The power of a lens is defined as:

The correct answer is: The reciprocal of its focal length in meters

Understanding the Power of a Lens:

The power of a lens measures its ability to converge or diverge light rays falling on it. A lens with a shorter focal length bends light rays more sharply, thereby having greater converging or diverging power.

Mathematically, the power of a lens ($P$) is defined as the reciprocal of its focal length ($f$) expressed in meters.

$P=\frac{1}{f\text{(in meters)}}$

Key Points:

1. Unit of Power: The SI unit of power of a lens is the dioptre (D), where $1\text{D}=1{\text{m}}^{-1}$.
2. Convex Lens: Since the focal length of a convex lens is positive, its power is also positive.
3. Concave Lens: Since the focal length of a concave lens is negative, its power is also negative.