1.

A convex lens of focal length f is placed some - where in between the object and a screen. The distance between object and screen is x. If magnification produced is m, the focal length of the lens is :

Answer»

`(mx)/((m+1)^(2))`
`(mx)/((m-1)^(2))`
`((mx+1)^(2))/(m)X`
`((m-1)^(2))/(m)x`

SOLUTION :(d) `(1)/(u) + (1)/(v) = (1)/(f)` (numerically)
`f = (uv)/(u+f)`
`f=(uv[(v+u)/(u^(2))])/((v+u)[(v+u)/(u^(2))])=((v+u)(v)/(u))/(((v+u)/(u)))`
`therefore "" f=((v+u)(v)/(u))/(((v)/(u)+1))`
`therefore v + u = x and (v)/(u) = m`
`therefore f = (x.m)/((m+1)^(2)).`


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