Question

Define the power of a lens and state its S.I. unit.

Hint:

Power of a lens increases when the focal length decreases: \[ P = \frac{1}{f} \] Shorter focal length → Greater power.

Answer 1

Concept: The power of a lens indicates the ability of the lens to converge or diverge light rays. A lens with a shorter focal length has greater power, while a lens with a longer focal length has less power.
Step 1:Definition of Power of a Lens} The power of a lens is defined as the reciprocal of its focal length measured in meters. \[ P = \frac{1}{f} \] where \(P\) = Power of the lens \(f\) = Focal length in meters
Step 2:S.I. Unit} The S.I. unit of power of a lens is the dioptre (D). \[ 1 \text{ dioptre} = 1 \, m^{-1} \] A convex lens has positive power, whereas a concave lens has negative power.