Question

# A convex lens of focal length 15 cm is kept in front of a concave lens of focal length 30 cm. What is the power of the two lens system?- 0.033 D + 0.033 D - 0.6 D + 0.6 D

Solution

## The correct option is B + 0.033 D Let P be the power of lens,  f be the focal length of lens f1 - focal length of a convex lens, P1 is the power corresponding to this lens f2 - focal length of a concave lens, P2 is the power corresponding to this lens Ptotal - Power of combination of lens  As per the sign conventions, f1=+15 cm f1=−30 cm (Ptotal) = P1 + P2              = 1f1 + 1f2   [As P=1f (in m)]              = 10.15 - 10.30              = 2−10.30              = 10.30 Ptotal = + 3.33 D

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