Question

Four students A, B, C and D performed the experiment
to determine the focal length of a concave mirror by
obtaining the image of a distant tree on a screen. They
measured the distances between the screen and the
mirror as shown in the diagrams given below:
(A)
(C)
(D)
The correct way to measure accurate focal length of
the mirror is:​

Answer 1

Answer:

The student needs a parallel beam of light falling on the concave mirror so that after reflection the reflected rays meet at a point indicating the position of the focus. We know that the light rays which are coming from a distant object of smaller (comparatively) size are parallel to each other and parallel to the principal axis as well. Hence the student should choose a distant tree or an electric pole. The image hence formed will be the location of the focus.