Convex mirrors are preferred as rear view mirror in vehicles as: 
* It forms an erect image of an object hence object becomes easily identified. 
* It forms a diminished image of object thus increases the field of vision.
Object that are far away from us are seen closer which helps us to take early decision while driving.
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Convex mirrors are preferred because they give an erect, though diminished image. Also they have a wider field of view as they are curved outward. Thus convex mirror enable the driver to view much larger area.
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