Question
Mon January 14, 2013 By: Rishav Singh

A convex lens of focal length 20 cm is placed at adistance of 24 cm from the screen.How far from the lens should an object be placed so as to form real image on the screen.

Expert Reply
Mon January 14, 2013

Given,

v(image distance on screen)= 24cm

f= 20cm

Using lens formula,

1/v-1/u=1/f

=> 1/v-1/f=1/u

=> 1/24-1/20 =1/u

=> 5-6/120= 1/u

=> -1/120 = 1/u

=> -120cm =u

Therefore Object distance must be 120 cm from the lens.

Now Magnification,

m=v/u

=> m= 24/-120

=> m= -1/5

=> m=0.2

Thefore image is real, inverted, diminished(smaller) 5 times and object in turn is placed 120 cms in front of the lens

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