A convex lens form a real image on a screen placed at a distance 60 cm from the object. When the lens is shifted towards the screen by 20 cm, another image of the object is formed on the screen. Focal length of the lens is ….. × 
cm.
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Ans. 4
Sol.
x + y = 60
y – x = 20
2y = 80 ⇒ y = 40, x = 20
= 
+ 
= 
+ 
f = 
cm
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