I'm in 10th grade and this question came in my physics test. Nobody was able to answer this question correctly except my physics teacher who says that the answer is 2m. My answer is that there should be no limit on how small the focal length needs to be in this case.
For example, if a convex lens of focal length 2cm is used to form an image on the opposite wall, the wall clock that is 2m away from the lens can be treated to be at infinity with respect to the lens and if the focus of the lens is kept at the opposite wall, an image of the object at infinity should form on the wall. My argument is that no matter how small the focal length becomes, as long as it's above zero, an image should form on the opposite wall. Please try to solve the problem and post the explanation.
I asked my teacher and he told me that while solving the equation through the lens formula, he had taken the object position to be infinite and the image distance at 2 meter.
If there were a minimum limit on how small the focal length could be, what would it be?