Why is the Sky Blue?

Janice Wisen-Finnerty
Jefferson Elementary
Blaine, Minnesota
based on an activity by D. Gioffre, Hillsborough Middle School, Hllsborough, New Jersey www.engineeringplanet.rutgers.edu
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This hands on lab helps explain why the color of our sky/upper atmosphere appears blue in color. Using common household materials, students will be able to simulate how light from the sun is scattered by our atmosphere to create blue light.

Learning Goals

Students will learn that light is made up of all colors.
Students will learn that some colors go through the atmosphere and down to earth and that blue light is scattered by atmospheric molecules.
Students will use the skills of questioning, observation, recording, and drawing conclusions based on observations.

Context for Use

This activity is appropriate for third grade. Class size shouldn't matter. Activity could be done individually or in small groups. The activity consists of short lecture and an activity. The activity should take no longer than 30 minutes. No special equipment is needed. Experimentation making rainbows would be helpful so that children understand that light is made up of different colors which can be separated.

Description and Teaching Materials

Objective: Students will investigate why the sky is blue and the sunset is red.

Pose the question: Why do you think that the sky appears blue during the day and the horizon appears red/orange at sunset?

Have children write their hypothesis in their science journal/notebook.

Conduct a directed discussion about light answering these questions:
1. What is sunlight? (answers: heat, light)
2. Explain that the sun produces white light, which is made up of light of all colors: red, orange, yellow, green, blue, indigo and violet. Each of these colors has a different wavelength and can be separated. Help students make the connection to the rainbow spectrum and that the colors are arranged according to their wavelength. Violet, indigo, and blue light have a higher wavelength than red, orange, and yellow.

3. Ask if anyone has ever noticed how water bends light. Ask for examples. (If you need to you can put a pencil in a clear glass of water to demonstrate how it looks like the pencil is bent.)

4. Help children draw the conclusion that the pencil didn't change but that the water changes the way the pencil looks.

5. Introduce the activity."We are going to "scatter the different wavelengths (colors in light) to observe what happens.

Fill a tall clear glass with water. Place a flashlight so that the beam shines through the glass. Add a tiny amount of skim milk. (I dipped the spoon in a glass of skim milk, coating the spoon, and slowly added and stirred it to the glass of water.) Stir until you can clearly see the beam shining through the liquid.

Look at the beam from the side of the glass and then from the end of the glass. You can also let the light project onto a white card, which you hold at the end of the glass. From the side, the beam looks bluish-white. From the end it looks yellow-orange.

If you have added enough milk to the water you will be able to see the color of the beam change from blue-white to yellow-orange along the length of the beam.

Students will write down their observations in their science journal/notebooks and then write a conclusion based on their results.

During your follow-up discussion and sharing, explain that we added milk for the light to reflect off of. What in the atmosphere would make light reflect or scatter? (Gases in the atmosphere.) What happens to the other colors in the light? (They travel down and hit the earth as heat and light.)

Teaching Notes and Tips


Students will be assessed based on their science journal/notebook entries. They should have a clear hypothesis, observations, and conclusion written.


3.III.C.3 The student will observe that the sun supplies heat and light to the earth. The lesson expands on the makeup of the sun's light and what happens when the light goes through our atmosphere.

References and Resources