<P> Some surfaces exhibit retroreflection . The structure of these surfaces is such that light is returned in the direction from which it came . </P> <P> When flying over clouds illuminated by sunlight the region seen around the aircraft's shadow will appear brighter, and a similar effect may be seen from dew on grass . This partial retro - reflection is created by the refractive properties of the curved droplet's surface and reflective properties at the backside of the droplet . </P> <P> Some animals' retinas act as retroreflectors (see tapetum lucidum for more detail), as this effectively improves the animals' night vision . Since the lenses of their eyes modify reciprocally the paths of the incoming and outgoing light the effect is that the eyes act as a strong retroreflector, sometimes seen at night when walking in wildlands with a flashlight . </P> <P> A simple retroreflector can be made by placing three ordinary mirrors mutually perpendicular to one another (a corner reflector). The image produced is the inverse of one produced by a single mirror . A surface can be made partially retroreflective by depositing a layer of tiny refractive spheres on it or by creating small pyramid like structures . In both cases internal reflection causes the light to be reflected back to where it originated . This is used to make traffic signs and automobile license plates reflect light mostly back in the direction from which it came . In this application perfect retroreflection is not desired, since the light would then be directed back into the headlights of an oncoming car rather than to the driver's eyes . </P>

How many ways can a surface reflect light