question_answer

Refractive index of water is 5/3. A light source is placed in water at a depth of 4 m. Then what must be the minimum radius of disc placed on water surface so that the light of source can be stopped? [AIPMT 2001]

A)        3 m                 

B)       4 m

C)              5 m     

D)                   \[\infty \]

Correct Answer: A

Solution :

Key Idea: The light from the source will not emerge out of water if angle of incidence is greater than critical angle.

As shown in figure,

Therefore, minimum radius R corresponds to.

In

or