Re: How do reflectors (bike) work

The best bike reflectors work using "retro-reflectors".  An ordinary
white surface reflects the light that hits it back in all directions.
An ordinary mirror reflects it in one direction, with the angle
of reflection equal to the angle of incidence.  That is if you
have a horizontal mirror and shine a light down into it from the
left, say, then the reflected beam goes up and to the right.  A
retro-reflector does a neat trick: it always reflects the light back
along the path it came in on.  So in the same situation, the reflected
beam would go up and to the left, towards the light.  This makes for
a very good bike reflector, because it reflects the headlights of
a car back towards the car, where the driver can see the reflection.
Road signs are often painted with retro-reflecting paint for the same
reason.  When you are in a car at nigt, road signs often look as
if they are lighted, when in fact it is just the retro-reflection of
you car's own headlights that is providing the light.

Retroreflectors are often made of many small spheres.  (Very small
in the case of paint.)  You could make one from the kind of clear
marbles used for holding plants in flower arrangements.


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