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

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 st line whose slope
`(1)/(f)=(b)/(c ) :. f = ( c)/(b)`.