Red Road Signs
Red light has a long wavelength, which means it’s scattered less by particles in the air than colours of light with short wavelengths (like green and blue). This means red road signs are more visible in fog!
The colours of the visible light spectrum. Light on the left (Red and Orange) have long wavelengths; moving across the spectrum, the wavelength of the light gets shorter. Violet light has a very short wavelength and scatters very easily!
Colours of the Visible Light Spectrum by Meganbeckett27 is licensed under CC BY SA 3.0When light hits particles in the air it scatters, which is why lights look fuzzy if you shine them through steam or fog.
The scattering of light by particles in the air is called Rayleigh Scattering. Different colours of visible light have different wavelengths, and scatter differently when they strike particles in the air; shorter wavelengths of light scatter more easily.
Blue and purple light have a short wavelength, while red and orange light have a long wavelength. We can see this in the world around us; the sky is lit by sunlight, which scatters as it passes through the atmosphere.
During the day the light travels steeply down into the atmosphere, through a relatively small amount of air. As sunlight contains a lot of blue light, we see the sky as blue as the sunlight is scattered. At sunset, however the sunlight is travelling sideways across the surface of the Earth through the atmosphere, travelling through a much larger amount of air and hitting a lot more particles. The shorter wavelengths of light (including blue) are scattered, so by the time the light reaches us we see only the remaining longer wavelengths of light - red and orange, which scatter less.
This makes red a very useful colour for warning signs and lights – they’ll be more visible in all weathers, because the light scatters less than another colour would!
For more information:
https://www.optics4kids.org/what-is-optics/scattering/why-is-the-sky-blue-why-are-sunsets-red
https://www.britannica.com/science/Rayleigh-scattering