Why Is the Sky Blue?

Contributor: Lindsey Congalosi. Lesson ID: 13114

Learn the answer to one of life's biggest mysteries! And then ask what if the sky were purple ... or red? What even is the color red? Why can it be so hard to see? So many questions. Get the answers!


Earth Science, Physical Science

learning style
Auditory, Visual
personality style
Beaver, Golden Retriever
Grade Level
Middle School (6-8), High School (9-12)
Lesson Type
Dig Deeper

Lesson Plan - Get It!


It's one of the first questions that many of us ask as children.

  • And yet how many people really know the answer?
  • Why is the sky blue?

Because it is!

Learn the real answer to this question, so you can impress the toddlers in your life (and everyone else)!

  • Why is the sky blue?

The simple answer is that it's not. Take a look at the image below.

blue sky

  • Looks blue, right?

Right. The sky looks blue.

But it isn't.

  • Confused yet?

Don't worry, you're not alone.

  • Colors are something most people see thousands of times a day, but what exactly is blue?
  • How does color even work?

To learn about color, we have to start with light. Light is energy. It's a form of electromagnetic energy. Electromagnetic energy is a type of energy that is reflected or released (emitted) from an object. Electromagnetic energy travels through space in the form of waves.

Watch Best Of Science explain The Electromagnetic Spectrum in this video:

The image below is the electromagnetic spectrum. It shows the different types of electromagnetic energy.

  • Can you see what the difference between each type of energy is?

electromagnetic spectrum

The factor that determines which type of electromagnetic energy is present is the wavelength of the light, or the distance between each wave.

Using the diagrams below, drag each of the types of electromagnetic energy pictured in order from largest wavelength (more spread out) to shortest wavelength (more smooshed together).

  • Why is wavelength so important when discussing energy?

The wavelength of a certain type of energy determines not only which kind of energy it is but also what it can do and how it can be used. As humans, our eyes are only able to see a small portion of the electromagnetic spectrum, which we call the visible light spectrum.

visible light spectrum

Notice that this image is the mirror image of the previous graphics. Here we have the longer radio waves on the right side of the spectrum. This doesn't change the meaning of what you're seeing in any way, but you do need to be mindful of how the particular graphic you're looking at is set up in order to read it correctly.

You've already ranked the types of electromagnetic energy by wavelength; now do the same thing with the colors of visible light in the diagram below.

Watch Light: Crash Course Astronomy #24 to learn more about the different forms of light and how we experience color:

  • If the sky appears blue to our eyes, then which wavelength of light are we seeing?
  • In other words, does blue have a relatively long or short wavelength compared to the other colors?

Now that you know what color is, you will need to understand how we see color. Light can be reflected or absorbed by objects. Depending on the object, different wavelengths (aka colors) of light are reflected from the object to your eye. The other colors are absorbed.

Take a look at the image below.

green plant

  • What color is the plant?

Assuming that you are not color blind, you can probably see that this plant is green.

  • But why does it appear green?

The plant is illuminated by sunlight. Sunlight appears to be a sort of yellowish-white, but it actually contains all colors. When the sunlight reaches the plant, all of the colors are absorbed except for the electromagnetic energy that our eyes perceive as green, which bounces off the plant and travels to our eyes.

Hence, a green plant.

green forest

Back to our blue sky. You may have correctly assumed by now that our eyes are seeing the sky as blue because of the wavelengths of light coming from it.

  • We've now answered part of our question: What is blue?

To figure out exactly why the sky is blue, though, we have to break down our question even further.

  • What is the sky?

Head over to the Got It? section to learn the answer to these questions and get a little bit closer to figuring out our original mystery: Why is the sky blue?

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