Here is more scientific support for my earlier post.
There is no good reason why fog lights are yellow.
an excellent explanation by
Prof. Craig Bohren of Penn State University:
"First I'll give you the wrong explanation, which you can find here and there. It goes something like this.
As everyone knows, scattering (by anything!) is always greater at the shortwavelength end of the visible spectrum than at the longwavelength end. Lord Rayleigh showed this, didn't he? Thus to obtain the greatest penentration of light through fog, you should use the longest wavelength possible. Red is obviously unsuitable because it is used for stop lights. So you compromise and use yellow instead!
This explanation is flawed for more than one reason.
Defect no. 1:
Fog droplets are, on average, smaller than cloud droplets, but they still are huge compared with the wavelengths of visible light. Thus scattering of such light by fog is essentially wavelength (therefore color) independent.
Unfortunately, many people learn Rayleigh's scattering law (without caveats) and then assume that it applies to everything.
They have ignored the fact that this law is limited to scatterers which are small compared with the wavelength
and only works at wavelengths far from strong absorption.
Defect no. 2:
To get yellow light in the first place you need a filter. Note that yellow fog lights were in use when the only available headlights were incandescent lamps. If you place a filter over a white headlight, you get less transmitted light, and there goes your increased penetration down the drain.
Then why did people try yellow fog lights at all?
One: the first designers of such lights were misled.
They did not understand the limitations of Rayleigh's scattering law
and did not know the size distribution of fog droplets.
Two: Someone thought it, useful to make fog lights yellow to signal to other drivers that visibility is poor and to please be careful.
Headlight designers have known for a long time
that
there is no magic color that gives great penetration!!! "Optics of headlights" is an article from the Journal of Scientific Instruments, published in October 1938 (Vol. XV, pp. 317-322).
Author: Prof. J. H. Nelson
Nelson notes that
"there is almost complete agreement among designers of fog lamps, and this agreement is in most cases extended to the color of the light to be used.
Although there are still many lamps on the road using yellow light, it seems to be becoming recognized that there is no filter, which, when placed in front of a lamp, will improve the penetration power of that lamp."
This was written 61 years ago. Its author uses a few words ("seem", "becoming recognized") indicating that perhaps in the 1930s, lamp designers imagined that yellow lights had greater penetrating power.
And it may be that because of this
that the first fog lamps were yellow.
Once the practice of making such lamps yellow began
it just continued because of custom."
I rest my case.
Ram