Why is the sky
blue and not violet?
College Park, MD -- (July 12, 2005)The hues that we see in the sky are not
only determined by the laws of physics, but are also colored by the human visual
system, shows a new paper in the American Journal of Physics.
On a clear day when the sun is well above the horizon, the analysis demonstrates,
we perceive the complex spectrum of colors in the sky as a mixture of white light
and pure blue. When sunlight enters the earth's atmosphere, it scatters (ricochets)
mainly from oxygen and nitrogen molecules that make up most of our air. What
scatters the most is the light with the shortest wavelengths, towards the blue
end of the spectrum, so more of that light will reach our eyes than other colors.
In the early 19th century, physicist John William Strutt, better known by
his title, Lord Rayleigh, wrote equations to show how light scatters in the sky.
And in recent years Raymond Lee of the U.S. Naval Academy, Annapolis, MD, measured
light coming from a noonday sky. Both equations and measurements showed that
peak amounts of violet light reach our eyes too. So what is happening?
Combining physics with quantitative data on the responsiveness of the human
visual system, Glenn Smith of the Georgia Institute of Technology in Atlanta,
points to the way in which our eye's three different types of cones detect color.
Cones are cells in the eye that detect color. They lie along the back of the
eye, the retina, amidst light and motion sensing cells called rods. The eyes
have three kinds of cones, each tuned to different ranges of color.
When a wave of light strikes the eye, the cones send a signal to the brain.
If a blue wave, with short undulations, enters the cone, the cone sends a signal
to see blue. If a red wave, with long humps, hits the cones, the brain sees "red." But
if a red wave and a green wave enter at the same time, the eye's various cones
send a signal that the brain interprets as the in-between color of yellow.
As Smith shows, the sky's complex multichromatic rainbow of colors tickles
our eye's cones in the same way as a specific mixture of pure blue and white
light. The cones that allow us to see color cannot identify the actual wavelengths
that hit them, but if they are stimulated by the right combination of wavelengths,
then it will appear the same to our eyes as a single pure color, or a mixture
of a pure color and white light.
And that is why the sky is blue – or seems so.
Smith, American Journal of Physics, July 2005.
Ben Stein
American Institute of Physics
301-209-3088
Glenn Smith
Georgia Institute of Technology
|
