Question

# A convex lens of focal length 30 cm is placed in contact with a concave lens of focal length 20 cm. What is the power of the combination of lens?

A
0.0167 D
B
10 D
C
1.67 D
D
+1.67 D

Solution

## The correct option is C −1.67 DGiven: Focal length of convex lens, f1=30 cm Focal length of concave lens, f2=−20 cm (Using new cartesian sign conventions, focal length of convex lens is positive and that of concave lens is negative) Power of a lens in dioptre is reciprocal of its focal length in metre. Power of convex lens, P1=1f1 Power of concave lens, P2=1f2 Power of combination of lenses in contact is the sum of individual powers. Hence, power of the effective lens is: P=P1+P2 P=1f1+1f2 P=130/100+1−20/100 P=−1.67 D Physics

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