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Why is the sky blue?

By: Juliana Rocha

Look out the window at the sky: What color is it right now? Looking up from the surface of our planet Earth, the sky takes on different hues depending upon the time of day. If the sun is out in the middle of the day, the sky will be blue; in late afternoon, the sky grows reddish; and at night, it’s simply black.

If you’ve seen photographs taken from outer space, you’ve probably noticed that the astronauts always see a very dark sky. So why do those of us down here see blue, orange, and red tones? Have you ever stopped to ask yourself why?

It’s because of how light is spread through the atmosphere. It might seem strange, but light is a form of energy that moves through space like a wave. That’s right: a wave! Except it’s a very tiny wave: to measure a wave of sunlight, for example, you have to divide a millimeter up into one thousand equal parts.

The popular saying “It’s not how big you are that counts” doesn’t hold true for light. Do you know why? Because the size of the wave described by this form of energy is precisely what defines its color. Shorter waves are blue while the longest are red.

Have you ever done an experiment with a prism? A prism is an object made of glass or crystal that serves to separate out beams of sunlight. You’ve probably heard that white light is a mixture of all other colors. Well, sunlight is white precisely because it is made up of wavelengths of all different sizes. With the help of a prism, we can see the different colors of beams that make up sunlight.

When sunlight reaches the Earth, it runs into an obstacle: the atmosphere—the huge mass of air that surrounds our planet. When these different lengths (and colors) of waves run into air molecules, they start spreading out in different ways. Shorter waves find it easier to spread. And what is the color with the shortest wavelength? That’s right: blue!

This same mechanism explains why the sky changes color. The atmosphere contains not only air molecules but suspended particles of dust as well. When these particles are smaller than the waves, they force the light to spread out even more. The blue waves spread so much that they end up diluted, letting us see longer waves like red and yellow.

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