The image of a small electric bulb fixed on the wall
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?

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:

$f_{\max }=\frac{d}{4}$

$=\frac{3}{4}=0.75 \mathrm{~m}$

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

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