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Question
The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall 3 m 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
Distance between the object and the image, d = 3 m
Maximum focal length of the convex lens = `"f"_"max"`
For real images, the maximum focal length is given as:
`"f"_"max" = "d"/4`
= `3/4`
= 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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