1.

In the displacement method, a convex lens is placed in between an obect and a screen. If the magnification in the two positions are m_(1) and m_(2) and the displacement of the lens between two positions is x, then the focal length of the lens is:

Answer»

`(X)/(m_(1)+m_(2))`
`(x)/(m_(1)-m_(2))`
`(x)/((m_(1)+m_(2))^(2)`
`(x)/((m_(1)-m_(2))^(2)`

SOLUTION :(B) `m_(1) = (v)/(u) m_(2) = (u)/(v)`
`(1)/(f) = (u-v)/(UV)`
`(1)/(f)=((u-v)(v+u))/(uv(v+u))`
`f = (uv(v+u))/(u^(2)-v^(2))`
`therefore "" f=(v+u)/(u^(2)/(uv)-(v^(2))/(uv))=(u+v)/((u)/(v)-(v)/(u))`
`therefore "" f = (x)/(m_(2) - m_(1))`.


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