question_answer 18)

The image of a small electric bulb fixed on thewall of a room is to be obtained on the oppositewall 3 m away by means of a large convex lens. What is the maximum possible focal length ofthe lens required for the purpose ?  

Answer:

Using the lens formula,and ..(1) Taking the sign convention into consideration, we have                    ?(2) Using this value ofv in eqn. (1), or For to be maximum, or from eqn. (2), Using these values of u and v in eqn. (1), or Shortcut. For a real image, least distance between object and image should be fourtimes the focal length i.e. i.e.