Question

Define Power of a Lens and state its S.I. unit.

Hint:

Power of a lens is the reciprocal of its focal length: \(P = \frac{1}{f}\). The S.I. unit of power is {Dioptre (D)}, where \(1D = 1\,m^{-1}\).

Answer 1

Concept: The ability of a lens to bend or converge/diverge light rays is known as the power of the lens. It indicates how strongly a lens can converge or diverge light. Power of a lens depends on its focal length; a lens with a smaller focal length has greater power.
Step 1:Definition of Power of a Lens.
The power of a lens is defined as the reciprocal of its focal length (in metres). Mathematically, \[ P = \frac{1}{f} \] where,

• \(P\) = Power of the lens
• \(f\) = Focal length of the lens (in metres)

If the focal length is small, the lens bends light more strongly and therefore has greater power.
Step 2:S.I. Unit of Power of a Lens.
The S.I. unit of power of a lens is Dioptre (D). \[ 1 \text{ Dioptre} = 1 \, \text{m}^{-1} \] Thus, a lens having a focal length of \(1\) metre has a power of \(1\) dioptre.

• Convex lenses have positive power.
• Concave lenses have negative power.