# a concave lens is in contact with convex lens.The magnitude of the ratio of their powers is 2/3.Their equivalent focal length is 30cm.What are their individual focal lengths(in cm).

a)-75cm,+50cm
b)-15cm,10cm
c)+75cm,-50cm
d)75cm,50cm

Eshan
3 years ago
Dear student,

P=1/f
$\dpi{80} \dpi{80} P_1/P_2=-2/3$
$\dpi{80} \implies f_1=-\dfrac{3}{2}f_2$
Also$\dpi{80} P=P_1+P_2$
$\dpi{80} \dfrac{1}{30}=\dfrac{1}{f_1}+\dfrac{1}{f_2}=-\dfrac{2}{3f_2}+\dfrac{1}{f_2}$
Hence find$\dpi{80} f_1,f_2$