Question

In order to determine the focal length of a concave mirror, a student obtained the sharp image of a distant tree on a screen. In order to obtain a sharp image of a window grill of the laboratory on the same screen, in which direction should the screen be moved?
(1) very close to the mirror
(2) very far away from the mirror
(3) slightly nearer to the mirror
(4) slightly farther away from the mirror

Answer 1

The concave mirror converges all the rays coming from the distant tree on its focus and forms a sharp image on the screen placed at the focal point of the mirror. So, in order to get the image of a grill of the window, which is a nearby object, the student needs to move the screen away from the mirror, as for any nearby object concave mirror always forms a real image beyond the focus. So, the screen should be moved away from the focal point of the mirror.

Hence, the correct answer is option 4.