Why is the sky blue?

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

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 has shorter wavelengths, letting it bounce around. Red light has a longer wavelength, so it travels further. The blue light gets trapped while the Red light continues moving. During sunrise and sunset, you see 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 white; since it 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 shorter than blue light. But it’s difficult for our eyes to see. In reality, the sky is purple during the daytime, but our eyes don’t perceive it. Violet light is too far into the ultraviolet light spectrum for us to see fully.

What is light?

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

Off to the Infrared

On a side note, the longer light travels through space it moves into the infrared spectrum. For this reason, most space telescopes look at deep space with infrared. They can see older light that we can’t perceive with the naked eye. Most images from the JWST and Hubble telescopes are run through filters to bring their light back into the visible spectrum so we can see them.

What color is the sky on other planets?

If you look at photos from the moon landings, you will notice that the sky is black. The reason is that there is virtually no atmosphere to scatter the light. On mars, the sky is primarily red, but if you look toward the sun, the sky would be blue around it, as pictured here. Venus has an orange sky due to its thick atmosphere. One has to wonder what other beautiful skies there are in the universe.