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.



The correct value for focal length will be obtained by the student:
(1) A
(2) B
(3) C
(4) D

Answer 1

A concave mirror converges all the rays coming from infinity on its focus, the distance of the pole of the mirror from the focus is called the focal length of the mirror.

If students obtain the image of a distant tree on a screen, then the distance of the screen fro the pole of the mirror would give the correct value of the focal length of the mirror. So only student C will get the right value because others are not measuring the distance of the screen from the pole of the mirror properly.

Hence the correct option is Option 3.