Question

An object 50 cm tall is placed on the principal axis of a convex lens it's 20 cm tall image is formed on the screen placed at a distance of 10 cm from the lens 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 

Answer 2

Answer: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 

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Explanation: