Three students A, B and C focussed a distant building on a screen with help of a concave mirror. To determine focal length of the concave mirror they measured the distances as given below:
Student A : From mirror to the screen , Student B : From building to the screen
Student C : From building to the mirror
Who measured the focal length correctly :

Options

A. Only A
B. Only B
C. A and B
D. B and C

Answer

A

Explanation

Focal length of concave mirror is distance from mirror to the screen. Therefore, student-A is correct.