A disc is placed on a surface of pond which has refractive index 5/3. A source of light is placed 4 m below the surface of liquid. The minimum radius of disc needed so that light is not coming out is
- 3 m
- 6 m
- 4 m
Answer
(2) 3 m
Solution
Light escapes only within a cone of semi-vertical angle C (the critical angle); the disc must cover the circle where this cone meets the surface.
[figure]
C = 1 = 35
C = 34
r = h C
= 4 34
= 3 m
Answer: (2)