Physics of a safety jacket

My cycling safety jacket exploits two different technologies to be visible.  I can’t resist wondering, are the two analogous?  What I find is that no, in a sense they are opposite.

One technology is directionality, which makes the silver strips on the vest shine bright in headlights. The trick here is that the fabric reflects light in the same direction it came from, making the strips appear magically bright to oncoming drivers.  (Traffic signs have used this technology for years.  I remember once as a kid being amazed when my father said, look, I can turn off all those signs up ahead on the highway just by switching off our car’s headlights!  My mother wasn’t pleased with this experiment.)

The other is fluorescence, which gives the fabric of the jacket its unnatural brightness.  The physics here is that ultraviolet light hitting the fabric is converted by quantum mechanical effects to a visible wavelength and radiated out again.   So my jacket emits more yellow light than it receives.  Our eyes see the magic in this, and in the last year or two people have started wearing “neon” shirts and shoelaces.

Here is what I mean by saying that the two technologies are opposites.  Fluorescent fabrics work by bending incident light from one wavelength to another.  Reflective strips work by preventing the bending of incident light from one direction to another.

[21 October 2013]

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