The power of a lens is –5 D. Its focal length is:

Correct option is B

Sol: The correct answer is (b) –20 cm

Given: 

the power of the lens is -5 D

Formula used:

​$$P = \frac{1}{f}$$

Where:

P is the power of the lens in diopters (D).

f is the focal length in meters (m)

Solution:

​$$-5=\frac{1}{f}$$

$$f=\frac{1}{-5}$$

= -02m

So, the focal length of the lens is -0.2 meters

Convert meters to centimeters -0.2 $$\times$$ 100 = -20cm​