Light, Color, and Astronomy

Douglas Duncan & Gina Brissenden
American Astronomical Society
Education Office

Components

Components listed are the ones we use. 

·      1/4 sheet red filter—cut into 2” x 2” squares
Cat. No. Roscolux #27
1/4 sheet blue filter—cut into 2” x 2” squares
Cat. No. Roscolux #74
Grand Stage Company, Inc.
312-332-1603
About $8 (per sheet)

·      diffraction grating
Cat. No. PS-08A (5” x 9”)
Project Star
800-537-8703
$10.00

·      diffraction grating
Cat. No. PS-08B (5” x 6’)
Project Star
800-537-8703
$42.00

·      2” x 2” slide mounts
Cat. No. AAA 001
Wess Plastics, Inc.
631-981-4850
About $20 (per box 50, volume disc.)

·      2- 8” x 12” cardboard rectangles—not included
Slides Sets:

All slide sets noted here are available from the Astronomical Society of the Pacific at: 800-335-2624 or www.aspsky.org

1.        David Malin's "Splendors of the Universe" (especially set #1) from the ASP. Slide number 5 "NGC 6822" and slide number 8 " Cone Nebula".

2.        Hubble Space Telescope Slide Sets from the ASP : Set 12 ; slide number 19 " Mars Syrtis Major Region", slide number 13 "Lagoon Nebula", slide number 16 "Orion Trapezium" : Set 7 ; slide number 11 "Cygnus Loop", slide number 2 "Mars at Opposition", slide number 12 " NGC 6543 : Set 2 ; slide number 10 "Cygnus Loop".

3.        3. Mars Pathfinder Slide Set 180, slide numbers 46 and 13. Use red and blue filters in place of stereo glasses.

Assembling Diffraction Gratings

Note:  Handle diffraction gratings by edges, or wear gloves, to avoid leaving fingerprints on them.  One “large diffraction grating” and 15 “personal diffraction gratings” are included in your kit.

To assemble a large diffraction grating (components are in bold):

1.      Cut 8” x 4” rectangular hole from center of each cardboard rectangle.

2.      Tape or glue 5” x 9” diffraction grating over opening in one cardboard rectangle.

3.      Tape or glue second cardboard rectangle over large diffraction grating.

You now have a large diffraction grating!

To assemble personal diffraction gratings (components are in bold):

1.      With gloves on, cut 5” x 6’ diffraction grating into squares large enough to cover opening in slide mount.

2.      Pop open slide mount.

3.      Insert cut diffraction grating.

4.      Close slide mount.

You now have a personal diffraction grating!

Assembling a Slit-Slide

Note: One “slit-slide” is included in your kit.

To assemble slit (components are in bold):

1.      Open the empty slide mount.

2.      Cut two strips of blank paper large enough that each could cover about half of the opening in the empty slide mount.

3.      Place the strips of blank paper in the empty slide mount so that 1/4 to 1/3 of the slide opening remains clear between the two strips of blank paper.

4.      Close the slide mount.

You now have a slit-slide!

Light, Color, and Astronomy Activities

Following are a few suggestions for activities to try with students.  It is important that you set up each activity, then ask students to predict what will happen.  They should write these down and discuss their answers in groups or with the class.  Helping students become aware of their own conceptions in essential for their constructing scientifically accurate ones.

When group activity is called for, choose groups as follows.  Groups should have three people in them.  If class isn’t divisible by three, a group of four is better than a group of two.  Gender mix should be either two females and one male, all female, or all male.  Ability mix should be low medium and high in each group.  Roles should be assigned to each member: manager, skeptic, and checker/recorder.  The manager keeps the group organized and on task.  The skeptic questions decisions and ensures they aren’t made too quickly.  The checker/recorder checks for consensus and gets group approval before recording decision1.  Individuals will achieve greater conceptual understanding in this type of group environment.

White Light and Color

White: What color is it?

Make a big, bright spectrum on the wall of the room by placing the large diffraction grating in front of the lens of a slide projector which has a slit-slide in it.

1)   Ask students to describe the light before it passes through the grating and after it has passed through the grating.

·        What does the diffraction grating do to light? (separates its colors into a “rainbow”)

2)   Have students discuss the following questions and propose experiments to test their answers:

·        What is white light? (all colors)

·        Can a “rainbow” be made white again? (use mirrors to reflect colors on top of each other)

3)   Give each student a red and a blue filter. Show them a red filter and ask them to make predictions:

·        What will happen to the spectrum if you put a red filter in front of the light before it passes through the diffraction grating? (only the red comes through)

·        After it passes through the diffraction grating? (still only the red comes through)

·        What if you use a blue filter? (only the blue comes through)

·        What does a filter do? (allows limited wavelengths to pass through)

An Apple a Day

Make a big, bright spectrum on the wall of the room by placing the large diffraction grating in front of the lens of slide projector which has a slit-slide in it. Without letting students know what color it is, pass a red apple through the spectrum. Do the same with a green apple.

1)   Ask students to describe the apples as they pass through the spectrum:

·        How does the light from the spectrum change the appearance of the apples?

2)   Have students discuss the following questions and propose experiments to test their answers:

·        What color (or colors) are the apples?

·        How do we see things, like the apples, that don’t emit light?

Astronomical Slides and Filters

The slides we have recommended for this activity emphasize: nebulae, stars of different temperatures, and mars.  The first activity—Now You See It, Now You Don’t—can be used for nebulae and stars of different temperatures.  The Mars activity—Going Once, Going Twice, Sold to the Martian Astronomers in Group...—can be used with the Mars slide.  The spectrum activity requires gas emission tubes, which many schools own and which otherwise can often be borrowed from a neighborhood astronomer!  But first, a brief description of each category:

Nebulae

Nebulae are composed of hot gasses.  The brightest emission usually is from hydrogen and oxygen.  Hydrogen glows predominately red, and oxygen green.  The red filter will emphasize the hydrogen; the blue filter will emphasize the oxygen.  The spectrum activity will show this even more clearly.

Stars of Different Temperatures

Stars have different temperatures which cause them to appear different colors.  Stars which are cooler will be redder in color; stars which are hotter will be bluer in color.  Some slides have pairs of stars which are red and blue.  These will “blink on and off” as the slide is looked at through a red, and then a blue, filter.

Now You See It, Now You Don’t

1)     Before you project a slide onto the wall, have students look through the red filter, then project the slide.  Ask students to describe the object they see:

·        Are their stars?

·        Gas clouds?

·        Dust lanes?

2)     Now have students switch to the blue filter, and ask them to describe what they see.

·        In what ways is it different than it was through the red filter?

·        In what ways is it the same?

3)     Finally, ask students to predict what the image will look like without filter?

·        Why?

4)     Show students the slide without having them look through a filter.

·        How is it similar, and different, to what they predicted?

Diffraction Grating and Spectrum Activities

Give each student a diffraction grating, or have students work in small groups.  Spend some time looking at different elements in the gas tubes through the gratings.

1)   Ask students to describe the differences they see when looking at different elements:

·        Are all the elements the same? (No; different elements have different lines)

·        How are the colors seen through the diffraction gratings different from the colors seen through a filter?

·        Than in objects around the room?

Reflected vs. Intrinsic Colors

It is useful to summarize what we have seen, and to distinguish between objects which produce their own light, and objects which only reflect light.  The stars, sun, and nebulae produce their own light, and their spectrum gives us clues to what these objects are made of, and about their temperature, as we have seen above.  Apples, your clothes, and planets are examples of objects which reflect light.  Their color depends on both their composition, and the color of the light falling on them.

A very dramatic example of this can be shown with a “low-pressure” sodium lamp—used in many street lights (the ones that look very yellow).  These lamps put out a pure yellow color.  If you can get one (some hardware stores sell them for backyard lighting, for about $50), try this:

1)     Ask your students why red clothes look red, blue look blue, etc.  The answer is that red reflects red well, blue reflects blue, and we usually look at clothes under sunlight or other nearly white light which contains a mix of colors.  This means that a red shirt can reflect the red light which is present in sunlight.  So it looks red.

2)     Ask your students to predict how their clothes would look if you shined a pure yellow light on them.  Then do it!  Where did my color go!  You will find that all sense of color disappears.  There is only a sensation of bright and dark, which the brain interprets as shades of gray. (according to how well the clothes reflect yellow light).

Mars

Both the surface, and the atmosphere, of Mars are visible in the slide.  The atmosphere scatters blue light more than red, so the blue filter allows viewing of the atmosphere.  The red filter penetrates the atmosphere to allow viewing of the surface.  (Dust and molecules in the earth’s atmosphere also scatter blue light.  That is why our sky is blue!)

Going Once, Going Twice, Sold to the Martian Astronomers in Group...

1)     Have students look at Mars through the red, then the blue, filter:

·        In what ways are they similar?

·        Different?

2)     Ask students to try determine which part of Mars they are viewing through each filter.

·        How are they making these decisions? (the blue looks like cloud cover or haze?)

3)     Ask students why sunglasses are often tinted yellow or red (to penetrate atmospheric haze)

4)     Have students, in groups, write proposals to build a telescope based on their observations.

“Do-It-Yourself” Filter, Diffraction Grating, & Spectrum Activities

Work in groups to create new activities.  What you have learned about elements and color should be displayed in they final designs.

References

¹Heller, P. & Hollabaugh, M. (1992).  Teaching problem solving through cooperative grouping.  Part 2: Designing problems and structuring groups.  American Journal of Physics, 60(7), 637-644.