Why are fog lights yellow?

Why are fog lights yellow?

PROF WAQAR HUSSAIN

   It is matter of scattering of yellow light by fog droplets. Everybody knows that shorter wavelengths of light (violet, indigo, blue) experience greater scattering than the longer wavelength of light (red, orange, yellow). Greater is the scattering; lesser is the penetration of light in fog and weaker is the vision. So, to get greater penetration of light in fog, one should use longer 

wavelength possible. Red has the longest wavelength but its use is not suitable because it is used in stop lights. Ultimately, one goes for second option i.e. Yellow. So, yellow light undergoes least scattering in fog and thus have greater penetration so increases visibility. It is the reason why fog lights are yellow!

            Many believe that this explanation is flawed for many reasons. First, fog droplet is not so small to scatter the light. 

So scattering of light by fog is independent of wavelength. Second, to get yellow light, one needs a filter which is placed on white headlights. In doing so one gets less transmitted light so the goal of getting great penetration goes to gutter.

          Third, probably fog light came into vogue with idea of signalling to other drivers that visibility is poor and greater caution is required.

          Fourth, lamp designers might have thought that yellow light had greater penetration power. So, they started making yellow fog lights. Once it came into practice, now it is continuing because of tradition.

      Fifth, European ECE regulation 48 currently requires new vehicles to be equipped with head lamps emitting white light. However, yellow head lamps are also permitted throughout Europe and in many other countries like Japan, New Zealand, etc.

        Sixth, if yellow lights are useful then why most of cars in Europe and USA are using white fog lamps.

              In spite of above discussion, I feel use of yellow lights reduce number of accidents because yellow lights sends a warning and caution message. So drivers become more attentive and put extra strain on their sight to have greater visibility, reduce speed and make greater use of horn. As a result, many accidents are avoided. However, there is another advantage of yellow lights that they neutralizes the effect of shorter blue, violet lights which create difficulty for human visual system to work properly and also cause dazzling and glaring effect in fog. So, keep on using yellow till suitable light is not available.