Question

An object 50cm in length is placed on principal axis of convex lens .it's20cm tall image is formed on the screen placed at distance of10cm from the lens calculate the focal length

Answer 1

Hey..... I think this can be ur answer!!
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v= 10cm
h' = - 20cm
h = 50cm

h' / h = v/u
-20 / 50 = 10/u
u = - 4cm

Now,
1/f = 1/v - 1/u
= 1/10 - 1/-4
=1/10 + 1/4
=4 + 10/40
=14/40
f = 40/14
=2.85cm