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 distant object (preferably the sun) should be focussed on the screen. What should be done for this purpose?

To obtain a sharp image of a well-lit distant object (like the Sun) instead of nearby window grills, the student needs to move the mirror slightly towards the screen (or move the screen closer to the mirror).

Initially, the window grills are at a finite distance (beyond $C$). The image of an object placed beyond $C$ is formed between $F$ and $C$.

When focusing a much more distant object like the Sun (which is at infinity), the image is formed exactly at the principal focus $F$. Since the focus $F$ is closer to the mirror than the region between $F$ and $C$, the distance between the mirror and the screen must be decreased. Therefore, the mirror should be moved slightly towards the screen.