why is the sky blue and the sunset 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! 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! PAs 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
PP (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! PFor a fantastic picture, as well as fun facts, visit 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 greatPwebsites as well. PStay tuned for more cool science! When I was a kid I heard that the sky was blue because it was the reflection off the ocean. В The light from the Sun, which appears white, is actually made up of all colors of the rainbow, when the light enters the atmosphere the colors can become separated (Imagine looking at light through a prism).


В We know that bluer light travels in short, tight waves while redder light travels in longer waves. The shorter the wavelength, the more likely the light is to bounce off of an air molecule and become scattered. Blue light is scattered most in our atmosphere. I also heard, when I was a kid, that when you observe a sunset and you see the shift in color from blue to red that youвre actually seeing the sunвs rays being filtered through the pollution. В Well, that sounds dismal. В Itвs also not entirely true. One of the main factors in determining a sunsetвs color is the Earthвs atmosphere. The atmosphere is made up mostly of gases as well as some other molecules and particles thrown in for good measure. В The most common gasses in our atmosphere are nitrogen (78%) and oxygen (21%). The remaining single percent is made up of water vapor and lots of tiny solid particles like dust, soot ash, pollen, and salt from the oceans. В There are also trace gasses like argon present. Also, depending on where you live, youвll have to factor in that volcanoes can put large amounts of dust particles high into the atmosphere and pollution can add different gases or dust and soot to the air as well. The atmosphere of the Earth can be thought of like a filter on a camera lens. Light from a light bulb or the Sun may look white, but it is actually a combination of many colors. When you see light filtered through a prism youвll see this white light split up into its separate colors, i. e. wavelengths.


В White light is the colors of the spectrum blended into each other. And a rainbow that you see in the sky is actually a natural prism effect as rain drops split those different colors up. В The colors have different wavelengths, frequencies, and energies. Violet has the shortest wavelength. Red has the longest wavelength. В The shorter the wavelength means the more powerful the frequency. So letвs put it all together in how light acts in the air surrounding our planet. Light moves in a straight line until it is messed with (be it gas, dust, ash, etc. ). Once something interferes and gets in the way of the light wave itвll scatter that light in different directions. The probability of light to be scattered by a molecule is proportional its wavelength, so shorter wavelengths of light are scattered much more often than longer wavelengths. In the case of air molecules, the molecules are much smaller than the wavelength of the scattered light, this is called. As the Sun sets later in the day, the light becomes less and less direct, think of what causes your shadow to be longer in the afternoon than during mid day. During mid day, sunlight is shining almost directly down through the atmosphere, while at the end of the day it is shining through more atmosphere. As the white sunlight travels through more atmosphere, more of the shorter wavelength colors are scattered away from our line of sight. Until finally as the Sun is about to set below the horizon, only red (the visible light with the longest wavelength) remains.

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