A luminous point object is moving along the principal axis of a concave mirror of focal length 12 cm towards it. When its distance from the mirror is 20 cm its velocity is 4 cm/s. the velocity of the image in cm/s at that instant is

Options

A. `6`, towards the mirror
B. `6`, away from the mirror
C. `9`, away from the mirror
D. `9`, towards the mirror

Answer

Converging; real

Explanation

`(1)/(f) = (1)/(v) + (1)/(u), (1)/9-12) = (1)/(v) + (1)/(-20) rArr (1)/(v) = (1)/(20) - (1)/(12)`
`rArr v = -30 cm`
Velocity of image
`(dV)/(dt) = -((V^(2))/(u^(2)))(du)/(dt) = -((-30)/(-20))^(2)4. = -9cm//sec rArr 9cm//sec` away form mirror.