Question

# A convex lens of power 5 D and a concave lens of power 7.5 D are placed in contact with each other. What is the: (a) Power of this combination of lenses? (b) Focal length of this combination of lenses ?

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Solution

## (a) Power of a convex lens is positive, and that of a concave lens is negative. Power of convex lens, P1=+5 D Power of concave lens, P2=−7.5 D Power of a combination of lens is equal to the sum of the individual powers. So, power of the combination will be: P=+5+(−7.5) P=−2.5 D (b) Focal length is equal to the inverse of power. So, the focal length of the combination is: f=1P f=1−2.5=−0.4 m

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