The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall 3m away by means of a large convex lens. What is the maximum possible focal length of the lens required for the purpose?

Given:


Distance between the object and the image, d = 3 m



Maximum focal length of the convex lens = fmax
For real images, the maximum focal length is given by,


fmax = d/4 …(1)


Where,


fmax = maximum focal length


d = distance between object and image


By putting the values in equation (1) , we get,


fmax = 0.75 m


Hence, for the required purpose, the maximum possible focal length of the convex lens is 0.75 m.


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