Question

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

• A. $\infty$
• B. 3m
• C. 6m
• D. 4m

Answer 1

Answer: (B)

3m

Answer 2

$\theta$ is the critical angle.
$\therefore \, \, \, \, \theta = sin ^{-1} (1/ \mu ) = sin ^{-1} (3/5)$
or $\, \, \, sin \theta = 3/5$
$\therefore \, \, \, tan \theta = 3/4 = r/4 \, \, or \, \, r=3 m$