Question

If f (in cm) is the focal length of a lens in centimetres, the expression for the power of lens (P) is:

Options

• $$\text{P} = \frac{1 \ \text{cm}}{f \text{ in cm}}$$

• $$\text{P} = \frac{100 \ \text{cm}}{f \text{ in (cm)}}$$

• f in cm × 100 cm

• none of the these.

Answer 1

$$\text{P} = \frac{100 \ \text{cm}}{f \text{ in (cm)}}$$

Explanation:

Power is the reciprocal of focal length in metres.

If the focal length is given in centimetres, $$\text{P} = \frac{100}{f\text{ (in cm)}}$$.