Question

If the power of a lens is +4 D, then the lens is a

• A. convex lens of focal length 25 cm
• B. concave lens of focal length 25 cm
• C. concave lens of focal length 40 cm
• D. convex lens of focal length 50 cm
• E. concave lens of focal length 20 cm

Hint:

A positive power indicates a convex lens and a negative power indicates a concave lens. The focal length is the reciprocal of the power.

Answer 1

The Correct Option is A

Step 1: The power \( P \) of a lens is related to its focal length \( f \) by the formula:

\[ P = \frac{1}{f}, \quad \text{where} \quad P \text{ is in diopters (D) and } f \text{ is in meters}. \]

Step 2: Given that \( P = +4 \, D \), we can find \( f \) as:

\[ f = \frac{1}{P} = \frac{1}{4} = 0.25 \, \text{m} = 25 \, \text{cm}. \]

Since the power is positive, the lens is a convex lens.

Thus, the lens is a convex lens with a focal length of 25 cm.