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A convex lens of power +6 dioptre is placed in contact with a concave lens of power 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, 50cm
Let the focal length of the convex lens be f1 and that of concave lens be f2.

Given : power of convex lens , P1=+6 dioptre

power of concave lens , P2=4 dioptre

Using power of lens=1focal length(f) where f is in metre

Let focal length of the combination be F

now 1F=1f1+1f2

1F=6+(4)

thus F=50cm

As F comes out to be positive, hence the combination of the two lenses is a convex lens.

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