Here are some samples of some fog and driving lights that I sell on eBay.
Why are fog lights yellow? Does it help that they are? Does it matter if you are driving in city or countryside?
My understanding is that it is important for fog lights to be one color
(rather than white, which is all colors) because the different
wavelengths(colors) of visible light scatter off the fog droplets
differently. This phenomenon is known as "dispersion," because the
different colors of light in an image will separate from each other,
causing the image to "disperse." If you illuminate the road with only one
wavelength (color) of light, the images of the objects you see will still
become somewhat blurry because of the scattering of light by the fog, but
at least you won't have extra problems from dispersion.
So, if we want to use just one wavelength of light, which wavelength
should we use? It turns out that light with short wavelengths scatters
more than light with long wavelengths (short to long: violet, indigo,
blue, green, yellow, orange, red). So, a long wavelength light will be best.
There's another thing to consider, too: our eyes are not equally sensitive
to all colors. It turns out that we are most sensitive to yellow and
green light. So, our best compromise between sensitivity for our
eyes and a long wavelength for least scattering is yellow light.
Now, I don't know what kind of light bulbs are used in fog lights, but
another consideration used in street lighting is cost and efficiency. You
may have seen some yellow street lighting in some places; this is
"low-pressure sodium vapor" lighting. The special thing about this light
is that it is almost entirely one (actually two very close together)
wavelength of yellow light, and that it gives the most illumination for the
amount of electricity. A big problem with this light, though, is that it
throws off color perception. Under sodium vapor light, something blue
looks gray. This makes it hard to, say, recognize your car in a parking
"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. 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 independent. Unfortunately, many people learn
(without caveats) Rayleigh's scattering law and then assume that it applies
to everything. They did not learn that this law is limited to scatterers
small compared with the wavelength and at wavelengths far from strong
The second flaw is that in order 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.
There are two possible explanations for yellow fog lights. One is that the
first designers of such lights were mislead because they did not understand
the limitations of Rayleigh's scattering law and did not know the size
distribution of fog droplets. The other explanation is that someone deemed
it desirable to make fog lights yellow as a way of signalling to other
drivers that visibility is poor and thus caution is in order.
Designers of headlights have known for a long time that there is no magic
color that gives great penetration. I have an article from the Journal of
Scientific Instruments published in October 1938 (Vol. XV, pp. 317-322).
The article is by J. H. Nelson and is entitled "Optics of headlights". The
penultimate section in this paper is on "fog lamps". Nelson notes that
"there is almost complete agreement among designers of fog lamps, and this
agreement is in most cases extended to the colour 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 at one time lamp designers
thought that yellow lights had greater penetrating power. And it may be
that because of this the first fog lamps were yellow. Once the practice of
making such lamps yellow began it just continued because of custom."
Please click below to look at all my driving and fog lights on eBay.