Question

The focal length of a lens having power 4D is

• A. 5 cm
• B. 15 cm
• C. 10 cm
• D. 25 cm

Hint:

To find the focal length from the power, use the formula \( f = \frac{1}{P} \), where \( P \) is the power in diopters.

Answer 1

The Correct Option is B

The power \( P \) of a lens is related to its focal length \( f \) by the formula: \[ P = \frac{1}{f} \] Given that the power is 4D, we can find the focal length as: \[ f = \frac{1}{P} = \frac{1}{4} \, \text{m} = 0.25 \, \text{m} = 25 \, \text{cm} \] Thus, the correct answer is option (2).