39. A convex lens and a concave lens, each having same focal length of 25 cm, are put in contact to form a combination of lenses. The power in diopters of the combination is ?
(a) 50
(b) ∞
(c) 0
(d) 25
Answer : (c) 0
\( f_1 = \frac{f_1}{1} + \frac{f_2}{1} \)
\( = \frac{25}{1} + \frac{-25}{1} \)
\( P = f_1 = 0 \)
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