Why are convex mirrors used instead of concave mirrors as rear view mirrors?
Convex mirrors always produce images that are virtual, upright and diminished.
According to the spherical mirror equation,
In the case of concave mirror, images of objects further from the focal point (
For convex mirrors, diminished images means that images appears smaller than the objects, which means that a larger field of vision could be obtained. This is also why a convex mirror is preferred over a planar one.
In summary, the driver needs a wide view of the road situation that are (relatively) far away, and convex mirrors are able to acheive that.