How Do Bicycle Reflectors Work?

Written by mary osborne
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How Do Bicycle Reflectors Work?
(Photo by Mary Osborne)

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Cube Corner Reflectors

Most newer bicycles have a red reflector on the back, under the seat, and a white reflector in front, just under the handlebars. Sometimes there are also one or two reflectors on the spokes of each wheel. It's hard enough for a motorist to see a bicyclist in the daytime, but nearly impossible to see one pedalling along the roadside at night without the aid of light-illuminating reflectors. Most bicycle reflectors comprise hundreds of cube corner reflectors that look like a honeycomb of hexagonal cells. These cells reflect light that shines on them back to its source, unlike a mirror, which reflects light, but throws off a glare as it does so.

How Do Bicycle Reflectors Work?
Hexagonal Cells

Total Internal Reflection

Like a faceted diamond that seems to sparkle with an internal fire when a light source is introduced, a cube corner reflector needs no external energy source other than light to produce light within it. The walls of the hexagonal cells are angled precisely right to allow for "total internal reflection" to occur. Light travels in a straight line, but the angles in a cube-corner cell are such that a beam of light bounces off one wall, then against another wall, and so on, continually retracing its path. Because the plastic of the hexagonal cell walls are dense enough, the light is reflected against them, and refracted, too, when it exits the reflector as a car's headlights begin to pass beyond the bicyclist, changing the angle of the light source.

Potential ProblemsWith Bike Reflectors

In order for bike reflectors to work effectively, the angle of the light source, as in a car's headlights, must shine directly at them. If the beam of light is at a more perpendicular angle, the light rays are refracted outside of the plastic cube-corner cells rather than being reflected inside them. Therefore, the headlights of a car crossing in front of a bicyclist will not cause the reflectors to become illuminated and a hazardous situation can occur. In fog or rain, light rays are spread and dispersed, causing the reflectors to become less effective. Dirty car headlights and dirty bike reflectors also can impair the function of bike reflectors. If a car's headlights or the reflectors on a bicycle aren't aimed properly, bike reflectors can't do their job to protect a bicyclist at night, which is why most serious bicyclists advocate the use of additional lighting, such as battery-operated headlamps and taillights.

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