A student obtained a sharp image of a burning candle, placed at the farther end of a laboratory table, on a screen using a concave mirror. For getting better value of focal length of the mirror, the subject teacher suggested him for focussing a well illuminated distant object. What should the student do?

Let us analyze the position of the image in both scenarios:

1. Initial Scenario (Candle at the end of the table):

• The candle is at a finite distance ($u$) from the concave mirror.
• Since the object is at a finite distance beyond the focus, its real image is formed at a distance $v$ which is greater than the focal length ($v > f$).
• The screen is placed at this distance $v$ to capture the sharp image.

1. New Scenario (Distant Object):

• A well-illuminated distant object is at infinity ($u \approx \infty$).
• The parallel rays from this distant object converge exactly at the principal focus of the concave mirror ($v = f$).

Since $f < v$, the distance between the mirror and the screen must be decreased to obtain a sharp image of the distant object.

• To decrease this distance, the student should move the mirror slightly towards the screen (or move the screen slightly towards the mirror).
• Therefore, Option B is the correct action.

Why other options are incorrect:

• Option A: Moving the mirror away from the screen increases the distance between them, which would make the image highly blurred.
• Option C: Moving both towards the object does not systematically address the relative distance reduction needed between the mirror and the screen.
• Option D: Moving only the screen towards the newly selected object (which is far away) would increase the mirror-screen distance, which is incorrect.