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?

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Solution

Given: The distance between the object and the image is 3 m.

The maximum focal length of the convex lens to obtain real image is given as,

f max = d 4

Where, the maximum focal length of the convex lens is f max and the distance between the object and the image is d.

By substituting the given values in the above expression, we get

f max = 3 4 =0.75 m

Thus, the maximum possible focal length of the convex lens is 0.75 m.

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