Why is the sky blue?

Why is the sky blue? The short answer is that when light from the sun hits our atmosphere, the light is scattered. Blue light has the shortest wavelength so it bounces around the atmosphere. This phenomenon is known as Rayleigh scattering.

Blue Sky

The long answer

Light from the sun is white as it hits the Earth. Tiny particles in the atmosphere scatter the light, blue and violet light have shorter wavelengths, letting it bounce around. Red light has a longer wavelength so it travels further. So the blue light essentially gets trapped while the Red light continues moving. During sunrise and sunsets you are seeing the Red wavelength that traveled further. The blue light has scattered away at this point.

Sun position

When the sun is directly overhead the light shining down is basically white, since the light travels the shortest distance, most of the light makes it to you. As the sun moves away from you the light begins to scatter. Since blue light scatters more, the sky becomes blue. As the sun gets even further away the blue light doesn’t quite make it to you so all you see is the red light that did make it through. Violet light is actually shorter than blue light. But it’s harder for our eyes to see. In reality, the sky is purple during the daytime but our eyes don’t perceive it. Violet light is a little too far into the ultraviolet light spectrum for us to see fully.

What is light?

visible spectrum

Visible light is just electromagnetic radiation that is between 380 to 750 nanometers long, so that we can see it with our eyes. Light leaving the sun is white. Which means it consist of all the colors of the visible spectrum. As you can see by the image above the light spectrum is massive. We only see a tiny portion of the light being emitted by the sun.

What color is the sky on other planets?

Mars sunset

If you look at photos from the moon landings you will notice that the sky is black. This is because there is virtually no atmosphere to scatter the light. On mars the sky is mostly red but if you were to look towards the sun, the sky would be blue around it, as pictured here. Venus has an orange sky due to its thick atmosphere.

NASA explains Why the sky is blue

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