B < θ R < 45 °
- Bθ B = θ R
- Cθ B > θ R > 45 °
- Dθ R > θ B > 45 °
Correct answer
C. θ B > θ R > 45 °
Step-by-step solution
As intensity of reflected light changes on rotating the polaroid this means reflected beam is polarised. By Brewster's law, n = tan θ n Blue > n Red ⇒ tan θ B > tan θ R ⇒ θ B > θ R Also, from geometry of figure, θ > 45 ° . So, we have θ B > θ R > 45 ° .