Question 19: NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments
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?
Let the object be placed x m in front of lens the distance of image from the lens is (3 – x) m.
Condition for image to be obtained on the screen, i.e.m real image. 9 – 12ƒ > 0 or 9 > 12ƒ or f < 0.75 m. so, maximum focal length is 0.75 m.
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NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments