Ncert Exercises & Solutions

9.19:
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?

Hint: The maximum distance between the object and the image is when the object is placed at C.

Step: Find the maximum focal length of the lens.
Distance between the object and the image, d = 3m
The focal length of the convex lens is maximum when the object is placed on its center of curvature. In that case, the image will also be formed at the center of curvature.
The distance between the object and the image $= 4 f_{\max}$

$\Rightarrow f_{\max} = \frac{d}{4} = \frac{3}{4} = 0.75 m$

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