Why does the sky turn blue during the day and red at sunset? The atmosphere scatters blue light more than other colors. You can model this effect with clear glue sticks and a small flashlight.
Shine a flashlight into one end of a glue stick. The end near the light glows blue. The far end glows yellow-orange. Tape more glue sticks together to lengthen the path. The colors shift further toward red as the light travels farther.
Add polarizing filters to see another property of scattered light. Rotate a filter between the flashlight and glue stick. One viewer sees a bright beam while another sees a dim one. This shows that scattering also polarizes light.
Hypothesis
The hypothesis is that the sky is blue because blue light is most readily scattered from sunlight in the atmosphere.
The atmosphere scatters blue light more than other colors because blue has a shorter distance between its wave peaks. You can model this with a clear glue stick and a small flashlight. Shine the flashlight into one end. The end near the light glows blue. The far end glows yellow-orange. Tape more glue sticks together to lengthen the path and the colors shift further toward red as the light travels farther.
Light scattering changes with distance. When you shine a flashlight into a clear glue stick, the end near the light glows blue because short blue waves bounce off tiny particles first. The far end glows yellow-orange because blue light has already scattered away, leaving longer wavelengths to pass through.
When light travels a longer path through a scattering material, short blue waves get bent away first and longer red waves pass through. You can observe this by shining a flashlight into a clear glue stick: the end near the light glows blue, while the far end glows yellow-orange. Taping more glue sticks together lengthens the path, and the colors shift further toward red, modeling how sunsets look red because sunlight crosses a thicker layer of atmosphere.
When light scatters through a material, it can line up to move in only one direction. You can observe this by placing a polarizing filter between a flashlight and a clear glue stick. Rotate the filter and the beam shifts from bright to dim. One viewer sees bright light while another, positioned differently, sees almost none. That difference shows that scattering through the glue stick has polarized the light.
Method & Materials
You will shine the flashlight into one end of a glue stick and hold the other end of the glue stick approximately one centimeter from the white background. You will then place two glue sticks end to end, and attach them together with the clear tape. You will also place a polarizing filter between the flashlight and the first glue stick.
You will need a Mini-MagLite or penlight, 4 to 6 glue sticks (must be the clear ones used in glue guns), a white background (paper, wall, or cloth), clear packaging tape, and two polarizing filters.
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We can conclude that the sky is blue because blue light is most readily scattered from sunlight in the atmosphere, just as blue light was most readily scattered from white light in the glue sticks. At sunset, the sun is low near the horizon, and light travels through a greater thickness of atmosphere before reaching our eyes than it does when the sun is higher in the sky, making the sunset red.
Why do this project?
This science project is interesting and unique because it uses a simple model to demonstrate why the sky is blue and sunsets are red.
Also Consider
Experiment variations to consider include using different colors of light and different lengths of glue sticks.
Full project details
Additional information and source material for this project are available below.
