Question

The unit of power of a lens is

• A. centimeter
• B. meter
• C. dioptre
• D. Watt

Hint:

The power of a lens is measured in dioptres and is equal to the reciprocal of focal length in meters.

Answer 1

The Correct Option is C

Concept:
The power of a lens indicates how strongly the lens can converge or diverge light rays. It is defined as the reciprocal of the focal length of the lens when the focal length is measured in meters. Mathematically, \[ P = \frac{1}{f} \] where:
• \(P\) = Power of the lens
• \(f\) = Focal length in meters The SI unit of power of a lens is called the dioptre. \[ 1 \text{ dioptre} = 1D \]

Step 1: Understanding lens power.
If a lens has a smaller focal length, it bends light more strongly and therefore has greater power. For example, if: \[ f = 1m \] then, \[ P = \frac{1}{1} = 1D \] Hence, the unit used to express lens power is dioptre.

Step 2: Checking the remaining options carefully.

• Centimeter and meter are units of length, not lens power.
• Watt is the unit of electrical power.
• Only dioptre represents optical power. Therefore, the correct answer is: \[ \boxed{(3)\ \text{dioptre}} \]