Question

Power of a lens is -2.5 D. The focal length and nature of the lens is:

• A. 40 cm; Concave
• B. 40 cm; Convex
• C. 4 cm; Concave
• D. 4 cm; Convex

Hint:

For a lens: - Power \( P \) is positive for a convex lens and negative for a concave lens. - Focal length \( f \) is negative for a concave lens and positive for a convex lens.

Answer 1

The Correct Option is A

The power \( P \) of a lens is related to its focal length \( f \) by the formula: \[ P = \frac{1}{f} \] where \( f \) is in meters. Given that the power is \( -2.5 \, \text{D} \), we can find the focal length as: \[ f = \frac{1}{P} = \frac{1}{-2.5} = -0.4 \, \text{m} = -40 \, \text{cm}. \] Since the focal length is negative, the lens is concave.