A retroreflector is an optical device that returns an oncoming beam of light back to its source. It can be as simple as a tiny glass sphere or a type of prism formed from glass or plastic. (For example, you can build a Glass Bead Projection Screen.)
While ordinary flat mirrors also reflect light, the light isn’t reflected back to the source but off to the side at the same angle the beam arrived. Only if the light beam is perfectly perpendicular to the surface of a flat mirror does it act like a retroreflector.
Retroreflectors are so much a part of everyday life that typically they don’t attract much attention. But they attract plenty of attention while driving at night, when they seem to be almost everywhere. They’re incorporated into the taillights of vehicles, safety barriers, traffic signs, and the painted stripes that separate lanes of traffic.
Recently while waiting at a traffic light on a dark night, I noticed seven brightly glowing traffic signs coated with retroreflective paint. These signs were illuminated by the headlights of my pickup, and they each reflected the oncoming light back toward me.
A nearly full moon was overhead, and it has retroreflective properties, too. That’s because the Apollo 11, 14, and 15 astronauts left arrays of precision retroreflectors on the moon. For 40 years, various observatories have pointed powerful laser beams at the moon and detected the light returned by those retroreflectors to accurately measure the distance between Earth and the moon.
Natural Retroreflectors
Retroreflection was observed long before artificial retroreflectors were invented. That’s because the eyes of many animals double as retroreflectors that glow at night when viewed from the same direction as a fire or lamp. When I was a Boy Scout I observed the eyes of alligators in a Florida lake, glowing bright orange in the light of a campfire. Years later I used a flashlight to find caimans in Brazil’s remote Cristalino River. Drivers notice the eyes of animals at night glowing brightly in the headlights of their cars.
The retroreflection exhibited by animal eyes is called eyeshine. A bright, head-mounted light provides the best way to observe eyeshine. Retroreflection from an eye occurs when some of the light focused onto the retina by the lens is reflected back through the lens, where it is refocused into a narrow beam that travels back to the source of light. While this occurs whenever the eye is opened, we notice it only at night when a bright light source held close to our eyes is pointed at the eyes of an animal. The retina alone is not a particularly good reflector, but in many vertebrate animals it is backed by a highly reflective layer of tissue called the tapetum lucidum.
The red glowing eyes of people in flash photographs is known as red eye. Human eyes lack a reflective tapetum, so human eyeshine is not nearly as bright as that of animals. The best way to eliminate red eye is to move the flash away from the camera’s lens. The light will be reflected back toward the flash, and most of it will miss the lens.
Manufactured Retroreflectors
For decades highway signs and painted stripes on roadways have been coated with tiny retroreflectors. The earliest and still the most common are clear glass beads, which can be sprayed or poured over freshly painted road stripes and highway signs. Various kinds of reflective sheets are also used to coat warning barriers and signs. Some employ a layer of glass beads, while others use sheets embedded with tiny plastic corner prisms called microprisms.
Microprisms are tiny versions of much larger retroreflectors cut from a corner of a solid cube of glass or silica. These reflectors are called corner cubes or cube corners, and they provide the best performance. The retroreflector arrays on the moon are silica corner cubes. Retroreflectors can also be made by mounting 3 mirrors to form an open corner of a cube.