Activity: Why the Sky is Blue

LEVEL
Ages 5-85

RATIONALE
An intuitive demonstration of how the atmosphere affects sunlight to create the sky colors we see.

LENGTH
10-15 minutes. (Preparation: 15-20 minutes, can be done well in advance.)

OBJECTIVES
Describe how the atmosphere causes a blue sky and a yellow Sun when the Sun is high in the sky, but a reddish Sun and sky when the Sun is near the horizon.

Make and use a Blue Sky model.

MATERIALS and PREPARATION

• Glue stick for a hot glue gun (see sources, below). You don't need the glue gun, just the glue stick.
• Cardboard for backing.
• Blue Sky template.
• Rubber cement or other glue.
• White playdoh for house (also see alternatives, below).
• Mini-mag light flashlights (also see alternatives, below).
• Exacto knife.

MAKE A BLUE SKY MODEL:

• Cut two pieces of gluestick: one 1.3 cm and one 8.3 cm. Try to make a smooth, straight cut.
• Glue the paper Blue Sky template to the cardboard.
• Find the marked points to set the hot glue stick. Use the rubber cement to glue the hot glue sticks in place. If there is writing on your pieces of glue stick, glue the writing side to the paper.
• Use a small piece of playdoh to form a house where marked on the pattern.
• If not using a mini-maglight, trim the cardboard so it is flush with the ends of the gluesticks, and prepare the flashlight as described below.

PROCEDURE

• Show an example of the blue sky model, pointing out the curve of the Earth and sky. (Note that the thickness of the atmosphere is greatly exaggerated. The atmosphere, compared to the whole earth, is about as thick as an apple's skin compared to the whole apple.)
• Explain/ask/discuss how white light is a mix of colors. Discuss the color of the sky during the day and near sunrise or sunset. (Notes.)
• If using mini-maglight, focus it so the light spot is about the same size as the end of the glue stick.
• Darken the room.
• Shine the flashlight straight at the house. Ask what color the light looks. Explain that the flashlight is simply white light, just like the light coming from our Sun.
• Shine the light through the short, vertical glue stick. (Make sure the light shines through the stick, not just on it.) What color is the glue stick? What color is the light on the "house"?. (Notes.)
• Shine the light through the long, horizontal stick. Now what colors do you see for the glue stick and "house"? (Notes.)

Sources, alternatives, and other details:

Glue sticks:
The ones I like best are available at Michaels floral & craft stores. I use the ones labeled HI & LO MELT; get the wider (approx 1/2 inch) glue sticks. A 4-inch glue stick is enough for one model, but if you're making many, you may wish to get the longer glue sticks. (Other brands can be used but you'll have to experiment with the lengths and modify the template accordingly.)

Template:
The template is provided above. (If you use another brand of glue stick, you'll need to adjust the template.) Note that we've exaggerated the thickness of the atmosphere compared to the Earth -- Earth's true atmosphere is more like the thickness of an apple's skin compared to the whole apple.

Mini-maglite/flashlight:
Mini-maglites are the perfect size and provide a bright white light. You can usually find them in hardware or variety stores. You can also use a regular flashlight. Take aluminum foil and cut a small hole slightly smaller than the end of the glue stick. Wrap this around the end of the flashlight so the hole is about in the middle. If using this with students, you may wish to secure the foil with tape or an elastic band. Fresh batteries will generally give you a whiter light.

Glue:
A glue-paste stick works well for attaching the template to the cardboard, but use any kind of glue that works for you. To attach the hot-glue sticks to the template, rubber cement or superglue work particularly well, but other kinds of glue will also work.

House:
White playdoh works well and is widely available. A piece of packing styrofoam, cut to an angle, also works quite well. We've also used Uhu Holdit (removable plastic poster-holder) or a small cone of white paper. Try to keep the house as white as possible so the colors will be more obvious.

Related Activities from The Universe at Your Fingertips resource notebook, published by the Astronomical Society of the Pacific:
J-4, Light and Color activities from the FOSTER Project.

Credits:
The initial and current versions were designed by Elizabeth E. Roettger. Project ASTRO Adler-Chicago staff and the Education staff of Adler Planetarium & Astronomy Museum contributed greatly to the intermediate development and write-up. The activity as a whole is based on ideas from the ISPP (an association of Chicago-area physics teachers who share ideas and demonstrations) and the Science Museum of Minnesota.

Last update: 09 Nov 1998, EER
URL: http://www.nthelp.com/eer/AENbluesky.html