The graph in Fig. shows how the inverse of magnification `1//m` produced by a convex thin lens varies with object distance `u`. What was the focal length of the lens used ?
.

Options

A. `(b)/(c )`
B. `/(ca)`
C. `(bc)/(a)`
D. `( c)/(a)`

Answer

D

Explanation

From `(1)/(v) - (1)/(u) = (1)/(f)`
`(u)/(v) - 1 = (u)/(f)`
`(1)/(m) - 1 = (u)/(f)`
or `(1)/(m) = ((1)/(f))u + 1`
which is the equation of a slit whose slope
`(1)/(f) = (b)/( c) :. F = ( c)/(b)`