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Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999

Why is the color red used for stop signs, danger signals, and brake lights?

I think the major reason has to do with how light gets absorbed in fog, particulates, haze, etc. We know that light is scattered (and thus attenuates or gets weaker) as it travels through a scattering medium composed of small particles.

As it turns out, red light is scattered (is weakened) less than other colors. In fact, of the light spectrum, blue is scattered the most and red the least. Why?

Lord Rayleigh was the first to discover that small particles in air scatter different light colors with different effectiveness; this effectiveness being proportional to the inverse of the wavelength to the power 4. Blue light which has the shortest wavelength is scattered the most and red, having the longest wavelength, the least. As such, at sunset, when the sun rays travel the longest distance, more of the red part of the Sun's spectrum reaches us. The other colors do not make it as well because they have been mostly scattered out along the way. The same explanation goes for the color of the sky. Away from the sun, we do not see the direct sunlight but the scattered light. Small air molecules scatter blue light more efficiently out of the direct sun rays, and that is what we see. Thus, the sky appears blue.

In summary, red light weakens the least in traveling in the air. That is why stop signs and other critical lights are red. There are some other reasons for using red light. One is perception and the other is effect of aging on our perception of colors. This is not my area of expertise but I would think that we have a sharper perception of red (and yellow) colors, even as we age. Our retina is covered with light sensitive receptors known as cones (mostly in the center of retina and responsible for color sensing) and rods (mostly on the edge of retina and responsible for perceiving shades, night vision, movement, but not color). There are three types of cones, each sensitive to one of three colors of red, blue, or green. I think that in the central part of the retina where we get sharpest images we have more of the cones that are collectively more sensitive to yellow and then red colors. I hope someone else will comment on this further.


Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory
Argonne, IL 60439

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