Why is the Sky Blue?
It is an age-old question posed by children with an inadequate explanation. The answer that it is caused by the short wavelength of light being scattered in the atmosphere is an excuse, used to provide an answer but not the reason.
Why would the short blue wavelength be scattered and not the longer wavelengths, when the shorter a wavelength is, the greater its energy and the more penetrating it is?
If the cause is the scattering of light, why, when you see the horizon of the Earth is there a sharp delineation between the black sky and blue sky but when in space when you look at the light coming from the Earth at night it is not blue?
If the gases in the atmosphere, nitrogen, oxygen, and argon, do not absorb or reflect visible light what is causing the scattering of the blue light?
The correct answer for why the sky is blue is that it is a result of how the nitrogen and oxygen in the atmosphere absorb and transfer energy coming from the sun. What wavelength of energy an object absorbs is determined by its bonds and size and what wavelength of energy the object emits is also a function of these characteristics.
Both the length of bonds of a molecule and the molecule’s size vary and objects do not absorb and emit “a” wavelength but a range of wavelengths. As the atoms of a molecule absorb energy they flex contracting and stretching changing the length of the bond and the size of the molecule.
A molecule will absorb energy both internally, by the atoms forming its bonds and as a molecule as a whole. When it absorbs enough internal energy the vibrations of the atoms across the bond exceeds the attractive force between the atoms forming the bond and the molecules splits.
Oxygen and nitrogen absorb energy from the x-ray and ultraviolet spectrum of light emitted by the sun. Since oxygen, with a double bond, is weaker than nitrogen with its triple bond, it will split into atoms before all the nitrogen bonds are broken.
This is why in the atmosphere above 100 km, where the energy from the sun is greatest, the percentage of nitrogen in the atmosphere decreases and the oxygen, in the form of oxygen atoms, increases. It is also the reason why in the upper atmosphere nitrogen and oxygen combine to form nitrous oxide.
What wavelength of energy the bonds, atoms, and molecules radiate depends on the rate of vibration across a bond and the velocity or energy of the objects as a whole. This energy is radiated in all directions and equalized with surrounding objects that are able to absorb these wavelengths.
As the altitude of the atmosphere decreases the density, or number of molecules, increases which results in the energy being distributed to more molecules and the velocity of those molecules decreasing. As the velocity of the objects decreases, the amount of energy emitted decreases and the wavelength of the energy increases.
In the stratosphere the absorption and radiation of energy by oxygen and nitrogen begin to convert ultraviolet light in blue visible light. Since blue light is not absorbed by the gases the progression of increasing wavelengths ceases with no green, yellow, orange, or red being produced and the sky is blue.
The sky is blue because the gases in the atmosphere absorb and radiate energy from the ultraviolet spectrum and converting it into visible blue light.
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Finn McCool
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Hi Herb,
You know that you now have to explain why the sky is red at night 🙂
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Charles Higley
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Whatever happened to Raleigh scattering in which light is absorbed and re-emitted, as it passes through the molecular micro emitters, is completely canceled in the lower atmosphere but incompletely in the thinner upper atmosphere where blue light, which is bent more than other visible light wavelengths, is incompletely canceled and also explains why blue light comes in from all directions from the sky. I have not heard anything that negates Raleigh scattering and requires the more complicated, complex explanation above. Occam’s Razor.
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Herb Rose
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Hi Charles,
I thought that molecules only absorb light of certain wavelengths and that oxygen, nitrogen, and argon do not absorb light in the visible spectrum including blue.
Isn’t it also true that the sky in the upper atmosphere is black and the blue is confined to the lower levels of the atmosphere?
Herb
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Jerry Krause
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Hi Herb,
The title of your posting asks: “Why Is The Sky Blue?” At the end of your posting you conclude: “The sky is blue because the gases in the atmosphere absorb and radiate energy from the ultraviolet spectrum and converting [convert] it into visible blue light.”
In Chapter 32. Section 5 (Scattering of Light0 of ‘The Feynman Lectures On Physics’, Richard Feynman taught his Caltech students an alternative explanation.
I suggest you read what he taught and explain why you consider your explanation is correct and that his explanation is not.
Have a good day, Jerry
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Herb Rose
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Hi Jerry,
When light is scattered it changes direction. When the scattered light is redirected it strikes other objects at different angles and again is scattered This would result in some light being redirected into space and the difference between the blue light striking the top of the atmosphere and the blue light striking the surface of the Earth would be significantly different than the difference of red light. The visible spectrum passes through the atmosphere without significant interference while 93% of the uv disappears in the atmosphere.
Herb
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Jerry Krause
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Hi Herb,
Maybe I was not clear. Do you claim that there is no natural phenomena, as described by Feynman, which involves the scattering of light (photons) by tiny particles of matter from the size of gas molecules up to the size of the tiny cloud droplets of the white clouds which we commonly observe from the ground by looking upward and from an airplane by looking downward?
Have a good day, Jerry
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Herb Rose
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Hi Jerry,
Scattering can be either from reflection or by absorption and re-emitting. Water droplets do both so they redirect light in all directions. If a gas molecule reflected blue light then the gas would appear blue.from the direction the light was coming from. If a gas molecule absorbed and re radiated blue light it would appear blue from all directions. Think of blue paint and a blue crystal. The N2, O2, and argon in the atmosphere neither reflect or absorb visible light because the wavelength is too long, just as they don’t absorb or reflect radio waves, so they do not scatter light.. Water droplets do absorb and reflect light, scattering it (rainbow) because of the size of the droplets.
If you had ever done any Scuba diving you would know that you need a flashlight to see the true colors of corals because water absorbs the longer wavelengths (reds) while the blue penetrate deeper..
Have a good day,
Herb
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Jerry Krause
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Hi Herb,
Consider the following from ‘follow mosasic-experdition.org.
Feb 6, 1894. 79o57’N. Nansen’s diary comment was: “A strong sun-glow above the horizon in the south: yellow, green, and light blue above that: all the rest of the day sky deep ultra marine. I stood looking at it, trying to remember the Italian sky was ever bluer; I do not think so.”
Jan 8, 1894 78o58’N. “I got quite a start when I came on deck and saw a strong red light just above the edge of the ice in the south. It twinked and changed color; … It was Venus, which we see today for the first time, as it has till now been beneath the horizon. It is beautiful with its red light.”
Feb 6, 2020. 87o37’N. “Nansen’s diary entry of Jan 8 inspired us already a month ago to have a look at the stars and planets we can expect to see as we were keen to spot Venus.”
What they saw relative to both Nansen’s Venus and twilight word descriptions,, can now be seen in a MOSAiC photograph.
Have a good day, Jerry
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John Nicol
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Correct Jerry. Like most of the contribution attempts by Herb, this is another which is totally physically wrong. It is not difficult though beyond the limits of this reply, to show conclusively that Rayleigh scattering is proportional to the inverse of the fourth power of the wavelength. It is NOT a quantum mechanics phenomena. It is a “classical Physics” phenomena dependent on the polarization of the charges in a molecule which causes radiation according to the classical laws associated with an accelerating charge – the acceleration in this case being provided by the incoming radiation.
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Jonas
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Hi John,
I do have exactly the same view as you regarding Rayleigh scattering.
I would be interested to have your comments on CO2 and earths outgoing radiation.
I think one can apply exactly the same equations as for Rayleigh radiation. The effect of greenhouse gases should be scattering – not “backradiation”.
Some of the scattered radiation will come back to earth.
I think it would be refreshing if people talked about scattering instead of “backradiation”. Scattering is a defined concept in physics. Backradiation is not.
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Ross Nixon
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The sky isn’t blue, it just appears that way because our eyes pick up that colour more strongly. The sky is actually slightly more into violet.
I googled the interwebs and confirm that:
“In the visible spectrum, blue light has a wavelength of 450-495 nm while violet has a shorter wavelength of 380-450 nm. Violet light does indeed get scattered more than blue light, however the lower portion of the spectrum for sunlight consists of blue light and eyes are much more sensitive to blue light than violet light.”
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Herb Rose
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Hi Ross,
That would fit with the hypothesis as ultraviolet would also be lengthened to violet.
All molecules absorb radiated energy. We know O2 absorbs uv and creates O3. Why wouldn’t longer waves, like blue light just pass through the O2 without being affected just as infrared, radio, and shorter waves like gamma do? Scattering involves an interaction either by reflection, absorption and refraction, or absorption and re-radiation in all directions. Do blue spotlights become blurrier than red spot lights with increasing distance?
Herb
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T. C. Clark
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Never ignore red light because blue light may appear in your rear view mirror. Whatever happened to Indigo light….it used to be nestled in there between blue and violet but seems to have gone away? Feynman also explains with quantum mechanics…I have to go with Feynman on why the sky is blue…and sunsets reddish.
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Herb Rose
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Hi T>C>,
Sunrise and sunset red light I don’t know.
Do you believe that N2 and O2 absorb uv?
Do you believe that since they absorb energy they must radiate energy?
Do you believe that shorter wavelength have more energy than longer wavelengths and doesn’t this require that as a molecule loses energy the wavelength it emits must become longer?
These premises lead me to the conclusion that absorbing and emitting of energy would cause increasing wavelengths.
Herb
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T. C. Clark
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Feynman talks about electrons and photons as well as molecules and atoms. The reddish sunsets are ROY….red orange yellow…or lack the blue light which has been largely scattered away by more atmosphere being in the path of sunlight near sunset than at midday.
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Herb Rose
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Hi T.C.,
They are launching infrared telescopes into space so they can see further into space than visible light telescopes. They need to be above the atmosphere so the IR isn’t absorbed by water.The reason you can see further with IR is not because it has more energy and can travel further but because the longer the wavelength the fewer things things there are that will absorb that band of radiation. (radio telescopes another example) Is the lack of blue light due to scattering or because more particles are able to absorb shorter wavelengths?
When the sun or moon are on the horizon they appear larger even though they are not. During a super moon it appears even larger than a normal full moon even though the atmosphere the light travels through hasn’t changed.
When you watch a sun setting it approaches the horizon then appears to pause on the horizon before setting quickly, even though the rotation rate of the Earth doesn’t change.
There is something other than scattering that cause these optical illusions.
Herb
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Jerry Krause
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Hi Herb,
Optical illusions are just that–illusions. So there is no physical effect; hence no cause.
Have a good day, Jerry
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T. C. Clark
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Feynman said that it can get very complex. He tried to offer explanations that can be understood by more people. In astronomy, you get the red shift from objects in deep space traveling away from the observer and infrared does penetrate dust clouds better than visible light. The human eye has its shortcomings but we make devices that turn invisible radiation into visible. You can view lots of Nightvision products on Youtube….hunters attach cameras and show feral hog hunting etc.
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jerry Krause
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Hi T.C.,
Thank you for sharing. I hope it isn’t an illusion that I follow your comments.
Have a good day, Jerry
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Herb Rose
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Hi T.C.,
I believe night vision is a result of amplification of light, not turning invisible light into visible light.
As for red and blue shift, why does some light have both a red and blue shift? Is the source both moving towards and away from us?
Herb
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T. C. Clark
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Herb, I suggest you search Wikipedia/night vision. Some night vision products are not available to the public….only military/police. There are autos with infrared cameras to aid night driving. The Germans had a night vision infrared sniper system in WW2 but it required an illumination infrared source so if the enemy had an infrared detector, the sniper was an illuminated target…..ooops. Never heard of your mysterious light that both red and blue shift….ask Jerry.
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Bevan Dockery
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Do You lot mean to say that the science is not settled ?
How am I to explain the results that I achieve in analysing climate data if the physics keeps changing? Then again, it is always going to be changing, isn’t it ?
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Manuel
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Quite the interesting topic for discussion. Science is forever attempting to confuse the masses. Look to the word of God and all will be clear.
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