Since v+u=d⇒v=d−u From lens formula f1⇒f1⇒f=v1−u1=d−u1−−u1=(d−u)ud=ddu−u2…(i) Differentiating eq. (i) we have dudf=d1(d−2u) for maximum value of f1=dudf=0 ⇒d1(d−2u)=0⇒d=24⇒u=2d putting these value in eq. (i) f=4d
A bulb is located on a wall. Its image is to be obtained on a parallel with the help of a convex lens. If the distance between parallel walls is ' d ' then the required focal length of the lens placed in between the walls is:
Held on 30 Apr 2002 · Verified 9 Jul 2026.
only 4d
only 2d
more than 4d but less than 2d
less than or equal to 4d
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