Question: Why is the sky blue?
Answer: It isn’t. Not the same way that bluebirds, sapphires, and the members of Blue Man Group are, anyway. To see why, we need to first talk about what exactly we mean when we say that something is ‘blue’ or ‘red’ or any color at all.
To talk about color, we must first talk about light. Light, at the smallest scale, is broken up into photons, individual particles or pieces of light. Those photons each have a certain amount of energy, and a corresponding frequency, that determines their color. In physics, there are only six colors in the visible spectrum (red, orange, yellow, green, blue, and violet) with many more outside what we humans can see (radio, microwaves, infrared, ultraviolet, x-ray, and gamma). Red light has a lower frequency than yellow light which has a lower frequency than blue light.
So where do the other colors come from? If all we have are those six colors to choose from, how do we get browns and pinks and even white? What may surprise you, especially if you’re artistically inclined, is that you get brighter, lighter colors by adding multiple colors together. White light is light which contains frequencies of all the other colors at once. That is why you can see every color when in a room lit only by white light–the white light contains all the other colors. Notice how this is different than what you get if you add together different pigments, like paints. If you add pigments together you end up with a dark, blackish color. We’ll talk about why these two are different in a second, but first we must discuss what it means for an object to “be” a certain color.
For the most part, color in the natural world is a reflective property. This is because most objects don’t emit their own light (some things, like butterfly wings, are colored by diffraction but that’s a different story entirely). They only reflect light that hits them, redirecting it into our eyes. However, most objects don’t reflect all of the light that hits them, only certain frequencies or colors.
When you look at an object and see white, it means that object is reflecting all colors of light back at you. Black objects reflect little to no light (this is why wearing a black shirt in summer makes you warm). When you see a blue object, it means only blue light is reflected off of it, with all other colors being absorbed.
When we say an object is blue, we mean that it only reflects blue light. A blue object absorbs every color except for blue. This is why mixing pigments gives you darker colors. Each pigment only reflects certain ranges of colors, and if these ranges don’t overlap, less light will be reflected, giving you a darker, gray or black color.
With all of this said, we can now return to our original question: What color is the sky? To answer this, we need to know what exactly the sky is. And for the most part, it’s air. Mostly nitrogen, about 20% oxygen, some carbon dioxide, and other trace gases. It’s the same stuff that surrounds you all the time. It’s the same air that you’re breathing right now.
So look around. What color is the air around you? What colors of light are being absorbed by the “empty” space in front of you? Unless you’re living in a heavily polluted area, the answer should be clear. Literally.
Air is transparent. And thus, so is the sky.
Revised Question: Why does the sky appear blue?
Answer: You might think that was all a little bit pedantic, but the distinction between the sky looking blue and the sky being blue is crucial to understanding why the sky looks the way it does. The sky isn’t colored by reflection the way most other objects are. It’s a different process entirely, one you may have experienced yourself without realizing.
What lights up the entire sky is the sun. And the sun, despite what you may think from looking up at it here on Earth, is white. Not yellow, not orange, but white.
What you’ll notice in the picture above is that, when viewed from space, the light from the sun doesn’t spread nearly as much as it does here on Earth. In fact, even with the glare, you can see the total blackness of space as little as ten degrees away from the sun.
What allows the sun to color the entire sky is a process known as Rayleigh Scattering, and it’s exactly what it sounds like. If you think of light as a stream of tiny particles (or photons), then you should be able to imagine those particles colliding with things like molecules in our atmosphere. As a result of those many collisions, the light gets scattered, spreading out in all directions so that, no matter where you look, you can see it.
If you have ever played with a laser pointer you’ve experienced this before. Shine a laser pointer at a wall and all you’ll see is a red dot. You won’t be able to see the beam itself. But give the light something thick like smoke or fog or chalk dust, and the beam will scatter off of it, becoming visible to all.
So the scattering of light can explain why the sky has color, why it’s not just pitch black around the sun. But why is it blue?
I will try to stay away from doing too much math (there was enough of that in my last post), but there is an equation which governs the amount of scattering that occurs via this process:
It’s a messy formula, and if you’d like more information about what each variable represents you can check here, but the important part is the dependence on frequency. The higher frequencies of light scatter more than the lower frequencies. This is why you can see blue and violet all over the sky, but red and yellow only when you look directly toward the sun. The lower frequencies don’t scatter as much.
Only a few more questions to clear up. If higher frequencies of light scatter more, why is the sky blue instead of violet? The answer is because the sun emits more blue light than it does violet. The violet does scatter, but it is overwhelmed by all the blue light around it.
Why is the sky red during sunrise/sunset? At these times, when the sun is reaching the horizon, the blue light scatters too much, having to pass through too much of the atmosphere to reach us. Once all of the blue and purple light has been scattered away, all we’re left with are those beautiful red, orange, and golden sunsets.
Well, this was a fun one. What I like about questions like this is how trying to answer one question opens up–and then answers–many more. For example, you may have thought that it would be simple to explain why the sky is blue, but in the process of doing so we got to talk about what color is, how things get their color, and the scattering of light in a medium. These are the kinds of questions I like most, where the answer takes a winding road through different areas and disciplines. It’s because of questions like this that I started teaching. Unfortunately, given our prescriptive and standardized attitude toward education, it’s rare that kids be given the chance to really explore a topic like this in school.
That about wraps it up for this week. If you have any other questions you want answered, let me know.