Let There Be Lights—Part 10
Go to <Last, Index, Next>A Scientific Aside On Yellow Versus White Fog Lights
It is a fallacy that a single color of light significantly improves the ability to see in fog — many still believe it to this day. An excellent discussion of this is provided by Professor Craig Bohren of Penn State University at the USDOE Ask A Scientist website. Lynn follows Professor Bohren's explanation here. (Lynn's PhD thesis in physics involved quantum electrodynamics and the calculation of Rayleigh scattering from atoms.)
The misconception that a single wavelength of light lets you see farther in a fog stems from a misunderstanding of classical Rayleigh scattering, which predicts the strong dispersion/spreading of scattered light based on its wavelength. If this were true for fog, the light of different wavelengths from an object would be dispersed to different angles, adding more blur to the image one sees. But the assumption of classical Rayleigh scattering needed for this prediction to be valid, that the wavelength of the light is large compared to the size of the scattering objects, is violated for fog. Fog droplets are huge compared with the wavelength of visible light and generally do specular dispersion of the light — all wavelengths are scattered equally. Visible light has wavelengths of about 400-700 nanometers; fog droplets have sizes of the order of 10-100 microns (10,000-100,000 nanometers, 0.4-4 thousandths of an inch), about 10-200 times bigger than the wavelengths of visible light.
There are things one can do to improve vision in a fog but using a single wavelength of light is not one of them. In fact, by using a filter to create the single color light, you are actually decreasing the amount of light and reducing your ability to see. It does helps to move the light source away from your line of sight, the reason that fog lights should be placed low on a vehicle.
In October, 1938, J. H. Nelson published a paper entitled "Optics of headlights" in the Journal of Scientific Instruments (Vol. XV, pp. 317-322). In a section on fog lights, Nelson writes "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."
In spite of this scientific fact being know at least as early as 1938, you can still find many sources of information today that claim that yellow fog lights help you see better than white lights. (See, for example, this discussion on the color of fog lights at ThePartsBin.com.) When you read such drivel in the future, just remember that it is scientific bull feces.
Contact
with your comments or questionsCopyright © 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Lynn Kissel
Last updated: May 24, 2009