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Magnification by Thin Lenses

A convex and ...

Question

A convex and a concave lens separated by distance $d$ are then put in contact. The focal length of the combination

A

Decreases

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B

Increases

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C

Becomes

0

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D

Remains the same

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Solution

The correct option is A Decreases
$\frac{1}{f_{r}} = \frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{d}{f_{1} f_{2}}$
So when are brought in contact $d = 0$ , hence focal length $f_{r}$ will decrease .

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