Question

1.The image of a small electric bulb fixed in a wall on the room is be obtained on the opposite wall 3 m away by means a large convex lens what is the maximum opposite focal length of lens required for the image form.?

Answer 1

Solution :
For a real image on the wall, the minimum distance between the object and image should be 4f.
∴4f=3m
=> f=34m
=0.75m

HERE IS YOUR ANSWER
HOPE YOU UNDERSTAND

Answer 2

Solution :

For a real image on the wall, the minimum distance between the object and image should be 4f.

∴4f=3m∴4f=3m

=> f=34f=34mm

=0.75m