Question

Two lenses of focal length \( f_1 = 10 \, \text{cm} \) and \( f_2 = -20 \, \text{cm} \) are kept as shown. The resultant power of combination will be

• A. \( -10 \, \text{D} \)
• B. \( 5 \, \text{D} \)
• C. \( 10 \, \text{D} \)

Hint:

The total power of two lenses in combination is the sum of the individual powers.

Answer 1

The Correct Option is B

Step 1: Power of Combination.
The power of a lens is given by: \[ P = \frac{1}{f} \] For two lenses in combination, the total power \( P_{\text{total}} \) is the sum of their individual powers: \[ P_{\text{total}} = P_1 + P_2 = \frac{1}{f_1} + \frac{1}{f_2} \] Substituting the given focal lengths: \[ P_{\text{total}} = \frac{1}{10} - \frac{1}{20} = 5 \, \text{D}. \] Step 2: Conclusion.
The correct answer is (B), \( 5 \, \text{D} \).