Find the focal length of equivalent mirror if concave surface of a plano concave lens is silvered. Radius of curvature of concave surface is R and refractive index of material of prism is `mu`.

Explanation

To find the focal length of the equivalent mirror when the concave surface of a plano-concave lens is silvered, we can follow these steps:

### Step 1: Understand the System
We have a plano-concave lens where the concave surface is silvered. This means that the concave surface acts as a mirror. The plano-concave lens has a radius of curvature \( R \) for the concave surface and the other surface is plane (infinite radius of curvature).

### Step 2: Focal Length of the Plano-Concave Lens
The formula for the focal length \( f_L \) of a plano-concave lens is given by:
\[
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