Pink mountains, red sunsets
Q: Why is the setting sun red? Why do mountains turn pink in the setting sun?
Mountain rime ice reflects a golden sunset, in the German Alps. Photo
courtesy of Bernd Haynold and Wikipedia.
A: Any west-facing mountain turns golden, then pink as it reflects light from
the setting sun. The sun becomes dimmer and redder as it sets and the mountain
reflects this red light.
At sunset the red-orb sun hovers on the horizon, changed from the
blinding-white disk we see at noon. Air, smoke, and dust cause this amazing
transformation. When the sun is low, its light must go through more atmosphere
(about 38 times more) than when it's overhead. A sunray beaming through thick
atmosphere loses violet, blue, and green light on the way. Red light shines
through relatively unhindered. That's the main reason why the white sun turns
red at sunset.
Light is made up of all colors: some better at traveling straight to your eye
than others. An incident sunbeam coming into our atmosphere may smash into an
air molecule or particle of dust or smoke. If it does, its blue light is three
times more likely than red to scatter. The sunbeam re-radiates blue light in all
directions. Less of the original light gets through because of this scattering.
The light that does get through is red.
 How
the red light gets through. Image courtesy of Science Made Simple, image
modified by author.
Water and ozone enhance the reddening effect by absorbing violet, blue, and
green light.
The beauty of the show transcends the mechanics. As the sun dips towards the
horizon, the sunbeam path through the atmosphere gets longer and longer. And the
sun changes from yellow to wondrous colors of peach, magenta, orange, pink, and
finally red. The show changes daily with varying amounts of dust, smoke, and
water in the atmosphere. Never the same watermelon pink from day to day. When
the show's over, the deep blue shadow you see moving up the mountain is the
Earth's shadow.
(Answered April 11, 2001; updated Aug. 30, 2007)
Further Reading:
USATODAY.com graphic: What gives the sky its color
Matt McIrvin: Why the
sky is blue-dipole scattering
Science Made Simple:
Why is the sky blue
Color and Light in Nature by D. Lynch and W. Livingston
(Answered September 1997; updated Sep. 11, 2007)
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