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A convex lens of focal length 40 cm is kept in contact with a concave lens of focal length 25 cm. What is power of this combined lens system.

$(a)\;+6.50\;D \\ (b)\;-6.50\;D \\ (c)\;+1.5\;D \\ (d)\;-1.5 \;D $

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$P_1 =\large\frac{1}{f_1 \;in\; meter}$
$\qquad= \large\frac{100}{40}$$=2.5 D$
$P_2 =\large\frac{-100}{25}$$=-4D$
$P=P_1+P_2$
$\quad = P_1+P_2=-1.5 D$
Hence d is the correct answer.
answered Jan 17, 2014 by meena.p
 

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