A screen is placed at a distance 40cm away from an illuminated object. A converging lens is placed between the source and the screen and it is attempted to form the image of the source on the screen. If no position could be found, the focal length of the lens:

Explanation

The lens formula (1/f = 1/v - 1/u) relates the focal length (f) of the lens to the object distance (u) and the image distance (v). Here, the image distance is 40cm since the screen is placed at this distance. If no position could be found to form an image, it suggests that the sum of the object and image distances does not accommodate the focal length. For a focal length greater than 10cm, the object distance required to form an image on the screen exceeds the available space, making it impossible to form an image. Therefore, the focal length must be greater than 10cm for this situation to arise. Other options are incorrect because they suggest focal lengths that would allow the lens to form an image on the screen.