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Grades K–2 · Everyday Life

Why does a bicycle reflector shine back?

A bicycle reflector can look bright when a lamp shines toward it. The reflector is not a little lamp. It sends back some light that has arrived from somewhere else.

J29 visual explanationA separate lamp supplies light to an illustrative prismatic reflector. Tiny angled faces can redirect some light toward its source. Overall arrows are not a microscopic ray trace, brightness calculation or visibility guarantee.A reflector sends arriving light backPrismatic reflector · overall directions, not a ray traceLampLight: brightTiny angled facesArriving lightLight returns toward sourceSome light returns; the reflector makes no light.

In the diagramA reflector sends arriving light backPrismatic reflector · overall directions, not a ray traceLampLight: brightTiny angled facesArriving lightLight returns toward sourceSome light returns; the reflector makes no light.

Lamp light reaches the prismatic reflector, which redirects some light toward the lamp; the reflector does not make its own light.

Inside this kind of reflector are many tiny faces set at angles. Light can bounce from one face to another and head back toward the place it came from. This is called retroreflection.

An eye near the lamp can receive some of that returning light. With no incoming light, the reflector does not shine by itself. Its useful brightness also depends on where the lamp, reflector and viewer are.

About this explanation

Starting knowledge: Recognizes a lamp and can trace an outgoing and a returning arrow with adult reading support.

Sources

  • Smithsonian National Museum of American History — Stimson Reflector
  • National Institute of Standards and Technology — To the Moon and Back … in 2.5 Seconds
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