Question

Two converging lenses of focal length 20 cm and 40 cm are placed in contact. The effective power of the combination is

• A. 5.25 D
• B. 7.25 D
• C. 2.75 D
• D. 7.5 D

Hint:

When two lenses are in contact, their powers simply add algebraically: \( P = P_1 + P_2 \)

Answer 1

The Correct Option is D

Power of a lens \( P = \frac{100}{f} \) (in cm) \ For focal lengths \( f_1 = 20 \) cm and \( f_2 = 40 \) cm:\ \[ P_1 = \frac{100}{20} = 5 \, D, \quad P_2 = \frac{100}{40} = 2.5 \, D \Rightarrow P_{\text{total}} = P_1 + P_2 = 5 + 2.5 = 7.5 \, D \]