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Question
It's said that one of the primary reasons why blue light is seen part of the morning & most of the day is because the Elector-magnetic color spectrum (ECS) is the "shortest" wave length of the ECS; yellow, red & sometimes violet, on the other-hand, are seen during dawn & dusk.  What exactly does that mean? Way does the length of the light wave determine what color of light is shown in the sky? Furthermore, why isn't green light almost never seen in the sky?  hope u can help
         Thank you

Answer
Dan,
The sky is blue because of Raleigh scattering, see
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html

All light is just photons, massless "particles" at different wavelengths and frequencies.
The entire electromagnetic spectrum from shortest wavelength to longest wavelength is as follows:
Gamma rays, X-rays, ultraviolet, visible light (what you call the color spectrum), infrared
(fancy name for heat), and radio (includes TV, FM, AM, shortwave radio, and microwaves). Some think that gravity waves should be included below the radio spectrum, but we just don't know as yet.
The only difference between an infrared photon and a gamma ray photon is it's wavelength.
Also, all photons exhibit both a particle and a wave property at the same time... I know this is hard to visualize, but take my word for it, it's true.

Now to color in the visible light part of the electromagnetic spectrum, which is actually a very small region of the entire electromagnetic spectrum. All colors are just the way our retina and brain interpret different wavelengths of light, ranging from the longer wavelengths like red, down to the shorter wavelengths of blue and violet. Now you can have emitted light (like the sun) and you can have reflected light, like your green shirt or the moon.
Emitted light generally (except for monochromatic laser light) is composed of all wavelengths which the human eye interprets as white light.  (The color black is simply the absence of all light).  The sunlight, composed of all wavelengths, strikes the Earth's atmosphere, and most of the other wavelengths (except for blue) are absorbed and scattered. Thus we see the sky as a blue color.
Now as for reflected light, your green shirt (or the dyes in it) absorb all wavelength colors EXCEPT for green... thus our eye interprets your shirt as reflecting the green wavelength.  And this is true for all reflected light.  If an object absorbs no light and reflects it all, we see it as white... if it absorbs all wavelengths, then we perceive it as a black shirt.
And that's how it all works. All wavelengths are being absorbed, and the one reflected is the color you perceive with your human eyes.
Oh, sky color  -  when storms are present, color can be seen also. A "green colored" sky is sometimes prominent when there is a tornado in the area... grass, pollen, leaves, and weeds are tossed into the sky and observer can observe a green colored sky. Also rain can be red or reddish orange if a violent thunderstorm comes in off sandy desert regions, the color coming from the iron in the fine sand particles can cause a reddish-orange. In Siberia, blue and green colored snow can fall from pollen stirred up in the air. So various particulate matter can and does color rain and winds and even snow, under semi-rare occasions.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA  

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Tom Whiting

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I can answer most questions involving general science and engineering, except those involving electronics and computers.  My strong points are all the Earth Sciences, especially Meteorology and Astronomy, and any related subjects to Astronomy.  My weak point is Electronics Technology.   No astrology questions please, or questions about alleged UFO identifications.

Experience

My hobby has been Astronomy for over 50 years. I currently ownand use a 30 inch portable telescope.   I taught at the University Level for 13 years.  My personal interest in life, in general,  has always been science oriented.

Organizations
President - Erie County Mobile Observers Group for over 15 years.Member of allexperts.com astronomy catagory for over 8 years.

Publications
Astronomy Technology TodayErie Times Newspaper - I wrote the "Over Erie Skies" columnfor 11 years.

Education/Credentials
BS  Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PAAlso retired USAF pilot, 20 years.

Awards and Honors
I was the discoverer of the Mini-Coathanger Asterism up in Ursa Minor.  I earned 6 Air Medals with USAF and retired after 20 years of service with the rank of Major, with over 2100 hours flying time.I also earned the rank of Eagle Scout in 1956.

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