A police inspector is chasing a thief who is running away in a car with a speed 3 m/s. The speed of police jeep is 12 m/s. Then the speed of image of police jeep as seen by thief in the rear view mirror when the police jeep is at a distance of 30 m is (value of focal length of the rear view mirror is 15 m)

Here police jeep is the object. Its velocity w.r.t. mirror is V₀ = 12–3 = 9m/s
[velocity of mirror = velocity of car or thief = 3 m/s]
velocity of image of jeep as seen by thief (V_I) = velocity of jeep w.r.t. mirror itself

${\mathrm{V}}_{\mathrm{I}}=-{\left(\frac{\mathrm{f}}{\mathrm{f}-\mathrm{u}}\right)}^2{\mathrm{V}}_0$

$=-{\left[\frac{+15}{+15-(-30)}\right]}^2\times 9$

$=-1 \mathrm{m}/\mathrm{s}$

here f is +ve as the rear view mirror in the vehicles is convex.