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Power and Thin Lenses in Contact

A convex lens...

Question

A convex lens is in contact with a concave lens. Focal length of both lenses are equal. What is the focal length of the combination ?

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Solution

$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{1}{f} - \frac{1}{f} = 0$
$∴ f = \frac{1}{0} = \infty$
or $f = \infty (i n f i n i t e)$

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