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

+6.67D

−6.5D

−1.5D

+6.5D

A

−1.5D

B

+6.5D

C

−6.5D

D

+6.67D

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Solution

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Given,

f1=+40cm

f2=−25cm

Power, P=1f

The power of combination,

P=1f1+1f2

P=100+40cm+100−25cm

P=−1.5D

The correct option is C.

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