Question
Thu October 27, 2011 By:
 

an object and a screen are placed 80 cms apart. there are two positions at which a convex lens forms a real image on the screen.the magnification in the two cases being 3/2 and 2/3. calculate the focal length of the lens

Expert Reply
Wed November 09, 2011

For lens at L1, u=-x,

V=80-x and m1=-3/2

M=v/u=-3/2
or, 80- x / -x = -3/2

X=32cm.

U=-32cm and v=48cm substituting

Now, 1/f = 1/v - 1/u = 1/(48) + 1/(48)
or, f = 19.2 cm

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