Rear view mirrors are used in vehicles to get the images  of the approaching objects clearly.the field of view of the convex mirror is more,eventhough they appear in small in size

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A convex mirror shows small images of vehicles that follow behind the vehicle.  So it is easy to observe the type of the vehicle and its speed.
    A concave mirror does not form the virtual images until the vehicle comes nearer with in its focus.

Using a lens, virtual image formed will be of less intensity, as some of the light passes through.

So only a convex mirror is suitable.