Question

The unit of power of a lens is

• A. metre
• B. metre $\times$ second
• C. per metre
• D. metre/second

Hint:

The power of a lens is measured in diopters (D), and its unit is per metre (m$^{-1}$), which is the reciprocal of the focal length in meters.

Answer 1

The Correct Option is C

Step 1: Understanding the Power of a Lens
The power of a lens is defined as the reciprocal of the focal length. It is given by the formula: \[ P = \frac{1}{f} \] where \( P \) is the power in diopters (D) and \( f \) is the focal length in meters.

Step 2: Unit of Power
Since the power is the reciprocal of the focal length, the unit of power is given as per metre (D). The unit of focal length is meter, so the power has the unit of per metre (D).
Thus, the correct answer is option (C).