Ask A Scientist Physics Archive

name         Mrs. D.
status       educator
age          40s

Question -   A student in my class wants to know why the sun appears
to be orange at sunset?
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Short answer is, sunset is orange because the sky is blue.
Bluish and orangish are opposites.  Complementary parts of white light.
Sky blue is white biased to the short wavelength end of the spectrum.
Sunset colors (yellow, orange, red) are white biased towards the long wavelength end of 
the spectrum.

Air tries to be perfectly clear, but it cannot quite do it perfectly.
It's made of empty space with molecules and temporary clumps of molecules bouncing around 
in it.
So air is a slightly bumpy ride for light to go through.
The bumps are faint and very small, so longer wavelengths of light
(orange, red, and near-infrared) sail over them with little disturbance.
But light rays with short wavelengths, like blue and UV, feel the bumpiness more
and gradually get bounced left and right randomly.
We call this "scattering" because it is not absorption.  "Rayleigh scattering" in 
particular.
The light is not absorbed or lost, it is just mixed up, steered in all directions.
( Google  blue sky scattering ,  or  http://www.why-is-the-sky-blue.org/why-is-the-sky-
blue.html)

Imagine always-white sunlight, in space, hitting the top of earth's atmosphere at a slant.

The blue light scatters in all directions,
 - down at the people on earth's surface right below the place it hits, making their sky 
 blue instead of black,
 - sideways at anybody nearby flying a plane or standing on the ground looking just above 
 their horizon,
 - up at astronauts orbiting overhead (Our blue planet is dark water tinted lighter blue 
 by the sky in front of it!)
Meanwhile the red light sails through along its original straight line, going to the eye 
of the person watching the sunset.

This wavelength selectivity is not 100%, but it is pretty strong.
For a given short distance through air, the percentage scattered goes as (1/wavelength) 
to the 4th power.
There is always a little of the red light scattered, and a little of the blue light 
transmitted straight through.

At sunset the sunlight must go through about ten times more atmosphere than at noon to 
reach your eye,
so this splitting of blue and orange is much stronger at sunset than at noon.
Near sunset the sky above you is bluer as well as the sun being redder.

Atmospheric particulates (smog, volcano dust, large-fire smoke) also make sunlight redder,
 when they exist.
Their amount is variable from year to year and even day-to-day.
But they are different because they are actually brownish, like orange glass, they 
actually absorb light, blue light more than red.
If these particles are small enough they can also do some of the losses scattering, too.
I am not sure of the exact percentage split between effects of these particulates and the 
molecular scattering described above.
But both make the setting sun redder than the noon sun.

Sorry, I have given you the long story and left you to figure out which part to tell your 
students.

Jim Swenson
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