Question

# A convex lens of power +6 dioptre is placed in contact with a concave lens of power âˆ’4 dioptres. What will be the nature and focal length of this combination?

A
Concave, 25cm
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B
Convex, 50cm
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C
Concave, 20cm
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D
Convex, 100cm
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Solution

## The correct option is B Convex, 50cmLet the focal length of the convex lens be f1 and that of concave lens be f2.Given : power of convex lens , P1=+6 dioptrepower of concave lens , P2=−4 dioptreUsing power of lens=1focal length(f) where f is in metreLet focal length of the combination be Fnow 1F=1f1+1f21F=6+(−4) thus F=50cmAs F comes out to be positive, hence the combination of the two lenses is a convex lens.

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