A camera has a single converging lens with a fixed focal length f. (a) How far should the lens be from the film (or in a present-day digital camera, the CCD chip) in order to focus an object that is infinitely far away (namely the incoming light rays are parallel with the principal axis of the system). (b) How far should the lens be from the film to focus an object at a distance of 2f in front of the lens

a) Due to the characteristic that a converging lens focuses light rays from infinity and parallel to its main axis. Therefore, the lens should be placed at a distance "f" from the film, in this way it will form the image of the object placed at infinity in said film.

b) Since the converging lens produces an image of an object placed at a distance of 2f, the lens must be placed at the same distance (2f), so that this object that is placed at a distance of 2f is focused.

Explanation:

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.