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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