At-Home STEM Activities: Why is the Sky Blue?
This week we’re diving into wavelengths as we focus on optics and spectra. Let’s start off with something that you may have wondered about, the color of the sky. You might think it has something to do with our atmosphere, and you’d be right- it’s a result of the interaction between our atmosphere and the light from the Sun. But how does it work?
The gases that make up our atmosphere contain atoms moving around energetically. When they hit a light wave, they reflect it- this scatters the light in all directions. That’s why we don’t see a black sky during the day- the Sun’s light is reflected within our atmosphere- but do see one at night when the light from the Sun is gone from the sky.
Since blue light is a shorter wave than red, it has a higher frequency and so has a better chance of encountering an air molecule. Image courtesy of NASA
So why a BLUE sky? This has to do with the size of the gas molecules and the different wavelengths coming from the Sun. Since oxygen and nitrogen atoms are very small, smaller than the Sun’s wavelengths, they have a much greater chance of meeting and reflecting the shorter wavelengths than the longer ones. So indigo, blue, and green are most commonly reflected- the result is that we see a blue sky which ranges from deep blue when the air is clear to paler blue when there are particulates like water droplets, pollution, etc.
You can recreate this scattering of blue light at home with just a few things:
-a clear container for water
-water
-milk
-a spoon or something else to stir
-a flashlight (I used the app on my cell phone)
Fill container with water
Holding the flashlight on one side of the container, stir in milk a little at a time, until you can see the light beam coming through the water *this takes less milk than you think- the first time I tried this, I put in too much and that made it difficult to see anything
Notice how the color of the milky water changes from near the flashlight to the far side- it goes from blue at one end to yellow at the other. You’re seeing blue because those shortest waves are being reflected by the atoms in the milky water, and then yellow at the far side because the red, orange, and yellow wavelengths are the ones most often making it to that point. *Although this worked for me when I held the flashlight to the side, it was much more obvious when I placed the flashlight underneath the jar and compared the bottom with the top.
