# 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

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by shyamSule

2015-04-28T19:49:44+05:30

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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.