MadSci Network: Physics |
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
Try the links in the MadSci Library for more information on Physics.