Two thin lenses of focal lengths f1 and f2 are placed in contact with each other

Question

Two thin lenses of focal lengths

f_{1}

and

f_{2}

are placed in contact with each other. The focal length of the combination will be given by:

Options:

A . f1f2f1−f2

B . √f1f2

C . f1f2f1+f2

D . f1+f22

Answer: Option C
:
C
The power of a lens is defined as the reciprocal of its focal length (in metres). When there is a combination of two lenses in contact, the effective power of the combination is the sum of the individual powers.
Total Power,

P = P_{1}

P_{2}

\frac{1}{f} = \frac{1}{f_{1}} + \frac{1}{f_{2}}

f = \frac{f_{1} f_{2}}{f_{1} + f_{2}}

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