Wednesday, September 14, 2011

Ask Dr Mama R: Why is the sky blue?

I have a preschooler who asks lots of questions. LOTS. As a scientist, I love trying to figure out how to answer in a way that is appropriate for him. Except when he doesn’t care about the answer and is asking “why” over and over just because it is fun for him, in which case I just frantically search for something to say that will get him to shut the hell up.

Anyway. Back to the fun questions. I really do like helping him with those. The trick is to give him just enough information to answer his question but not go over his head.

I expect that your kids might also have a bunch of good questions that might be hard to answer in a pre-school appropriate (or grade-school appropriate way), and I thought it might be fun to start a regular (or semi-regular, depending on how many questions your kids have) column here where I try to break it down for you. If you have some questions, please email them to DoctorMamaR (at) and I’ll get to work.

Today’s question (courtesy of N, age 2.5): Why is the sky blue?

Grown-up/High School Answer: To really understand this we have to start with light itself, which you can think of as a wave. Different colors of light have different wavelengths: blue light has a shorter wavelength while red light has a longer one. The white light that comes from the sun includes all of these colors.

Thanks, NASA

This white light passes through our atmosphere, which is full of tiny particles (air, water, dust, pollen, pollutants, etc). The incoming light is scattered (deflected from its original path) by these tiny particles via a phenomena called Rayleigh scattering. Rayleigh scattering affects light with smaller wavelengths much more than light with longer wavelengths. What this means for us is that blue/indigo/violet light is scattered, causing the sky to appear blue.

Grade school answer: The atmosphere contains a lot of really small stuff, like water and air molecules and dust and pollen particles. This stuff is way too small to block the light from the sun, but it is big enough to of get in the way. As the light from the sun passes through the atmosphere, the blue light bounces off of these particles while the rest of the light passes through. When you look up at the sky, you can see this blue light.

Preschool Answer: When light shines down from the sun, the blue light bounces off of the air and water in the sky, making the sky look blue.

Hope that was helpful!

Dr. Mama R has a PhD in Materials Science and Engineering, which she puts to good use teaching undergraduates and her two little kids. One of these groups is much more interested in boogers, both wonder why the sky is blue.


  1. I love this idea! My answer is always, "Because blue is Mama's favorite color." Egocentric much?

    Also, isn't it nasty how much undergraduates are into boogers? They need to grow up already!

  2. Carla, I like your answer way better than mine. I may steal it for the future. :)

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