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Why is the sky blue?

We know why the water in the ocean and at sea is blue but water in your hands or in a glass has no color. It is simply the reflection of the sky. (Although as Mike Lopez has mentioned, it’s not only that. Check his post also on why is the sea blue.) But why is the sky blue? Is there any material up there that makes the sky blue? But at night the sky is black and you can see far away stars.

I started my career as a chemist where I have a college degree in chemistry and also took up my master’s degree as well and I have this overall interest in science, although I know there are a lot of people that does not appreciate explanations that are too scientific that sounds like another language to them, I’ll try to explain this in the most simplest way I can.

Let’s talk about color first, all colors come from light. So no light, no color. Since light gives of a range of waves and each wave has a measure of wavelength. To help you imagine this, a wave is something that goes up and down moving to a certain direction, and the longer it takes for a wave to go up and back down has a longer wavelength and the faster it is, the shorter the wavelength.

Image taken from Wikipedia about waves.

So in light sources, or even sound sources, these waves come out. Although you do not see anything up and down coming out and even if it is not really going up and down, this is the best way to explain the behavior of light as a wave. (And we will not discuss in this article how waves behave as a particle, that is totally a different story.)

And our eyes are designed to only see waves of a certain wavelength, and these are generally at 400nm to 700nm, but there are some people that could see beyond this range and there are also some that see below this range (the color blind people). The unit nm is nanometers which is x10^-10 meters. Just think of the wavelengths to be very very small. Light from the sun or any light source, a lamp, flashlight, etc. gives of a range of waves in all wavelengths of the visible region (400nm to 700nm), and when this light hits anything, let’s say a red car, some waves go through, some waves are absorbed, some are diverted (refractions) and bounced back (reflections). So in a red car when light hits it, all wavelengths either go through or are absorbed but only the wavelengths of red are refracted and reflected which goes back to our eyes so we see red. So again, no light, no colors.

So when the sun shines on the atmosphere, what does that tell us? All wavelengths are absorbed or goes through the atmosphere and only the blue wavelengths are refracted and reflected so we see a blue sky. The atmosphere is predominantly oxygen (O₂ or oxygen we breath and O₃ or ozone) and nitrogen. So waves from the sun, when they hit oxygen and nitrogen molecules, the blue is refracted and reflected and scattered all over the sky. Let me try to explain this further, here are the range of wavelengths of colors I saved from Wikipedia:

You can see red has the longest wavelengths and violet has the shortest wavelengths. Now to picture this better, imagine the red wave as a person riding a bicycle in a zigzag fashion (a wave) and it takes a long time for it to move from left to right and back to left (long wavelength).

And imagine a person riding a bicycle in a zigzag fashion (a wave) and it takes a short time for it to move from left to right and back to left (short wavelength).

Now imagine them biking through a dense forest with a lot of trees (oxygen and nitrogen molecules) that they might bump into, each tree represented by a circle below.

If the red biker (red wavelength) and blue biker (blue wavelength) bike through the same forest with trees (sky with oxygen and nitrogen molecules), more red bikers will get to pass through and more blues bikers will probably bump a tree.

Now I hope you get the picture of why more blue wavelengths will hit oxygen and nitrogen molecules of the sky simply because their wavelength is shorter. And every time the blue wave hits an oxygen or nitrogen molecule, it gets reflected or refracted and is scattered all over the sky, so that is why we see the blue sky.

Why do we still see blue objects under the sun?

Question: Does it mean that If the waves with the wavelengths of blue are reflected and refracted by the sky, when the waves going down on objects we see on the ground should not have any blue?

Answer: No. Each oxygen and nitrogen molecule are very very far apart. Majority of the waves of all kinds all go through. But since the wavelength of blue is shorter, it has a higher probability of hitting a molecule in the sky.

Why don’t we see a purple sky?

Question: If the blue wavelength is short that increases the probability of colliding with a molecule in the atmosphere, why don’t we see a violet sky where violet has a shorter wavelength than blue and has even a higher probability of colliding with a molecule in the sky?

Answer: Most probably, the sky is more violet than we actually see. Violet is in the range of 380nm to 450nm. While blue is from 450nm to 495nm. But normal human eyes can see at a range of 400nm to 700nm. So we could only see 50 wavelengths of violet and 45 wavelengths of blue, and majority of the violet wavelengths you see are already at the near blue region. So we see more blue than purple skies.

Why is the sky red sometimes?

Question: During sunset and sunrise, the sky may appear red, or sometimes orange or yellowish. How come this happens?

Answer: The sun is farther away and sunlight has to pass through more layers of the atmosphere. To picture this, I made an illustration below:

Since at sunset or sunrise, sunlight passes though more amount of sky, it gives more opportunities for all the waves to hit more molecules of oxygen and nitrogen. That even the red wavelengths start hitting them also.

While I was on the plane flight from the United State to the Philippines, I was able to see the sunset at a higher view of about 40,000 feet just where the blue sky is separated by the red sky at the horizon, there is a small band of all colors of the visible region spectrum. In other words all colors of the rainbow, in a very narrow band of colors. Unfortunately, I left my camera in my bag which I did not have with me while at my seat on the plane. But on my flight back to America from the Philippines, I prepared my camera for sunset to get this spectrum, but unluckily, I had to go to the restroom near sunset and when I came out, sunset was over since sunrise and sunset occurs faster than normal in plane flights going on the opposite end of the earth since you add or subtract a day, depending on what direction you are going and you pass by sunsets and sunrises quickly, a bit faster than normal.

The original title of this post was: The Science Outside the Airplane Window. Where I supposed to also talk about: Why clouds are white? Why do many fluffy clouds have flat bases? What are jet streams? What is turbulence? But the post got so long and I limited it to why is the sky blue and I will just talk about the other things on some other post.

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