Blue Sky

Why is the Sky Blue?

We’ve all wondered: ‘why is the sky blue?’ Perhaps you’ve googled it before. But, I doubt you’ll have had an explanation quite like this…

Bienvenu to this, my first blog post. Bet you didn’t know I spoke French did you? Why not check out my About page to find out some more and then come back?

To those who kept reading, well played. To those who have just come back from the about page, I apologise for the shameless plug. As you may now know, I don’t speak French, although I am trying to learn.

Right, I seem to have strayed from the topic a little bit there, where were we?

Ah yes, the sky. Why is it blue? Well, let’s find out:

(P.S. If you want a summary, then you can scroll to the end 🙂 )

Background

Light is an electromagnetic wave, as I’m sure you know. This means it comprises an electric field oscillating perpendicular to a magnetic field. For the sake of this argument, we neglect the effect of the magnetic field. This is justified since the magnetic field is much weaker anyway.

We will be talking a lot about dipoles in the following, so what are they?

Well, an electric dipole is simply a positive charge and a negative charge separated by some (small) distance. Note, I add small since two charges very far from each other would have little effect.

Electric Dipole with Field Lines
A simple electric dipole; the black lines represent the electric field between the charges.

These dipoles can radiate light if the distance between the charges changes, or if the strength of the charges changes.

Why is this? From Maxwell’s equations, we know that a changing electric field will lead to a propagating electromagnetic field, thus radiating light.

If you’re unfamiliar with Maxwell’s equations, Wikipedia has a very in-depth article here.

So, if the size of the charges or the distance between them changes, then so will the electric field between them, which will radiate energy.

The atmosphere

Now, Earth’s atmosphere is made up of mostly Oxygen and Nitrogen molecules.

As you might have guessed from my discussion of dipoles, the incoming sunlight turns these overall neutral molecules into dipoles; it polarises them.

This is because the electric field of the sunlight moves the positive and negative charges in opposite directions, creating a separation between them.

But, since this electric field is oscillating, then the distance between the charges is always changing, thus these dipoles radiate.

Light Scattering off of Air Molecules
The scattering of light off of the air molecules in the atmosphere.

Dipole Scattering

It can be shown that these dipoles scatter with power according to the following diagram (black line):

Graph of Scattered Power Distribution
Graph showing the scattering intensity for air molecules. The combination (black) is strongly peaked at blue wavelengths.

Notice that the overall effect is the combination of the sun’s scattering (blue line), which is strongly peaked near yellow/orange (this is why the sun is that colour), and the dependence on the wavelength of scattered light (red line).

If you don’t know what that symbol next to wavelength is, it’s the Greek letter lambda. It’s the physics symbol commonly used for wavelength.

This results in a strong peak at the blue part of the EM spectrum (black line); this is why the sky is blue.

The scattering of light off of particles this way is known as ‘Rayleigh Scattering’.

Red Sunsets

You may have looked up at the setting sun, not directly I’d hope, and wondered, “why is the sky turning red?”

Well, think about it like this. At midday, the Earth faces the sun at right angles, which, as shown below, results in a very short path for the light to travel.

(Obviously, the Earth can’t “face” the sun at an angle if it’s virtually a sphere, but its tangent at the location in question will be normal to the light. So there, you don’t need to go to the comments to expose me.)

This means that there is not much scattering, and hence the light at midday is a bright blue.

In the evening however, the Earth is at an angle to the sun, which means that the light travels a lot further.

(Look, I’m sorry if you’re offended by me using the term “at an angle” loosely, but it’s much simpler to understand in my opinion. If you don’t like it, then here: the tangent to the Earth at the location in question is almost parallel to the sunlight.)

In this distance, it encounters many more air particles, and so the light scatters more than at midday.

Light Travels Further in the Evening
The length that the light travels in the evening (right) is longer than at midday (left).

The result is that most of the blue light is scattered away, and the remaining, unscattered light reaching us is the leftover red light.

Summary

I will summarise this post, and remove some of the more technical jargon as follows:

The light from the sun is scattered off of particles in the Earth’s atmosphere.

This scattering, known as Rayleigh Scattering, is strongly peaked in the blue region of the spectrum, and so the light which reaches us is mostly blue.

In the evening, the light comes in non-perpendicular, so the light travels a longer distance in the air, which leads to more scattering.

This scatters off the blue light, and the remaining light which reaches us is in the red end of the spectrum.

Well, that’s it, my first post. I hope you liked it, and that you learnt something.

If you have any suggestions, or anything’s unclear, please feel free to leave a comment or get in touch with me here.

Have a browse round my other articles if you found this interesting, I’m sure you’ll love those too.

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