A small illuminated bulb is at the bottom of a tank, containing a liquid of refractive index `mu` upto a height H. Find the expression for the diameter of an opaque disc, floating symmetrically on the liquid surface in order to cut- off the light from the bulb.

Explanation

To solve the problem of finding the expression for the diameter of an opaque disc floating symmetrically on the liquid surface in order to cut off the light from a bulb at the bottom of a tank, we can follow these steps:

### Step-by-Step Solution

1. **Understanding the Setup**:
- We have a small illuminated bulb at the bottom of a tank filled with a liquid of refractive index \( \mu \) up to a height \( H \).
- The light from the bulb will spread out in the form of a cone.

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