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A lens of focal length 12cm forms an erect image three times the size of the object. What is the distance between the object and image? 
Asked by bhu.joshi54 | 14 Jun, 2022, 09:21: PM
answered-by-expert Expert Answer
If the image is magnified and erect means, the lens is of converging type.
 
Magnification m = v/u = 3 
 
where v is lens-to-image distance and u is lens-to-object distance.
 
hence we get v = 3u  ................. (1)
 
We have lens equation :- (1/v) - (1/u) = 1/f  ............ (2)
 
By applying cartesian sign convention, because object and image both are left side of lens,
 
we get from eqn.(1) and eqn.(2) as ,   begin mathsize 14px style negative fraction numerator 1 over denominator 3 u end fraction plus 1 over u space equals space 1 over 12 end style
By solving above equation, we get u = 8 cm
 
hence from eqn.(1)  v = 3×8 cm = 24 cm
 
Distance between image and object = ( 24 - 8 ) = 16 cm
Answered by Thiyagarajan K | 14 Jun, 2022, 10:53: PM

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