Question

to determine the focal length of concave mirror , a student focus a distant object using concave mirror, the best object can be
1 a distant tree
2) class room window
3) Sun

Answer 1

a distant tree. 

hope it helps!!

Answer 2

Hope this helps you...
(2) class room window. The focal length of the mirror can be calculated correctly only when we know the object distance and the image distance correctly for experiment. In case of a tree, electric pole, distant building, grills etc the object distance are not known correctly. Also in any of these the object distance is not infinite. So, the best option is class room window. The distance of the window of the mirror can be measured easily using a scale and a sharp image can be formed on a screen. Measuring the image distance we can easily calculate the focal length of the concave mirror.