MHT CET 2021 Physics Solved Paper

(b)The given situation is shown below

From the figure we can see the light from the LED will not emerge out of the water, if at the edge of the disc, the incidence angle is greater than critical angle.
‌ i.e., ‌‌i>C
‌ or ‌‌sin‌i>sin‌C
Now, if R is the radius of disc and λ is the depth of the LED, then
sin‌i=‌

R

√R2+h2

and sin‌C=‌

1

µ

From Eq (i), we have
‌

R

√R2+h2

>‌

1

µ

⇒R>‌

h

√µ−1