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:

Step-by-step solution

Since v+u=d v=d-u From lens formula aligned 1 f & = 1 v - 1 u 1 f & = 1 d-u - 1 -u & = d (d-u) u f & = d u-u^2 d ( i ) aligned Differentiating eq. (i) we have d f d u = 1 d (d-2 u) for maximum value of f₁= d f d u =0 aligned & 1 d (d-2 u)=0 d=24 & u= d 2 aligned putting these value in eq. (i) f= d 4