Why is the sky blue? The simple answer

Why is the sky blue? The simple answer
Posted on: July 28th, 2023

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.

Why is the sky blue – The long answer

The sun emits white light, a mix of all colors in the spectrum, as it reaches Earth. This light encounters Earth’s atmosphere, filled with tiny particles, and undergoes a process known as scattering. This process occurs when the light rays collide with the atmospheric particles, causing the rays to spread out in different directions.

The scattering of light is more effective for shorter wavelengths, such as blue and violet, due to their higher frequency, allowing them to bounce around more within the atmosphere. On the other hand, light with longer wavelengths like red, orange, and yellow travels further because they scatter less.

While both blue and violet light scatter more, the sky appears blue to us, and not violet. This is due to a couple of factors. One, the sunlight reaches us more at the blue part of the spectrum, near violet, and second, our eyes are more sensitive to blue light and less sensitive to violet. On top of that, some of the violet light gets absorbed in the atmosphere and then re-emitted in different directions, reducing its direct visibility.

Sun position

This phenomenon is most pronounced when the sun is at its zenith, directly overhead. The sunlight then has to travel the shortest distance through the atmosphere to reach us, minimizing the amount of scattering and thus allowing most of the light spectrum to reach us, which we perceive as white light.

However, as the sun’s position changes and it moves closer to the horizon during sunrise or sunset, the sunlight has to traverse a longer path through the atmosphere. This increased distance intensifies the scattering effect, with the short-wavelength blue and violet light getting scattered out of our line of sight. What remains are the longer wavelengths—red, orange, and yellow—that have undergone less scattering. This is why we often see these warm colors painting the sky during sunrise and sunset.

Our sky appears blue during a clear day due to the scattering of sunlight by the atmosphere. The blue color of the sky is the result of a particular type of scattering called Rayleigh scattering, which favors shorter-wavelength light, such as blue and violet light, over longer-wavelength light, such as red and yellow. However, because our eyes are more sensitive to blue light and because sunlight reaches Earth’s surface more at the blue part of the spectrum, the sky appears blue instead of violet.

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.

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