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

(a) From, `1/v-1/(-u)=1/(+f)`
`v=(uf)/(u-f)orv/u=m=f/(u-f)`
`rArr1/m=(1/f)u-1`
Cpmparing this with equation of straight line, 1/f is the slope of line, which is b/c.