Blue Skies Ahead

Question: Why is the sky blue?

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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 photonsindividual 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 pigmentslike 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.

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Courtesy NASA

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:

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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.

About Schools and Being Educated

As the holiday weekend comes to a close, the day that millions across the country dread silently approaches. That’s right. For many, the first day of school approacheth.

The undeniable fact is that far more students dislike school than enjoy it. This is more than just about any fact or statistic depresses me to no end. It shouldn’t be this way. There is no one party responsible for this—neither the ungrateful students nor restrictive schools are entirely to blame. Both sides share responsibility, and both must make concessions to fix this.

First, let’s address the schools.

The concept of fairness and equality in education is a great and noble goal. Unfortunately, the practices associated with that principle often fall far short of their goals. Many policy makers mistake “the same quality of education for all,” with “the same education for all,” which has disastrous effects for everyone involved. This where restrictive and inflexible curricula and standards that favor rote memorization over true and fundamental understanding come from.

Standardization is one of the worst things to happen to education, for the simple reason that only measurable artifacts can be standardized. But learning and understanding often can’t be quantized across a large and diverse population. In today’s environment, schools and teachers are implicitly instructed to favor the product over the process simply because products are easier to measure.

But in education, the process is all that matters.

Especially in the increasingly digital world we live in today, the memorization and regurgitation of facts is becoming an antiquated skill. While I would prefer that students be able to recite from memory Newton’s Second Law or the date Magna Carta was signed (F=ma and 1215, in case you were wondering), today it’s so easy to look things like that up on smart phones that there seems to be no point in even bothering to memorize them. I’ll address the issues with that later, but it’s certain that no one’s life or career will ever depend on their ability to recite trivia from memory any more, if at all they ever did.

Schools should focus not on how much students can memorize, but on how well they can use the resources presented to them. School should be a place for the enrichment of thought and cultivation of understanding, not a place to sit for hours on end and have facts jammed into your head.

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The rigid structures of the school day, the emphasis on shallow assessments, and the passive way in which all too many students are taught to learn are slowly but surely killing education. If we want to thrive as a country and as a culture, they need to change. That said, there’s no simple solution to this problem, no matter which politicians or policy-makers say otherwise.

Now for the students.

You don’t know how lucky you are. That’s not your fault, and I certainly didn’t when I was your age. Given the way things are now, it seems impossible that a little over a hundred years ago there was no mandatory education.

If you go to school, I don’t care what happens in the rest of your life you are privileged. Less than 1% of the human population has ever had access to the kind of education at your fingertips every day. I’m not trying to make you feel guilty, but think about that for a bit. For most of human history, the opportunity to sit in a classroom and do nothing but learn was restricted to only the wealthiest and highest born children. That’s how far we’ve come.

You are privileged because you’re not forced to spend every minute of every day struggling to support yourself and have enough food to make it through the day. Even if you work part-time jobs, the fact remains that five days a week you are given a time and place in which to only think about exploring the world around you and discovering yourself.

Because that’s your primary job as a student. As you are now you’re incomplete. Take the time during the school week to figure out who you are and what you want. Learn to write, learn to think, learn to explore and be self-sufficient.

Yes tests can be tedious and frightening, and sure the rules and regulations can seem restrictive and counterproductive, but that will always be the case, even after you’ve graduated and moved on. Now is your chance to learn how to deal with that pressure while being free from the many responsibilities that will saddle you when you’re older.

If you find yourself wondering when exactly you’ll use the things you’re learning in school, maybe you never will. That doesn’t detract from the value of having learned them. A secondary education is more about learning how to learn than about the particular facts and formulae you end up learning. You’re learning skills, practicing on what might be trivia so that later you can use them on things that will be more directly relevant to your life.

And never forget that your education isn’t free, even if you attend public school. We, the adults of your community, pay through taxes for you to receive the best education possible. This isn’t charity, and it’s not for your benefit. It’s for ours. We are investing in your education so that you can go on to do great things that will make our lives better and longer in the future. We pay for you to be educated so that you can cure the diseases that would otherwise kill us as we age, or clean up the disaster we’ve made of the planet so we don’t choke to death on all the smog, or even so you can invent the device that heats up butter just enough to make it spreadable but not enough to melt it.

No matter who we are, there will inevitably come a day when we need you, and for that reason we want you to be as prepared as possible for when that day comes. Remember that the next time you’re debating the value of the things you’re learning in school.

By all means enjoy yourselves, but also take full advantage of this time in your life where your major responsibility is to learn and grow and develop as a human being. Trust me, it won’t last nearly as long as you think it will.