The graph shows how the magnification `m` produced by a convex thin lens varies with image distance `v`. What was the focal length of the used ?

Options

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

Answer

D

Explanation

For a lens `m=(f-v)/(f)=-(1)/(f)v+1`
Comparing it with `y=mx+c`
Slope `=m=-(1)/(f)`
From graph, slpe of the line `=(b)/(c)`
Hence `-(1)/(f)=(b)/(c)implies|f|=(c)/(b)`