An object kept on the principal axis and infront of a spherical mirror, is moved along the axis itself. Its lateral magnification m is measured, and plotted versus object distance `|u|` for a range of u, as shown in figure. The magnification of the object when it is placed at a distance 20cm in front of the mirror is

Options

A. `-1`
B. 1
C. 8
D. 20

Answer

A

Explanation

When `m=2,u=-5cm`
`2=-(v)/(-5)rArr v=10cm`
`(1)/(f)=(1)/(10)+(1)/(-5)rArrf=-10cm`
Now when `u=-20cm`
`-(1)/(10)=(1)/(v)+(1)/(-5)rArrv=-20cm,m=-(-20)/(-20)=-1`