Why yellow fog lights?

[hayzin]

For a long time, I have been wondering why fog lights are yellow. Most people say because they are better visible in foggy conditions. Well I did some research. Here is what I found:

Despite common assumptions, the yellow color of fog lights does not improve visibility in fog.

Misconceptions

The belief that yellow fog lamps contribute to superior lighting in foggy conditions is based on a misinterpretation of physical laws about the behavior of light which hold that colors at the shorter wavelength end of the spectrum disperse more easily in interactions with small particles. Longer wavelength colors like yellow were thought to be less subject to this effect.

Reality

In fact, the large size of fog droplets negates any significant difference in scattering effects among colors.

Filtering

"Yellow" fog lights are actually just filtered white lights. Since the filtering naturally reduces the emission of light, it does not boost its power to cut through mist.

Origins

The use of yellow as a color for fog lights may have started because of a genuine misunderstanding about its ability to illuminate foggy roads. After that it may have simply become traditionally accepted. An alternate theory holds that yellow is associated with danger and indicates hazardous conditions.

Conclusion

Car experts Tom and Ray Magliozzi recommend white fog lights as an alternative to yellow.

Please feel free to comment and discuss.

[fullyloaded2]

have u ever noticed the difference in visibility when u switch between low beam to high beam.... the visibility drastically reduces as doing so highlights an enormous wall of tiny white droplets of water in front u.....

what yellow light does is make any changes behind the wall of fog appear in a vague outline and give a bit more time to make necessary changes...... this is just my observation.....

[hayzin]

have u ever noticed the difference in visibility when u switch between low beam to high beam.... the visibility drastically reduces as doing so highlights an enormous wall of tiny white droplets of water in front u.....

what yellow light does is make any changes behind the wall of fog appear in a vague outline and give a bit more time to make necessary changes...... this is just my observation.....

Yes that might be true. But upon my readings, it seems that everyone has their own experience with these colors.
Thats why I wanted to look at it from a more technical point of view.

[zunny]

the color of fog/mist is whitish/greyish. Where yellow lites mite not give u more visibility in terms of looking ahead but due to being a contrasting color it does differenciate from its surroundings n thus the oncoming person can see the yellow color better. Maybe the fog lite were not made for the driver of that vehicle on which it is mounted rather for the other to see it coming better

[hayzin]

Maybe the fog lite were not made for the driver of that vehicle on which it is mounted rather for the other to see it coming better

Agreed!

[sguls2]

I found an interesting article that discusses this issue, article is pasted below:
Copied from: Fog lights

There is no good reason why fog lights are yellow. Here is an
excellent explanation provided by Professor 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. 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
absorption.

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."

Also, take a look at the following web site:
Do Fog Lights Really Work?, Alaska Science Forum

Dr. Lawrence D. Woolf
General Atomics

[Horse_rider]

sab wave length ka chakkar hy bhai, yellow light got improved wave length. its all about wave length.

[F1]

simply yellow light doesnot scatter because of its wavelength which helps to penetrate where as normal white light scatters .and the reason they are mounted low is that most obstacles are grounded and lower level illumination makes it easier for the driver to see them. Also fog is less loser to ground because of the heat of earth.

[hayzin]

@All

Thanks guys for all your input. I looked up wavelengths of all visible colored lights. Turns out that red has the strongest, followed by orange, and then yellow.
Check this:

<table summary="used for formatting." cellspacing="8"><tbody><tr valign="top"><td>Our eyes are sensitive to light which lies in a very small region of the electromagnetic spectrum labeled "visible light". This "visible light" corresponds to a wavelength range of 400 - 700 nanometers (nm) and a color range of violet through red. The human eye is not capable of "seeing" radiation with wavelengths outside the visible spectrum. The visible colors from shortest to longest wavelength are: violet, blue, green, yellow, orange, and red. Ultraviolet radiation has a shorter wavelength than the visible violet light. Infrared radiation has a longer wavelength than visible red light. The white light is a mixture of the colors of the visible spectrum. Black is a total absence of light.
Earth's most important energy source is the Sun. Sunlight consists of the entire electromagnetic spectrum.

</td> <td>
<small>(Wavelength image from Universe by Freedman and Kaufmann.)</small></td> </tr> </tbody></table> <table summary="used for formatting." cellspacing="8"><tbody><tr> <td>Violet Light

The visible violet light has a wavelength of about 400 nm. Within the visible wavelength spectrum, violet and blue wavelengths are scattered more efficiently than other wavelengths. The sky looks blue, not violet, because our eyes are more sensitive to blue light (the sun also emits more energy as blue light than as violet).</td> <td> </td> </tr> <tr> <td>Indigo Light

The visible indigo light has a wavelength of about 445 nm.</td> <td> </td> </tr> <tr> <td>Blue Light

The visible blue light has a wavelength of about 475 nm. Because the blue wavelengths are shorter in the visible spectrum, they are scattered more efficiently by the molecules in the atmosphere. This causes the sky to appear blue.</td> <td></td> </tr> <tr> <td>Green Light

The visible green light has a wavelength of about 510 nm. Grass, for example, appears green because all of the colors in the visible part of the spectrum are absorbed into the leaves of the grass except green. Green is reflected, therefore grass appears green.</td> <td></td> </tr> <tr> <td>Yellow Light

The visible yellow light has a wavelength of about 570 nm. Low-pressure sodium lamps, like those used in some parking lots, emit a yellow (wavelength 589 nm) light.</td> <td></td> </tr> <tr> <td>Orange Light

The visible orange light has a wavelength of about 590 nm.</td> <td> </td> </tr> <tr> <td>Red Light

The visible red light has a wavelength of about 650 nm. At sunrise and sunset, red or orange colors are present because the wavelengths associated with these colors are less efficiently scattered by the atmosphere than the shorter wavelength colors (e.g., blue and purple). A large amount of blue and violet light has been removed as a result of scattering and the longwave colors, such as red and orange, are more readily seen.</td></tr></tbody></table>