Fog lights aid visibility in foggy or misty conditions and are traditionally yellow to minimize scatter. However, the size of water molecules in fog means that the wavelength of the light is irrelevant. Yellow may have been chosen for its connotations with caution. Modern fog lights are often halogen or high intensity discharge lamps.
Fog lights are lights mounted on the front of a car or truck that aid visibility in foggy or misty conditions. They come in a variety of colors and intensities, but virtually every vehicle has them, and their assistance can be crucial for navigating in bad weather.
Traditionally, fog lights are yellow, and the reason why is the subject of some speculation. A common claim is that they should be a single color light rather than white light, to minimize scatter as the light hits the water vapor and scatters in different directions. It’s often said that while red might be a more ideal color, it already has connotations for drivers: It’s used for both brake lights and stop lights. Yellow, then, seems to be the next best option, because it has the next longest wavelength of visible light.
The problem with this idea, which seems scientifically plausible, has to do with the size of the water molecules in the fog. The water vapor molecules are large enough that scattering does not occur significantly, making the wavelength of the light irrelevant. The first car companies to use fog lights may not have been aware of the impact that the size of the water vapor molecules would have on the scattering, so they believed that by choosing yellow lights they were minimizing the blur emitted by the lights. . Most likely, yellow was chosen because of its connotations in the West with caution. Yellow lights and yellow signs are used to indicate that a driver should slow down, look for obstacles, generally use increased levels of caution, precisely the activities one wishes other drivers to exercise when driving in heavy fog.
Many modern fog lights are halogen lights, which allows them to burn at much higher temperatures than traditional lights, while maintaining a low burn rate. Halogen lights tend to have a tighter beam than incandescents, which provides an added benefit. A special class of fog lights seen on some new cars are high intensity discharge lamps. These lights can use a number of different minerals to operate, including mercury, sodium, and halide. These are often distinguished by a purple hue, rather than the more traditional yellow color.
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