A student obtains a sharp image of the distant window (W) of the school laboratory on the screen (S) using the given concave mirror (M) to determine its focal length. Which of the following distances should he measure to get the focal length of the mirror?
(A) MW (B) MS (C) SW (D) MW- MS"
(B) MS
Explanation
The focal length, $(f)$ of a concave mirror is the distance between its pole $(P)$ and the principal focus $(F)$. So, to get the focal length of the mirror, the student should measure the distance between mirror and screen because here the object (window) is at distant, which is taken as at infinity, and when an object is at infinity the image is formed at focus i.e. between screen and mirror. So, the distance between the screen and the mirror should be measured.
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