Question

A converging lens of focal length 5 cm is placed at a distance of 20
cm from a screen. How far from the lens should an object be placed so
as to form its real image on the screen ?

Answer 1

The object should be placed 20/cm from the lens so as to form its real image on the screen

Mirror equation

The mirror equation is given according to the expression below;

1/f = 1/u + 1/v

where

f is the focal length

u is the object distance

v is the image distance

Given the following parameters

f = 5cm

v = 20cm

Required

object distance u

Substitute

1/5 = 1/u +1/20

1/u = 1/5-1/20
1/u = 3/20

u = 20/3 cm

Hence the object should be placed 20/cm from the lens so as to form its real image on the screen

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