Re: Still confused about yellow fog lights....

First, let me clarify an item...  The new lamp systems use xenon arc lamps, not 
xylene.  Xenon is a rare gas that doesn't interact much with anything 
chemically, so that it should be (and is) very long life because nothing bad 
happens to the gas.  Instead of heating up a filament of tungsten wire with 
halogen gas around it, xenon between two electrodes is actually ionized/heated 
and produces the output light.  [Xylenes are organic chemicals which are 
excellent solvents and are sometimes added to gasoline to help clean out fuel 
injectors and carburetors, but which are never to my knowledge used in 
illumination systems.]

Your confusion about yellow fog lights is not surprising, since a lot of the 
reasoning behind the selection of yellow as a fog light color had nothing to do 
with physics.  I've heard two reasons cited as the rationale behind the use of 
yellow fog lamps:  (1) yellow lamps suffer less backscatter into the eyes of the 
car's driver than blue or green, allowing them to see better in the fog; (2) 
yellow lamps are more readily seen by an oncoming driver, so that a car with fog 
lights on will be seen more readily than white lights would be. 

The first point is true for light scattered off very tiny aerosol particles in 
fog -- blue light is scattered out of the path from the sun to the viewer of a 
sunset, leaving white light minus blue to reach the observer... so he/she sees 
yellow/red -- but is pretty irrelevant for the kind of yellow lights used for 
fog lamps.  Those lamps were generally made by putting yellow filters over a hot 
tungsten filament incandescent lamp, so that a lot of light is lost in trying to 
make the yellow light -- reducing the brightness available for the driver to 
see, which is not such a good idea.  Using a halogen gas fill increases the 
brightness of the bulb, but the same general principle of reduced brightness 
still holds.  Additionally, the size of water drops in fog is generally not 
appropriate for producing the kind of scattering I mentioned above -- if so, 
headlights would appear very red to oncoming traffic, which they don't.

The second point, that the yellow color is more visible to oncoming traffic is 
also true, but irrelevent as well.  The point of fog lights is to allow the 
car's driver to see better, not to be seen better -- if you wanted to be seen 
better, a flashing strobe would be more effective, but you don't see much of 
those in the fog light business.  

Most likely, the real reason that people have used fog lamps that are yellow is 
that they look so cool.  Realistically, most people do not drive a lot in foggy 
conditions -- certainly not enough in most cases to justify the cost of 
expensive additional lights.  Both my wife's SUV and my sport luxury car have 
yellow fog lamps on them, but we've never used them...  They came as standard 
equipment on the vehicles, probably because the manufacturers thought they'd 
look more bold or aggressive.  Since it's more expensive to get standard package 
equipment taken off a car than to just leave it alone, we've got the lights as 
artifacts of somebody's idea of style.

Your observation that refraction occurs within aerosol water drops is true, but 
it turns out that it's the surface scattering rather than dispersive refraction 
(which refers to the fact that different colors are bent more on refraction, 
leading to them being dispersed in angles) that results in the non-problem which 
yellow lamps were supposed to solve.  The dispersive refraction effects may lead 
to rainbows being visible under certain conditions -- I think everybody's seen 
them on sunny days when you spray a garden hose in a very fine mist around you 
-- but I don't think I've ever looked for or seen one at night in fog or rain.  
It might be fun to get a very finely misted garden hose, stand with your back to 
a headlamp and your head at about lamp level, and spray a very fine mist in 
front of you.  You might just be able to see a rainbow at night.  Fun, but not 
relevent to the problem of fog lamps.

To wrap up my discussion of fog lamps, I think that the reason why folks are now 
moving toward xenon arc lamps is that they're just so darned bright that they 
put out tons of light which you may be able to use to see.  People have realized 
that it's very unlikely that you'll get so much backscattering of the light -- 
whatever the color -- from the aerosol water in fog that it will prevent you 
from seeing something that you could see in lower light.  That would require a 
bizarre conspiracy of events in which the object being viewed has very low 
contrast in general... which would make it unlikely that you'd see it anyway.

Thanks for your very good question...  it got me thinking about stuff I haven't 
considered for years!

Steve Guch