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Q & A: Why is the sky blue?

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Most recent answer: 07/03/2013
Q:
Why is the sky blue?
- Jose M. Aponte (age 9)
Chicago, IL USA
A:
Light is what's called an electromagnetic wave. It pulls any particle that has electric charge back and forth as the wave passes by, sort of the way that a water waves pushes things up and down as it passes by.  These waves pull back and forth on the electrons in the molecules in the air, because the electrons have electric charge. When the electrons move back and forth, they send out new waves, spreading out in all directions. Those waves are the light we see from all parts of the sky.

Light from the sun is white, which means it's made up of a whole range of different colors. Each color corresponds to some rate at which the electric field wiggles back and forth. Blue is the fastest back-forth wiggle we can see, red the slowest. We say blue is the highest frequency and red is the lowest frequency.

So why does the high frequency blue light get scattered around by the air more than the low frequency light? The reason has to do with how the moving electrons respond to the waves and radiate new waves. Because the electrons are not heavy and are stuck tightly in their molecules, the distance that they wiggle depends only on the strength of the wave hitting them, not on its frequency, at least for visible light. For a given distance of wiggle, you get more radiation the faster the wiggle happens. So the high frequency blue light scatters more, and that's mostly what you see coming from the sky on a clear day.


It's actually a bit more complicated if you worry about details (like physicists do). If you are interested, have a look at




Mike W. (with a link from EJ)

(published on 10/22/2007)

Follow-Up #1: Blue skies and red sunsets

Q:
I read the answer to "why is the sky blue?", but then why are sunsets and sunrises red? Professor Lewin suggests that rays of light have to pass through more air in the evening and morning. If the atmosphere is roughly spherical, why is there more air to pass through? Also, if blue light scatters the most, which makes the sky blue, shouldn't it also scatter the most at sunrise and sunset?
- Joe M (age 50)
Waynesboro, VA
A:

Hi Joe,

Here's a crude picture I made in paint which shows how light from the sun has to pass through more air when the sun is near the horizon (aka, near sunset).

The composition of the earth's atmosphere is fairly constant, so blue light scatters the most all day and all night. So why does traveling through more air lead to a red sky, not a blue sky? Wikipedia () gives some details, and one of my favorite images of all time:

File:Why is the sky blue.jpg

Here, inside a single gemstone, we see blue skies and orange sunsets. What is this magical gem? Oh right: blue glass. What's happening here is simple: the glass scatters blue light strongly in all directions. So whatever angle you look at it from, it looks blue. Longer wavelengths, like red and orange, don't scatter much at all. Therefore, they pass right through, as you can see in the picture (the shadow tells you where the light source is, so you can verify that the orangle light all passed straight through).

Similarly, in the atmosphere, the sky looks fairly blueish most of the day because the blue light is scattered in all directions, including into your eyes, while the red light from the sun passes straight through the atmosphere, hits the earth, and is absorbed. It looks redish during sunsets because the blue light is scattered so strongly that most of it doesn't even make it through the atmosphere, instead being absorbed in the sky as heat or (reflected back to outer space). In this case, the red light is all that gets through, so the sky becomes both darker and redder.

Hope that makes sense now!

On a related note, some people wonder why the sky doesn't look purple (since shorter wavelengths scatter more, and purple's wavelength is shorter than blue's). I think the answer is simple: the human eye is much better at seeing blue than it is at purple. But I've never actually tested this: it would be fun to look at a blue sky with a spectrum analyzer!

David Schmid


(published on 07/03/2013)

Follow-up on this answer.