A convex lens is placed between an object and a screen which are at a fixed distance apart for one position of the lens. The magnification of the image obtained on the screen is `m_(1)`. When the lens is moved by a distance d the magnification of the image obtained on the same screen `m_(2)`, Find the focal length of the lens.

Explanation

If D is the distance between the object and the screen d the separation of the two position of lens throwing two images on the screen then
`m _(1) = ((D + d))/((D- d))` and `m_(2) = ((D- d))/((D+d ))`
so `m_(1) - m_(2) = (d )/( f ) rArr f= ( d) /( m_(1)- m_(2))`