Retroreflectivity is the property of returning light along the same
path which it was sent, the
incedent path. This action is opposed to sending it off at an opposite angle, like a (flat)
mirror, or
scattering it in all directions, like a
white t-shirt.
Corner reflectors and
concave spherical mirrors are both retroreflective. One's
reflection from the first can just have a
magnifying mirror effect if the spherical nature isn't very pronounced, whereas the second will always look like an exactly reversed
mirror image to the observer.
Sylvanshine and heiligenschein are two common optical illusions caused by the retroreflectivity of droplets of water.