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

C

Explanation

As `(dV)/(dt)=-(V^(2))/(u^(2))(du)/(dt)`
Now, `1/f=1/u+1/v`
`=1/(-12)=(-1)/20+1/V`
`rArr v=-30 cm//s`
`:. (dv)/(dt)=-9 cm//s`
i.e., 9 away, from the mirror.