A point object on the principal axis at a distance `15cm` in front of a concave mirror of radius of curvature `20cm` has velocity `2mm//s` perpendicular to the principal axis. The magnitude of velocuty of image at that instant will be

Options

A. `2 mm//s`
B. `4 mm//s`
C. `8 mm/s`
D. `16 mm//s`

Answer

B

Explanation

`(1)/(f)=(1)/(v)+(1)/(u) implies (1)/(-10)=(1)/(v)+(1)/(-15) implies(1)/(v)=(2-3)/(30) , v=-30cm` .
`m=(I)/(O)=-(v)/(u)=-(-30)/(-15)=-2`
(dI)/(dt)=2(d theta)/(dt)=4 mm//sec` .