A convex mirror is preferred over a plane mirror because while driving, the driver needs to get a large view. Convex mirror show a very large view because they collect a large amount of light in a small space to form an upright and diminished image. That is why in rear view mirrors there is written that the objects in the mirror are closer than they appear.
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Convex mirrors are used as rear -view [wing] mirrors in vehicles . these mirrors are fitted on the sides of the vehicles ,enabling the driver to see traffic behind him/her to facilitate safe driving .convex mirrors are preferred because they always give an erect,though diminished ,image. also they have a wider field of view much larger outwards .thus,convex mirrors enable the driver to view much larger are than would be possible with a plane mirror.