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  • Why Is the Sky Blue?

  • In the middle of a bright, sunny day, the sky is usually a shade of blue.

  • Why blueand not purple, green, or orange?

  • Well, it's all because of how the Sun's light reaches Earth!

  • If you see light from the Sun hitting the ground, it probably looks like plain white

  • light.

  • However, that white light is made up of all the colors of the rainbow.

  • You can actually separate and see all these colors if you shine sunlight through a specially

  • shaped crystal called a prism.

  • Light energy travels in waves.

  • And different colors of light are created by different types of waves.

  • For example, when our eyes recognize the color red, what we're actually seeing is long,

  • stretched-out light waves with peaks that are far apart.

  • And when our eyes recognize the color blue, we're really seeing shorter, choppier light

  • waves with peaks that are close together.

  • The different colors of light are all determined by how stretched out the light waves are.

  • Together, all the colors make up the spectrum of visible lightthe light we can see.

  • So, why do we only see blue light when we look up in the sky on a sunny day?

  • The blue color we see comes from sunlight hitting Earth's atmosphere—a layer of

  • gases that gives us air to breathe and keeps the planet warm enough to live on.

  • When sunlight reaches Earth's atmosphere, it is scattered in all directions by the gases

  • in the air.

  • The types of gases in Earth's atmosphere mostly scatter the shorter, choppier waves

  • of blue light.

  • So, when we see a blue sky, we're really just seeing all of these blue light waves

  • scattering in our atmosphere.

  • But wait a minute: why is the sky a different color at sunset?

  • Well, as the Sun gets lower in the sky, its light is passing through more of the atmosphere

  • to reach you.

  • So, even more of the blue light is scattered away before it gets to you.

  • This allows more of the orange and yellow light to pass straight through the atmosphere

  • and directly to your eyes.

  • However if the whole sky is glowing red, it could be due to particles of dust, pollution,

  • or smoke in the air.

  • These particles also scatter lots of blue light, which can lead to a very red sky.

  • NASA's Earth-observing satellites monitor how many of these particlescalled aerosolsare

  • in our air.

  • The information from these satellites help forecasters keep an eye on particles in the

  • air and make sure the air is safe to breathe.

  • In general, a blue sky is good news.

  • And now you know why!

  • Find out more about our home planet at NASA Space Place.

Why Is the Sky Blue?

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Why Is the Sky Blue?

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    Sandra posted on 2021/11/02
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