• # question_answer 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 convex lens. What is the maximum focal length of the lens for this purpose? A) 0.5 m             B) 0.75 mC)      1.0 m             D)      1.5 m

To obtain a real image on a screen, the minimum distance $({{X}_{\min }})$ between the object and the screen is 4f, where f is the focal length of the lens. Conversely, for a fixed distance between the object and the screen, the convex lens should have a maximum focal length of ${{f}_{\max }}=\frac{{{X}_{\min }}}{4}=\frac{3}{4}=0.75\,m.$ Hence, the correction option is (b).