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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
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B
10 D
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C
1.67 D
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D
+1.67 D
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Solution

The correct option is C 1.67 D
Given:
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+120/100
P=1.67 D

Physics

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