Question

Class X Sciences
[papers from previous years]

To determine focal length of a concave mirror a student obtains
the image of a well lit distant object on a screen. To determine
the focal length of the given concave mirror he needs to measure
the distance between:

(A) mirror and the object
(B) mirror and the screen
(C) screen and the object
(D) screen and the object and also mirror and the screen

Answer 1

The distance between the pole of the mirror and the image of a distant object is called the focal length of the concave mirror.


1/f = 1/ u + 1/v        for a mirror.

If the object is distant,    u >> f  or  u >> v.    so  1/u  <<  1/v  or  1/f.
so we ignore it.

  1/f = 1/v    =>    f = v.

v is the distance between the mirror and the screen.
u = distance between the mirror and the object.
f = focal length of the mirror.