Color Red and Danger
Why is the color red used for stop signs, danger signals, and brake
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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Update: June 2012