13. An image is formed on a screen by a convergent lens. If the top half of the lens is then covered what will happen to the image?
(A) the image is dimmer but otherwise unchanged
(B) the image becomes half as big
(C) only the top half of the image is produced
(D) only the bottom half of the image is produced
(A) the image is dimmer but otherwise unchanged
(B) the image becomes half as big
(C) only the top half of the image is produced
(D) only the bottom half of the image is produced