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Two thin lenses of focal lengths $f_1$ and $f_2$are in contact , the equivalent focal length is

$(a)\;f_1+f_2 \\ (b)\;f_1 \times f_2 \\ (c)\;\frac{f_1f_2}{f_1 +f_2} \\ (d)\;f_1 -f_2 $

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For combination of lenses
$\large\frac{1}{F} =\frac{1}{f_1}+\frac{1}{f_2}$
$\large\frac{1}{F} =\large\frac{f_1+f_2}{f_1+f_2}$
$F= \large\frac{f_1f_2}{f_1+f_2}$
Hence c is the correct answer.
answered Jan 22, 2014 by meena.p

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