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CBSE Class 12 · 2013 · Geometrical OpticsShort Answer

A convex lens of focal length

f_{1}

is kept in contact with a concave lens of focal length

f_{2}

. Find the focal length of the combination.

Solution

For thin lenses in contact the powers add. Taking the convex lens

+ f_{1}

and the concave lens

- f_{2}

:

\frac{1}{f} = \frac{1}{f _{1}} + \frac{1}{( - f _{2} )} = \frac{f _{2} - f _{1}}{f _{1} f _{2}}

f = \frac{f _{1} f _{2}}{f _{2} - f _{1}}

CBSE Class 12 Board · 2013 · Subjective

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