# 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.

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

$P_1 =\large\frac{1}{f_1 \;in\; meter}$
$\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.