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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