Question

To determine the focal length of a concave mirror, a student focuses a distant object using the concave mirror. The best object can be:

(a) Sun (b) A distant tree (c) Classroom window (d) All of these

For XIII class CBSE.

Answer 1

d ) all these ok. The distance of the window should be considerably large compared
     to  the focal length of the mirror.
   Other wise: a distant tree and sun should be used.

Answer 2

All three can be used. But the sun rays can help determine more precisely.