IIT JEE , AIEEE Forum - Physics , Optics

diksha

In the displacement method , a convex lens is placed in between an object and a screen .If the magnification in the two positions be m1 and m2 and the displacement of the lens between th two positions is X , then foal length of the lens is

a) X / (m1 * m2)

b) X/ |m1 - m2|

c) X/ |m1 + m2|

d) X / (m1-m2)²

diksha

no one..plz help

rooney

Question is wrong.
Reasons
1> f = (D²-X²⁾/4D
So, to get the answer we need to replace D in terms of m₁ and m_2.D =x₁+ x₂m₁= (x₁/x₂)
m₂= (x₂/x₁₎Not possible to eliminate x₁and x₂ in 1st equation and get value of D in terms of the magnifications.

2> Assume a case where D= Distance b/w lens and screen = 4 f and lens is kept midway i.e. x₁ = x₂ = 2f
So, X = 0. m₁=m₂=1
Put these values in any of the 4 options. You get f = 0 which is false.

krishna.gopal

I agree with rooney but rooney there isa flaw in your reasoning in last part.
If X=0 and m1=m2=1 then we cannot eliminate second answer as there f is not equal to zero but it is 0/0 which can have finite value

rooney

Right Sir.

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