An isotropic point source is placed at a depth h below the water surface. A floating opaque disc is placed on the surface of water so that the source is not visible from the surface. What is the minimum radius of the disc? Take refractive index of `"water"=mu`.

Explanation

To solve the problem of finding the minimum radius of the opaque disc placed on the water surface such that an isotropic point source at a depth \( h \) below the surface is not visible, we can follow these steps:

### Step 1: Understanding the Critical Angle
The critical angle \( \theta_c \) is the angle of incidence above which total internal reflection occurs. For light traveling from water to air, the critical angle can be determined using Snell's law:

\[ \sin \theta_c = \frac{1}{\mu} \]
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