Why is the sky blue and sunsets red

Believe it or not, I get asked this question all the time. While most people assume they know the answer, the truth is, we often don’t remember what we learned as a kid (or we never learned it). A profound question indeed, and not as simple as you might expect. Well, let’s get to it!

Índice Show

What makes a red sunset?
Is the sky blue on other planets, too?
Related Resources for Educators

The reason the sky is blue has to do with a concept called Rayleigh Scattering. Let’s say we have a gas, and we shine light on it. The light will hit the gas molecules, and get absorbed. Then, the gas molecules will emit the light back out at the same wavelength (color!), but in a new direction. Interestingly enough, blue light (shorter wavelength) gets absorbed more than red light (longer wavelength). This scattering of incoming light is Rayleigh Scattering!

Have you figured it out yet? Now imagine the sun, shining light towards the Earth. Remember, this is white light, which means it contains light of all visible colors at once! As the white light from the Sun hits the Earth’s atmosphere (a gas!), the molecules absorb and emit the blue light, and scatter it all around. So when you look up at the sky, you’re seeing all of this scattered blue light. But why is the sky paler near the horizon? At the horizon, light has to pass through more of the atmosphere, and in the same way blue light is scattered towards you, it ends up getting scattered away again, and so the other wavelengths are the ones that get through, since they are less affected by scattering. Here’s a nifty diagram I found at http://www.sciencemadesimple.com/sky_blue.html :

(As you can see, the blue lines represent the paths of blue light beams)

This horizon effect is the same one that causes the sunset! As the sun is setting on the horizon, the light once again has to pass through more of the atmosphere in order to reach you, and so the blue light gets scattered away, and the redder light is what reaches you. But why are some sunsets more spectacular than others? In addition to the atmosphere itself, dust particles and water particles in the atmosphere can contribute even more to the scattering of shorter wavelength light beams, and so you get a dazzling display of red, orange, and yellow hues. The more particles in the atmosphere, the more variety you can end up with–and so the sky will look like a majestic painting.

So why is this cool? Indeed, it’s nice to know why the sky can be so beautiful, but it’s not the only reason Rayleigh scattering can be cool. A friend of mine recently asked me why the Moon was so red, even though it was not a lunar eclipse (that’s a whole different cool phenomenon). When you see the moon, you’re really seeing sunlight that’s reflected off of the moon’s surface. When the Moon is near the horizon, or if there are lots of these particles in the air, the light reflected off the Moon gets scattered the same way sunlight is scattered during the day. This causes the Moon to appear the color of the sunset! For a fantastic picture, as well as fun facts, visit http://apod.nasa.gov/apod/ap050922.html

Hope you enjoyed my first entry! For more information, I’d advise checking out the two sites I mentioned in more detail. Of course, a google search should bring you to some great websites as well. Stay tuned for more cool science!

This entry was posted in Astronomy, Physics and tagged light, moon, scattering, sky, sunset. Bookmark the permalink.

A lot of other smart people have, too. And it took a long time to figure it out!

The light from the Sun looks white. But it is really made up of all the colors of the rainbow.

When white light shines through a prism, the light is separated into all its colors. A prism is a specially shaped crystal.

If you visited The Land of the Magic Windows, you learned that the light you see is just one tiny bit of all the kinds of light energy beaming around the universe--and around you!

Like energy passing through the ocean, light energy travels in waves, too. Some light travels in short, "choppy" waves. Other light travels in long, lazy waves. Blue light waves are shorter than red light waves.

All light travels in a straight line unless something gets in the way and does one of these things:—

reflect it (like a mirror)
bend it (like a prism)
or scatter it (like molecules of the gases in the atmosphere)

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

Closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead. As the sunlight has passed through all this air, the air molecules have scattered and rescattered the blue light many times in many directions.

Also, the surface of Earth has reflected and scattered the light. All this scattering mixes the colors together again so we see more white and less blue.

What makes a red sunset?

As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.

Sometimes the whole western sky seems to glow. The sky appears red because small particles of dust, pollution, or other aerosols also scatter blue light, leaving more purely red and yellow light to go through the atmosphere.

Is the sky blue on other planets, too?

It all depends on what’s in the atmosphere! For example, Mars has a very thin atmosphere made mostly of carbon dioxide and filled with fine dust particles. These fine particles scatter light differently than the gases and particles in Earth’s atmosphere.

Photos from NASA’s rovers and landers on Mars have shown us that at sunset there is actually the opposite of what you’d experience on Earth. During the daytime, the Martian sky takes on an orange or reddish color. But as the Sun sets, the sky around the Sun begins to take on a blue-gray tone.

The top image shows the orange-colored Martian sky during the daytime and the bottom image shows the blue-tinted sky at sunset. Both images were captured by NASA’s Mars Pathfinder Lander. Credit: NASA/JPL

Our World: Sunsets and Atmospheres

The sun produces white light, which is made up of light of all colors: red, orange, yellow, green, blue, and violet.

Light is a wave, and each of these colors corresponds to a different frequency and therefore a different wavelength of light.

The colors in the rainbow spectrum are arranged according to their frequencies: Violet and blue light have higher frequencies than yellow, orange, and red light.

When the white light from the sun shines through the earth’s atmosphere, it collides with gas molecules. These molecules scatter the light. The shorter the wavelength of the light, the more it is scattered by the atmosphere. Because its wavelength is so much shorter, blue light is scattered approximately ten times more than red light.

In addition, the frequency of blue light, compared to red light, is closer to the resonant frequency of the atoms and molecules that make up the air. That is, if the electrons bound to molecules in the air are pushed, they will oscillate with a natural frequency that is even higher than the frequency of blue light. Blue light pushes on the electrons with a frequency that is close to their natural resonant frequency, which causes the blue light to be re-radiated out in all directions in a process called scattering. The red light that is not scattered continues on in its original direction. When you look up in the sky, the scattered blue light is the light that you see.

Why does the setting sun look reddish orange? When the sun is on the horizon, its light takes a longer path through the atmosphere to your eyes than when the sun is directly overhead. By the time the light of the setting sun reaches your eyes, most of the colors of light have been scattered out. The light you finally see is reddish orange.

Violet light has an even shorter wavelength than blue light: It scatters even more than blue light does. So why isn’t the sky violet? Because there’s just not enough of it. The sun puts out much more blue light than violet light, so most of the scattered light in the sky is blue.