I get a great satisfaction from being able to answer my son’s questions about the world around him. As you’ve probably figured out, I love explaining scientific concepts to anyone interested in learning them. But just because I understand a concept doesn’t mean it’s easy to explain. I’ve known lots of brilliant scientists that were absolutely horrible teachers. Part of the problem is that sometimes even simple questions require an understanding of many other concepts in order to grasp the answer. A question as simple as “why is the sky blue?” requires that we ask – and answer – a few other questions before we can understand the answer we are seeking. Sometimes we need to rephrase the question itself in order to help us approach an answer, so let’s do that here. What we are really asking is “why is the Earth’s sky blue and not some other color?” If we understand the answer then we should be able to understand why robots on Mars show that the sky on that planet is red, because truly understanding a concept in Science can lead to understanding something else entirely.
To understand why Earth’s sky is blue we first need to understand a few things about light and how we Earth-born creatures see. Our eyes are natural cameras; in fact, the eye is where scientists got the idea for lenses and cameras in the first place. In our eyes (and the eyes of most Earth creatures) are molecules that react to three different colors of light: blue, green, and red. Our eyes measure the relative brightness of each different color and our brain processes that into colors that are not purely blue, green, or red. For instance, orange is mostly red but with a bit of green light mixed in, so our brain balances the amount of red and green and identifies orange. Our eyes see these three primary colors for a simple reason: our eyes evolved on this planet. Our Sun does not emit every kind of light, and it emits some colors more than others. Our eyes evolved to make the best of the exact spectrum – or relative amount of each color – of light that our Sun gives off. From space, our Sun appears pure white because our eyes are perfectly capturing all the colors given off, and our eyes have evolved so that all those colors in that exact abundance appears white since that will best illuminate our world. Imagine if our world always had a red tint to it, it would be difficult for us to distinguish predators that might be hunting us because everything would look red. Worse, what if we only saw in black and white? Earthly eyes evolved to make the most of the light that we have.
The light from our Sun appears white in space, but if you go outside and steal a quick look at the Sun you might notice it has a reddish yellow color. The reason that the Sun appears slightly red on the Earth’s surface is the same reason that the rest of the sky appears blue.
If we shine white light into a prism we get a rainbow out the other side. This is because all those colors are part of that white light. In white light all those different colors are traveling in the same direction. Different colors carry different amounts of energy, which is why staring into a bright blue light seems more intense than staring into a bright red light – the blue light has more energy. A prism makes the different colors go in different directions depending on their energy. An effect like this is called “scattering.”
Rayleigh scattering occurs when light of a specific energy hits molecules or dust particles in the air. Depending on the size of the particles the light can either continue unaffected or bounce off in a different direction. For a given size particle, higher energy (blue) light scatters much more efficiently than lower energy (red) light. When the Sun’s white light hits the oxygen (O2) and nitrogen (N2) molecules that make up a big part of our atmosphere, the higher energy (blue) light scatters away. This blue light continues bouncing around the sky, hitting air molecules and dust particles, some of it eventually ending up in our eyes. Meanwhile, the blue light having been removed, the remaining light from the Sun takes on a slightly reddish color.
What I’m describing is the picture when the Sun is high in the sky, but what about when the Sun is low to the horizon like at dusk and dawn? The Earth is roughly spherical, with our atmosphere forming a thin spherical shell around it. When the Sun is rising the light reaching our eyes must travel through more atmosphere than when the Sun is high overhead. Think of the Earth’s atmosphere as an Olympic swimming pool, which is three meters deep and fifty meters long. If you dive in one end and open your eyes under the water you can see the bottom of the pool without much trouble. Now look at the far wall of the pool, it is hazy because there is a lot of water between you and the wall that the light has to get through, which it can’t do without significant scattering off the water molecules.
Although the red color in sunlight scatters much less efficiently than the blue color does, given enough atmosphere the red light can scatter just fine. When the Sun is low on the horizon there is enough atmosphere that some of the red light spreads out into the sky, causing that sunset glow that we’ve come to expect as humans. There is nothing about our Universe that guarantees that a sunset must be red, and the fact that our sunset is a different color than our midday sky is due to the sensitivity of our own eyes, the spectrum of light from our Sun, and the makeup of our atmosphere. There isn’t anything special about the colors we get, we’re just lucky that they are pretty.
Mars receives the same light from the Sun that Earth does, though it’s dimmer by the time it gets to Mars because that planet is about one and a half times as far from the Sun as we are. The sky above Mars is red because dust particles in the much thinner Martian atmosphere are just the right size to very effectively Rayleigh scatter red light, so that color comes out predominantly. The rocks on Mars have a natural reddish brown color because of their makeup, and since the dust in the atmosphere is the same stuff that adds to the color. With so much red on Mars, any creature that evolved eyes there would almost definitely see a much wider range of reds then we Earthlings need to survive. As a result, blue as a color would probably be entirely invisible to them. In fact, the colors that we define are distinctly Earthling. Any creature that evolved eyes on any other world would not see red and blue as we do; they might not even see these colors at all. They might see purely in shades of infrared, like the Predator. (I presume that’s how the Predator sees. Frankly, I can’t make out anything in the Predator-vision scenes.)
Earth’s sky doesn’t always appear blue though. In places with high air pollution Rayleigh scattering off smog particles can give the sky a red or yellow tint. This effect became one of those things that clued people in to the idea that they were doing something bad to the environment, leading to emissions regulations in most developed countries. Though smog particles can also enhance the red light Rayleigh scattering at dusk giving a much grander sunset, so I guess it’s a matter of perspective.
The sky isn’t the only place we can see Rayleigh scattering. The picture at the top of this article is of a piece of opalescent glass in sunlight. Light scattering off the molecules in the glass give it a blue tint, while the sunlight streaming through takes on a red tone. You are seeing the exact same physical effect in that chunk of glass as you see when you look at the blue sky!
The next time you look up at that beautiful blue dome above us, remember that what you’re seeing is light of a specific energy bouncing around molecules too small to see until it finally enters your eye, which converts that energy into a signal the shoots into your brain, which in turn interprets it as a color that we agree to call blue. There is a lot of science flowing through that blue sky, and I think that makes it all the more beautiful.
You’re welcome to disagree with me on that last point, but I hope that I’ve turned you around to my way of thinking.
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