Define power of a lens and write its unit

Explanation

### Step-by-Step Solution

1. **Definition of Power of a Lens**:
- The power of a lens is defined as its ability to converge or diverge a beam of light rays that are incident upon it. For a convex lens, it converges parallel rays to a point (focus), while for a concave lens, it diverges parallel rays as if they are coming from a point (focus) behind the lens.

2. **Mathematical Expression**:
- The power \( P \) of a lens is mathematically expressed as the reciprocal of its focal length \( f \):
\[
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