Question

the image obtained with a convex lens is erect and its 4 times of initial length if the focal length is 20 cm find object and image distance

Answer 1

given,
I = 4O
or। I/O = 4/1 = -V/U
Hence, v= - 4u.........(1)
now from mirror formulae,
-1/20 = 1/4u + 1/u
= 5/4u
u = -25 cm
and hence from (1)
v= 100 cm.