A student obtained a sharp image of the grills of a window on a screen using a concave mirror. His teacher remarked that for getting better results a well lit distance object (preferably the Sun) should be focused on the screen. What should be done for this purpose?

1. Move the screen and the mirror towards the object
2. Move the screen and the mirror away from the object
3. Move the screen slightly away from the mirror
4. Move the mirror slightly towards the screen

(d) Move the mirror slightly towards the screen

Explanation

 To get the sharp image of a distant object (or, the object at infinity), the mirror should be adjusted so that the image will form at the focus $F$ or at the focal distance $f$ of the mirror. Because we know that when an object is at infinity or at a considerable distance, then the image forms at the focus, $F$ of the mirror. The nature of the image is real, inverted, and highly diminished.

Thus, a sharp image can be obtained, only when the screen will be at the distance of focal length $f$ of the mirror. Therefore, the mirror should be slightly moved towards the screen.