Pro-Human Extremist

Extremism in the defense of humanity is no vice

Why is the sky blue?

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I wrote my post on ten questions about the physical world on a whim, because it had been almost a month since my previous post, and I was backed up with exams at the end of our fall semester, and wanted something I could get out quick. But having done so, I figure I really ought to provide quick explanations to save people the trouble of hunting them up themselves. I’m also intrigued by the challenge of writing brief but reasonably complete answers to these questions.

Why is the sky blue? The reason it’s not black is because light reflects off the molecules of the atmosphere. Without that atmosphere, the only light we’d see in the sky would be what comes straight from the sun, stars, and planets, and the sunlight reflecting off the moon. The rest of the sky would be as black as at night, even though everything around us on earth would be lit up as bright as day.

But thankfully there is an atmosphere, and some of the sunlight passing through the layer of nitrogen and oxygen gas surrounding the earth hits the molecules of that gas and reflects off at an angle. We see the light that reflects at just the right angle to strike the light-sensing cells in our eyes. Enough light is reflected to light up every corner of the sky—not nearly as bright as the sun itself, but enough to make it appear far from black.

Why is it blue? The different colors we see are a result of how much energy is in different photons of light. Higher-energy photons look blue to us, lower-energy photons look red, and the colors of the rainbow between red and blue represent a range of energies from lower to higher.

Clouds look white to us because the tiny water droplets in them reflect all photons equally, so the light coming to us from clouds contains all the colors of the rainbow, and all colors taken together appear white. If the molecules of the atmosphere also reflected all photons equally, the whole sky would look white to us. But since higher-energy photons are more likely to reflect off the molecules of the atmosphere than lower-energy photons are, the sky looks blue.

Why do clouds look yellow and red at sunset? When the sun is close to the horizon just before it sets, the sun’s light is passing sideways through the atmosphere, which means it has to pass a greater distance through the atmosphere before it gets to our eyes. The atmosphere surrounding the earth is extremely thin relative to the diameter of the planet, so when the sun is right on the horizon, its light actually does pass through much more atmosphere than when it shines down from overhead.

As light passes through more atmosphere, more of the higher-energy photons are reflected in other directions, so the light that is left has a relatively greater number of lower-energy photons. The round orb of the sun that we see is made up of the photons that have come straight from the sun to our eyes. It turns more yellow and then orange as it sets because more of the higher-energy photons have been stripped out of the beam of light by reflecting off of molecules in the atmosphere. And since clouds reflect photons of all energies about equally, some of the yellow-orange light that strikes them reflects in our direction, making them look yellow-orange. They turn from yellow to orange to red as the sunset advances because more and more of the higher-energy photons are stripped out of the beam of light as that light passes more sideways through the atmosphere, as the sun sinks lower in the sky.

© Joel Benington, 2012.


Written by Joel Benington

January 3, 2012 at 4:15 pm

Posted in science

Tagged with , ,

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