Question

Three students A, B and C focussed a distant building on a screen with the 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 ;
(a) Only A
(b) Only B
(c) A and B
(d) B and C

Answer 1

A concave mirror always forms the image of a distant object at its focus.
 
In the given case the distance between the mirror and the screen is equal to the focal length of the mirror. Thus, the student A measures the focal length correctly. 

Hence, the correct answer is Only A