A small mirror of area A and mass m is suspended in a vertical plane by a weightless string. A beam of light of intensity I falls normally on the mirror and the string is deflected from the vertical by a very small angle `theta` Assuming the mirror to be perfectly reflecting, obtain an expression for `theta`

Explanation

To derive the expression for the angle \(\theta\) at which the mirror is deflected when a beam of light falls on it, we can follow these steps:

### Step 1: Understand the Forces Acting on the Mirror
When the beam of light strikes the mirror, it exerts a force on the mirror due to the change in momentum of the light. The weight of the mirror acts vertically downward, and the tension in the string acts along the string. The angle \(\theta\) is the small angle by which the string is deflected from the vertical.

### Step 2: Identify the Components of Forces
The tension \(T\) in the string can be resolved into two components:
- \(T \cos \theta\) acts vertically upward.
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