''Object in the mirror are closer than they appear'' in the side view mirrors of vehicles. what feature of the mirror requires this warning.
For the passenger side mirror, a planar mirror would not do, since it is further away from your eye, it would be very limited in the range of vision it provides. So, we need either a huge mirror (not a very good or elegant alternative) or a mirror that can 'compress' images to that smaller size. The latter has been chosen. The side view mirror is convex (curves outward) because a flat mirror in that position would not provide the range of visibility required for a driver looking from the side trying to look behind them. The curved mirror provides a "wide angle" which pulls more of the rear into the image. But by pulling more in, it needs to compress the image, and that means that what you're seeing is smaller than it would otherwise be. Since it's smaller, our brain interprets that as farther away.