A thin converging lens forms the real image of certain real object magnified m times.The magnification of real image become n when lens is moved nearer to object by distance x . find focal length of the lens

Options

A. `(xm)/(m-n)`
B. `(xmn)/(m-n)`
C. `(xmn)/(n-m)`
D. `(n-m)/(xn)`

Answer

C

Explanation

Here `m=f/(u-f)` or `u/f=((m+1)/m` in first case
In second case
`(u-x)/f=(n+1)/n` or `u/f-x/f=(n+1)/n` or `(m+1)/m -x/f=(n+1)/n)`