Question

an object 50 cm tall in placed on the principal axis of convex lens it is 20 CM tall image in format on the screen placed a distance of 10 cm from the lens calculate calculate focal length of lens

Answer 1

combine 2 formulae here:
magnification= image height/ object height
magnification= q/p
thus q/p = hi/ho
since we first find object distance p,
p= q.ho/hi
=10*50/20
p=25

now for focal length
1/f=1/q+1/p
=1/10+1/25
=0.14
f=1/0.14
f= 7.14
I guess