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Why is the sky blue?
Well, I mean, why is the sky blue on clear days after sunrise and before sunset? You may also have wondered why it’s yellow and orange and red and yellow at dawn and dusk. Most people probably have, and I bet a lot of folks assume the answers to those questions are really, really complicated.
What if I told you that not only do those questions have the exact same answer, but that it’s not all that complicated and you can replicate it in your kitchen? It’s true! Here’s how.
Materials
- Water
- Milk
- Clear glass cup
- Flashlight
Process
- Pour some water into the glass cup. Don’t fill it up. We need room for some milk.
- Shine the flashlight through the sides of the glass. The water should be clear. If it isn’t, well, then you have bigger problems then not knowing why the sky is blue.
- Now, start adding milk just a tiny bit at a time. Keep the flashlight light shining through the glass.
- When you’ve added just the right amount of milk you’ll notice a change: the liquid will turn blue!
- Turn off the flashlight. Is it still blue?
- Turn the light back on and bask in the blueness of your marvelous milkwater. Imagine you are Luke Skywalker on Tatooine and Aunt Beru has just passed you a glass of blue milk.
Summary
If you really wanted blue milk you would typically add some food coloring. Look back up at materials. Food coloring is not on the list. So if you added some, go back and start again. That’s cheating. So if there’s no food coloring, and water is clear and milk is white, why does a mixture turn blue when a flashlight shines through it?
The answer is something called Rayleigh scattering, and it’s the same reason the sky, which is made of air (which you might remember is also generally clear), looks blue during the day. And also why it looks all sorts of different colors at dawn and dusk. I know that sounds weird, but bear with me here.
There are three important things to know to understand Rayleigh scattering. Firstly, all matter in the universe is made of invisibly tiny particles called molecules. Secondly, white light from the sun is actually a mixture of all of the colors of the rainbow. Thirdly, light has different colors because it has different wavelengths, with colors like blue and violet having short wavelengths and colors like orange and red having longer wavelengths.
Rayleigh scattering is simply this: when light hits certain kinds of molecules, the molecules knock the photons of light around and scatter them based on their wavelengths. In the case of your milkwater and the sky, well, milkwater and air molecules scatter short wavelengths (blue and violet) more than longer ones (everything else). That means that red, orange, yellow, and green move to the earth more quickly, while a lot of the blue and violet photons stay up in the atmosphere (or inside the milkwater) making it look blue. In the case of the milkwater, we didn’t use pure milk because it just absorbs all the light; mixing a small amount of milk into water gave us just the right amount of milk molecules to Rayleigh scatter the light.
It’s worth mentioning that our eyes aren’t nearly as good at seeing violet as they are at seeing blue; if we had more receptors for violet wavelengths, the shy would look more bluish-purple to us. If that doesn’t blow your mind, I don’t know what will.
So what about sunrise and sunset? It’s still Rayleigh scattering, but it’s an interesting case. When the sun is rising or setting, it’s not straight up in the sky, which means it has more atmosphere to go through. The extra air scatters the short wavelengths out completely, and all that’s left for us to see are the longer ones: most noticeably orange and red.
To quote from one of my very favorite science books, Bad Astronomy by Dr. Phil Plait:
“The light coming from the Sun is like stuff falling from a tree. Lighter things like leaves get blown all around and fall everywhere, while heavier things like nuts fall straight down without getting scattered around. Blue light is like the leaves and gets spread out all over the sky. Red light is like the heavier material, falling straight down from the Sun into our eyes.”
So the next time you look at a beautiful blue sky, turn to a nearby adult and say “That color is the result of Rayleigh scattering and a relative lack of violet color receptors in the human eye! Cool, huh?” You’ll totally impress the heck out of them.
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