Why Is the Sky Blue?

I wonder...

Find It Out

As shown below, the mixture of different colors of light makes a very different result than if you were going to mix watercolor paints or your crayon colors. With light, a mixture of all the colors makes white light. The sun pours white light all over the earth, meaning the sun shines every color of light. We just can't see each individual color because it looks white to us. (This is why when you filter the light through a prism or rain, you can see all the colors in a rainbow.) Since the sun shines all the colors, we can see every color in the sunlight. Red shirts look red and blue jeans look blue. When you see a red shirt, you see red because the shirt reflects only the red beams from the sun and absorbs the blue, yellow, green, and all the other colors.

In this same way, the particles in the sky absorb the blue beams from the sun the most. Each different colored beam, or wave, has a different amount of energy. The addition or subtraction of energy makes some waves short and some waves long. The blue light has the shortest wave, so it is scattered the most by the particles in the air. Why, then, does the sky look blue instead of purple if violet has the shortest wavelength. Violet wavelengths are actually too short, and although we can see violet, our eyes are too sensitive to see this in air molecules, so we see the blue scattered the most. Also, the main three colors our eyes process are red, green, and blue. (This is why if you look very closely at the colors on an old-fashioned tv set, the little particles that make up the shapes are red, green, and blue.) Because of this, it is much easier for our eyes to see blue than it would be for them to see purple.

You might be wondering why the sky is no longer blue when there is a sunrise or sunset. Well, during a sunrise and sunset, the sun is farthest away from you. The farther the original white light is away from you, the more the light is scattered by the time it reaches your eyes. By the time you see the light, the blue light has been scattered so much that it is actually scattered away from where you are looking (where the sun is setting). Because the blue light is no longer visible, the remaining colors are the warmer colors- reds, oranges, and purples. This is why you see these colors when you look at a sunset.

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The Light Color Wheel

The light color wheel is very different from the paint color wheel that you learned about in school. In the light color wheel, a mixture of all the colors makes white light.

Try It Out

Blue Sky - Red Sunset

Sunset in the Water

To show your students how blue light is scattered more than the others, fill a tank with water and put a bit of milk in it. Mix it up and have the students shine a white light through the water. They will see when looking at the water from the side that the blue molecules scatter the light the most (are pushed farthest away from the light towards the edges). You will also see when looking at the light dead on that the milky water turns red or orange as the blue light is scattered away.

Prove it with a Prism

Take the kids outside and refract the sing light through a prism to make a rainbow, or take a field trip on a rainy day with your rain boots and see if they can find one themselves. Point out that the blue light (or purple) is refracted (or bent) the most. This is why the blue is scattered the most- it has the shortest wavelength! They may ask why the sky doesn't look purple then- simply, our eyes are less sensitive to violet light, so the sky looks blue to us.

Make Your Own Color Wheels

It's easy to make a color wheel with paints. Most students have learned how to do this, and if they haven't, it's quite easy to learn. Show your students how to mix blue and yellow to get green, etc. This may be a breeze for some who already know their colors. Encourage the students to see what color it makes when all of the colors are mixed (should turn out to be a neutral grey). Once everyone feels solid on the paint color wheel, explain that the light color wheel is different. Allow the kids to experiment with putting different colored gels over flashlights to see what colors they make. Allow them to hypothesize on why the color mixtures are so different than paint colors, and see if anyone realizes that the mixture of all the colors makes white light. Many students are new to the idea of light mixtures, so they may need some guidance for this one. It's always fun to have hands-on exploring with new concepts!