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 ?

Answer 1

Distance between the object and the image, d=3m
Maximum focal length of the convex lens= `f_(max)`
For real images, the maximum focal length is given as:
`f_("max") = d/4`
`=3/4=0.75m`
hence, for the required purpose,the maximum possible focal length of the convex lens is 0.75m.