Why is the Sky Blue?

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Why is the Sky Blue?

Why is the sky blue has been asked many times by many people for a long, long time. Most people are curious to this question, or even when the sun sets, why is the sky red?

When we see rainbows,

Rainbow Sunlight

Rainbow Prism Effect

we see all the colors coming from the Sun as the raindrops act as tiny prisms when lit up by the Sun which bends the light and separates it into the different colors.

Why is the Sky Blue?

But why are there all those different colors? The visible light that you see day to day is just a small part pf all the kinds of light energy beaming around the Universe, and even around you! Similar to energy passing through the ocean, light energy also travels in waves.

light, wavelengths

Visible Light

The differences in light is primarily due to the wavelength, or its range of wavelengths. Visible light, or the light we see includes the wavelengths that the naked eye can see. The longer wavelengths visible to the naked eye look red, while the shortest wavelengths are blue or violet.

The wavelengths in this image are not to scale as they are to small to see. Red light waves are about 750 nanometers, while blue or violet wavelengths are about 400 nanometers. A nanometer is one-billionth of a meter. A human hair is about 50,000 nanometers thick! So these visible light wavelengths are very, very small and not easy to graph for learning purposes.

Another important thing to understand why the sky is blue is about light. Light it travels in a straight line unless something gets in the way to either reflect it, like a mirror, bend it similar to what a prism or raindrop does, or scatter it, such as molecules of gas do in the atmosphere.

As the white light from our Sun enters our Earth’s atmosphere,

sunlight scattering

Sinlight Scattering

much of the red, yellow, and green wavelengths of light (mixed together and still nearly white) pass right through the atmosphere to our eyes. The blue and violet waves, however, are just the right size to get absorbed by the molecules of gas in the atmosphere, then spit out again, but in all directions. The air molecules scatter the blue light and allow other colors to pass through the atmosphere.

So what happens to all the “non-blue” wavelengths?

sunlight scattering

Sunlght Scattering on the horizon

They are still mixed together, unscattered by the atmosphere, so they still appear white. The scattered violet and blue light dominates the sky, making it appear blue. What happens to the violet? Some of the violet light is absorbed by the upper atmosphere. Also, our eyes are not as sensitive to violet as they are to blue.

As you switch from viewing the sky down closer to the horizon, the deep blue sky coloring fades to a lighter blue or white. The sunlight that reaches us from the horizon passes through much more air than the sunlight reaching us from directly overhead. The molecules of gas have re-scattered the blue light in so many directions so many times that less of the blue light reaches us.

So instead of our sky being green, orange, purple or green, that is why is the sky blue.

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