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Why is the Sky Blue?

Inquirying into Why SunLight is Clear and the Sky has Color

Jun 20, 2008

Understanding how the different wavelengths of the colors in sunlight causes the sky to appear blue as it enters our atmosphere, is an important science concept.

A common question every parent and teacher has heard from children at some point; why is the sky blue? Learning the correct answer is important. Because once children hear an answer several times, right or wrong, this answer becomes embedded in their brain. Convincing them otherwise, if they learned the wrong answer, takes a lot of evidence to undo the wrong.

Common Answers to Why the Sky is Blue

Children come up with a variety of answers when asked why the sky is blue. Their answers are a direct result of what they hear from peers, parents, and movies or television. Here are a few common answers children give:

The sunlight reflects off the oceans
The sky is blue because it is the bottom of space
Because of all the water in the sky
The sky reflects off the top of clouds which contain water

Sunlight has Colors?

The sky appears blue because the molecules of air in the atmosphere scatter the blue portion of light as sunlight passes through the atmosphere.

This answer will illicit another response from children – but light is clear because you do not look blue!

It is true light is clear, because of all the colors which make up sunlight. At this point, children will want proof. This can be proved by using a prism to separate the clear sunlight into the colors of the rainbow – ROYGBIV.

You have probably already predicted the next question children will ask. This question is, why isn’t the sky red, yellow, orange, green, etc.? This is typical and expected from children who are now questioning the evidence being presented to them. They are conducting inquiry-based science. For more about inquiry-based science visit Understanding Science Inquiry.

Inquiring into ROYGBIV

Children need to understand the colors of the rainbow have different wave lengths. This will help them understand why only blue light is scattered by the atmosphere. One way to help them understand is to have them do the following:

On a sheet of paper write ROYGBIV vertically for the colors of the rainbow, starting at the top left and going down the left side of the paper.
Next have them draw one complete wave about 3 inches wide after the R for red.
Next have them draw the another complete wave a little shorter after the O for orange.
They continue drawing waves a little shorter width every time all the wave down to the V for Violet.
The final wave width for violet should be about 1/2 inch wide.

When you describe shorter and longer wavelengths they have a picture to look at, which they personally drew. This is important for inquiry-based science teaching and learning – personal experience.

Understanding Why the Sky is Blue

Now describe how as light passes through the atmosphere, most of the sunlight gets through. However some of the blue light is scattered by the air, because its wavelength is shorter than the ROYG colors.

The next question is why nor Violet, because its wavelength is shorter than blue?

The answer to this is the human eye is strongly sensitive to red, green, and blue. This is where color vision for humans is derived. Humans cannot see violet very well; this is why the sky is not violet.

Connecting the prism and wavelengths is important to understanding why the sky is blue. Children need this evidence to support internalizing the new infomation they learned about sunlight and why the sky is blue.

The copyright of the article Why is the Sky Blue? in Homeschooling is owned by David R. Wetzel. Permission to republish Why is the Sky Blue? in print or online must be granted by the author in writing.