Q: A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is

(A) – 1.5 D

(B) – 6.5 D

(C) + 6.5 D

(D) + 6.67 D

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Ans: (A)

Sol: $\large \frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} $

$\large \frac{1}{F} = \frac{1}{0.40} + \frac{1}{-0.25} $

$\large P = \frac{1}{F} = -1.5 D$