Question

The same size images are formed by a convex lens when the object is placed at $20\, cm$ or at $10\, cm$ from the lens. The focal length of convex lens is ___$cm$.

Answer 1

$m =\frac{ f }{ u + f }$
$+ m =\frac{ f }{-10+ f } \,\,\,\, \cdots (1)$
$- m =\frac{ f }{-20+ f } \ldots (2)$
(1) $/(2)$
$-1=\frac{ f -20}{ f -10}$
$10- f = f -20$
$30=2 f$
$f =15 cm$