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Numerical

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.

Concept: Refraction at Spherical Surfaces and Lenses - Refraction by a Lens

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