Why is the Sky Blue?

Physics Explanation

Parents and teachers have traditionally made up lots of reasonable explanations.   Adults are supposed to know the answers to such simple questions, right?   Unfortunately, all of the explanations that I have ever heard are at least partly incorrect.

The correct answer is:   Only because of a remarkable coincidence.

The Earth's atmosphere is primarily made of Oxygen and Nitrogen.   It happens that both these elements are made up of atoms of about the same size.   It also turns out to be true that everything has lots of empty space between and around the atoms that it is made of.

Our Sun produces light which is actually a mixture of light of a lot of colors (called a spectrum, another subject).   You can prove this with a prism, which separates the colors of what we normally call white light or sunlight.   These various colors are actually just different sizes (wavelengths or frequencies) of waves of radiation (light).   Our eyes only recognize the total of all this light, and so it appears to us to be a bright yellow, almost white color.

It turns out that red light in the sunlight is the lowest frequency and therefore is the biggest wave (yet another subject!).   These big waves happen to be around twice as big as the size of the waves of blue and violet light also in the sunlight. When these big waves pass through the earth's atmosphere (or any other collection of oxygen and nitrogen molecules) they are not affected very much by the relatively small atoms of Oxygen and Nitrogen; so most red light continues straight on through.   (They are able to go around the atoms more than bumping into them.)

The blue (and violet) light in the sunlight is the highest frequency (of what we can see) and therefore has the littlest waves.   These smaller waves happen to be around half the size of the red waves just discussed.   When these small waves of blue light pass through the earth's atmosphere, many of them eventually "crash into" the relatively large atoms of Oxygen and Nitrogen. They tend to "bounce off" (are reflected or scattered) and then they go all directions including sideways and down toward us.

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A more scientific way of saying this is to say that the Oxygen and Nitrogen atoms are of a size that has a "natural vibration rate" (called frequency) that is closer to the rates of vibration of blue light. So the blue light can cause those atoms to start vibrating.   The vibrating atoms then give off blue light when they stop vibrating, and that new blue light can go in any direction.   (It's a little more complicated than that, but the general idea is correct.)

The really complete explanation of this is called the Rayleigh Theory of Scattering!

As sunlight is passing through the atmosphere horizontally above you, the RED light tends to go straight through, while the BLUE light tends to get bounced around (what is called "scattered") as described above. When you look up at open sky, you see this scattered light which is mostly BLUE.

Please notice that I keep saying things like "usually", "mostly", "tends", and not "always". Remember that there is a LOT of empty space between the atoms in the air. Some blue light CAN make it through a good distance of air. But, the farther that the light has to pass through air keeps making it more and more likely that the blue light will be scattered while a good amount of the red light will continue on through.

Why are Sunsets Red?

Just before sunset, when the Sun is very low in the sky, the light that has gotten to you has had to go through a LONG path through the earth's atmosphere. It went over the heads of a lot of people already, and some of the blue light was used up (scattered) making THEIR skies blue. More and more BLUE light was scattered out along the way, so the only light left which gets to you at sunset is primarily RED light. The lower the sun gets, the longer the path through the atmosphere, the more blue light scattered out, and the redder the Sun appears. (It does NOT actually change color!!) Since there is now virtually ONLY red light present, the sky also loses the brightness of the blueness of the sky. Now, when the remaining red light scatters (poorly) off the atoms, it makes the sunset sky reddish, especially clouds because they reflect (all colors of light) better.

White Clouds

Clouds include little condensed droplets of water. These droplets are a LOT bigger than atoms, and so they are not as transparent as oxygen or nitrogen gas. Therefore, ALL light colors tend to reflect off of those cloud droplets. During the day, that means that the light reflected off them appears bright white.

Dark Clouds

Normal good-weather clouds are only a few hundred feet thick. The droplets mentioned in the White Clouds section above are spaced fairly far apart. That normally allows some of the (white) light to pass between the droplets to then later bounce off the droplets farther inside the cloud. The normal droplet spacing allows some of the white light to get as far as droplets several hundred feet into the cloud. If the cloud is only that thick (and it usually is) then even the back side of the cloud appears white.

During storms, clouds become much larger and can be thousands of feet thick. If anyone sees the back side of such a cloud, or the underside of it, that cloud appears to be dark gray or black, depending on how thick the cloud is. It is actually no different than a "pretty, white" cloud, except thicker! It is interesting to note that this same dark cloud appears bright white to a distant observer who is on the same side of the cloud as the Sun is!

So it's only because Oxygen and Nitrogen atoms just happen to be a size comparable to the size of some of the waves of visible light, that the sky is blue! Neat, huh?

Q A question: If Oxygen and Nitrogen atoms happened to be a lot SMALLER, what color would the sky be?

(Answer: Nearly BLACK, while the Sun would look whiter than it does and there would be almost no dawn or dusk- it would just suddenly become day or night) The Oxygen and Nitrogen atoms would not be vibrated by any of the visible light, so nearly all would pass through, with very little being scattered by the sky. It would be likely that bright stars could be seen during the day!

Q Another question: If Oxygen and Nitrogen atoms happened to be a LOT BIGGER, what color would the sky be?

(Answer: Nearly BLACK again, with the Sun again seeming whiter than now.) The Oxygen and Nitrogen atoms would again not be vibrated by the various colors of the visible light, so again, all would pass through.

However, if the Oxygen and Nitrogen atoms were just a LITTLE bigger, the situation might be rather different. The sky might normally be reddish or whitish, with sunsets kind of bluish, and the Sun appearing sort of blue-white. The Oxygen and Nitrogen atoms would be vibrated by the RED light in the sunlight, with the blue light passing through.

The explanation of the BLUE SKY and RED SUNSETS are closely related.

BONUS: The oceans are blue primarily because they reflect the sky's blue light, which was scattered downward to it as discussed above. The water itself is not blue! (Yes, it contains some blue-green algae which add some color to it, but not enough for the impressive blue that our planet shows from space.)

ANOTHER BONUS: Have you ever seen an eclipse of the Moon? Did you notice that it looks very reddish then? You actually now know why!

Think about it!

The ONLY reason the Moon is not absolutely black during an eclipse is because the Earth has an atmosphere. As sunlight passes through the Earth's atmosphere, fairly close to the Earth, the light gets bent (the right word is refracted) because air has an Index of Refraction that is not exactly 1.0 like empty space has.

But now, think about the PATH of that light on its way to the Moon! It passed through thousands of miles of the Earth's atmosphere, because it had to pass so close to the Earth to even get to the Moon at all! And you learned above what happens to the different colors of light as it passes through an atmosphere of primarily oxygen and nitrogen, right? The blue light gets scattered out to appear as the blue sky, for a lot of people. And you already know what is then left. Only very red light! And THAT is the only light that is left to get refracted and then sent to the Moon during a Lunar Eclipse!

See how if you just think about things, you can figure out all sorts of interesting things? None of this was very difficult, was it?

First uploaded in May 1997.

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E-mail to: cj@mb-soft.com

C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago