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You will normally find more than one fog light on a vehicle, so there is usually at least two and they are situated on the front of a vehicle or truck. They are used to help visibility in foggy or misty conditions.
Just about every vehicle has them and they come in a variety of colors and intensities. It is sometimes very crucial that we have them, as we need them to help us find our way in that real bad weather.
Yellow is the normal standard color for fog lights. No one is quite sure why yet, but it is said that they need to be a single color light rather than a white light. The reason for this is to minimize dispersion as the light hits the water vapor and scatters in different directions. Many people seem to think that red might be a better color to use, but it seems that drivers may have a problem with that as red is already used for both stop lights and brake lights. Therefore, yellow would be the next best choice of color to use, because it has the next longest wavelength of visible light.
There is may sound a bit complicated to the not so scientifically minded, but the problem with this idea is that it has to do with the size of the water molecules in the fog. The molecules of water vapor are large enough that the dispersion doesn’t occur in any particular way, thus making the wavelength of the light totally irrelevant. Yellow lights were more than likely chosen because yellow is associated with caution.
Yellow lights and yellow signs are used to indicate that a driver should:
Slow down
Look for obstacles
So, it is used for increased levels of caution, which is exactly what drivers need to have when driving in heavy fog.
Now days, many fogs lights are halogen lights. This allows them to burn at a much hotter temperature than the traditional lights. Yet they still have a very low burnout rate. Halogen fog lights tend to have a tighter beam than incandescent fog lights, which is an added benefit.
There is also a special class of fog lights that are seen on some of the newer cars that are high-intensity discharge lamps. It is possible that these lights may use a number of different minerals for them to be able to operate.
These include:
Mercury
Sodium
Halide
You will be able to recognize these fog lights as they have a purplish hue, unlike the more standard traditional fog lights that are a yellow color.