Why Is the Sky Blue?

Ask Science reveals the dark conspiracy behind the age-old question of childhood: Why is the sky blue?

Lee Falin, PhD
3-minute read
Episode #30

What I’m about to reveal to you may come as a shock, but the truth must go forth. Throughout your entire turbulent life, one fact has remained constant: the sky is blue. Songs have been written about it, poems have reveled in its beauty, and innocent children have asked what makes it so. But the shocking truth of the matter is: the sky is not blue.

To unravel this conspiracy, we first need to understand the nature of light. In my episode on Interspecies Breeding, I mentioned that scientists really get a kick out of classifying things, or putting things into discrete groups.

As I mentioned in the episode What is Radiation?, if you were to take all of the different kinds of radiation in the universe and arrange them from lowest energy to highest energy, you would have what scientists call The Electromagnetic Spectrum or EM Spectrum, for short.

Near the center of that spectrum is the chunk of radiation that scientists have classified as “visible light.” That section is further broken up into sections called “colors,” which consists of red, orange, yellow, green, blue, and violet.

The Ups and Downs of Radiation

When we think about radiation, we can think about it as either acting like a stream of particles or acting like waves. Just like traffic on a busy street, when a single car behaves (or misbehaves) or an entire stream of traffic behaves (or misbehaves). 

What makes red light different from blue light is something called wavelength. As electromagnetic waves travel through the universe, they do so as a series of peaks and valleys (technically these are called crests and troughs, but peaks and valleys are easier to visualize). The distance between a pair of peaks or a pair of valleys is called the wavelength.

What we call red light is radiation with a wavelength of between around 620 and 750 nanometers. Just to give you an idea of how small a nanometer is, a sheet of paper is about 100,000 nanometers thick. Blue light on the other hand has a wave length of between 450 and 495 nanometers.

It turns out that blue and violet light have just the right size wavelengths to run into the particles that make up our atmosphere. As the light from the sun travels through the atmosphere, the blue and violet waves bounce off the particles, which makes the sky look blue.

Wait a Second…

Now you might be wondering, if the particles in the atmosphere can scatter blue and violet radiation, why doesn’t the sky look violet? Well the big secret is that it does, only our eyes aren’t sensitive enough to separate those two colors when they’re blended together. In fact, if you were a bee, a bird, or even a butterfly, your eyes would respond to a different set of wavelengths than human eyes do, so the sky would appear to be a different color.


So now you know the big secret: the sky isn’t really blue at all. Obi-Wan was right, “Your eyes can deceive you, don't trust them.” If you’re interested in learning more about how the sensitivity of our eyes makes us think the sky is blue when it really isn’t, there’s a great paper by Glenn Smith from the Georgia Institute of Technology on the topic.

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Sky image courtesy of Shutterstock.

Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Lee Falin, PhD

Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech.