(EER's homepage, Astronomer's Education Notebook)
(draft 2/1/94, revised 6/26/97 - still a draft!)
Elizabeth E. Roettger (with advice from Vivian Hoette <Adler Planetarium>, David Malin <Anglo-Australian Telescope>, and others <especially from the Jet Propulsion Laboratory>)
Description: Participants use colored filters to decode secret messages, look at rainbows, and learn how astronomers can decode information from the sky.
Just like astronomers, you can use colored filters to make discoveries about objects in space.
The activity described here is in an hour-long, lecture/participation format. For classroom use, I strongly recommend substituting the preparation work (steps 1-5) with activities from the GEMS Color Analyzers Teachers' Guide (see below for more information).
Materials/equipment (see below for details & sources):
1. Assemble the equipment, making the filter cards and slit slide if needed. (Instructions for making the cards and slide can be found below, under "Equipment, sources, hints, and details".)
2. Plan at least 15 minutes to test room and equipment - it can take some time to find the best way to project the spectrum and the best position for the mirror. Set up the slides and grating; put the colored objects out of sight.
(Substitute steps 1-5 with Color Analyzers for classroom use.)
1. Using light. Point out that almost all of what we know about places beyond Earth comes from light. Astronomers have many high-tech detectors, but we have detectors, too - our eyes. This activity uses our eyes and color filters, just like many astronomers use other detectors (CCDs, film...) and color filters. Pass around the filter cards and suggest that participants use them to look around the room.
2. Secret code sheet. Hold up the first secret code sheet. Ask the participants if they can read the message. Ask them how it looks through the yellow filter. How about the other filters? When most seem to have made an observation, ask what color revealed the secret code.
Cycle through the rest of the secret code sheets. Keep asking what color works best and what they see. Ask if they agree on the filter which usually reveals the code. Most people will select the red filter. Ask participants to speculate why they think red works the best.
3. Why does red work best? If the filter cards are not labeled, ask them to hold the card with their thumb near the red. (They won't be able to see the colors in the dark). Turn on the projector with the slit slide, and place the grating in front of the lens to make a rainbow (you'll have to aim the projector to one side); turn out the lights.
Get them to look through the filters at the rainbow. Point out where invisible IR and UV are. Walk through the colors with a white piece of paper, getting them to name the colors. Find the edges of what participants can see through each filter by asking them to look through each filter while you move a white piece of paper through the rainbow. Have them speak up when the edge of the paper becomes bright, and when it becomes dark again.
Summary: Filters allow some color(s) of light to pass through, but block other colors. For this set of filters, the colors that pass through are next to each other in the spectrum.
4. Where is pink? sky blue? Ask them to predict what will happen when you mix blue and green. Use the mirror to throw colors from one side above, and then into, colors on the other side. Point out that these colors are not single wavelengths of light, but that your eye detects only three ranges of light (colors) and your brain interprets them. Look through filters at the plain and mixed colors. Note that you can take, say, pink, and tell what wavelengths it's made of. Also note that there are two different yellows - one made by mixing wavelengths, and one that's only one wavelength. You can tell the difference by using the filters.
(Color TVs work by mixing red, green, and blue to produce all the colors you see. If you look at a color TV screen through a magnifying glass, you'll see that the picture is made up of dots of these 3 colors. Your eye blends them together.)
5. Where is an object's color? What happens when you put a colored object in the rainbow? Use colored paper or objects (red and green apples, big patterns, the shuttle code sheet) and walk them through the rainbow. Hold one in red. Does it look any different than through the filter? I usually hold a red apple in the green light and cackle, since it looks unappetizing. If someone has on some brightly patterned clothing, ask them to walk slowly through the rainbow. Go back and forth between red and green; typically, the bright and dark areas oscillate.
(There is a lot more in Color Analyzers about the difference between the color of light as given off by a glowing object and the color of light as reflected by and object. Objects that reflect light can at most reflect the colors that shine on them, hence the apple looks almost black when placed in green light.)
6. Color Astronomy. Project some astronomical slides. What do they look like through the different filters? What are we seeing? (Note: this is a good simulation of REAL astronomy: broad-band filter imaging -- in the daytime, in the classroom!) Notes on specific slides can be found below. Be willing to go back through the slides.
Note also that the particular slide projector and bulb will affect the colors you see, both in the rainbow and in the slides. The combination of this with the particular filters you're using may change the apparent science you get from the slides. I try to concentrate on the process - making logical inferences, talking about possibilities - rather than on facts.
Most of the slides we are using are actually 3 photographs or images added together -- each taken with black and white photographic film or plates, or with a black and white camera through colored filters. The three colors were then recombined to make these color images. It can take a great deal of skill to produce an image where the colors are balanced properly and each color is as clear as it can be.
7. Review. Take questions, then (if possible) give them a chance to play in the rainbow. Hand secret codes to people who seem reluctant. Keep looking at the world in as many ways as you can!
These are from an ASP slide set called "Splendors of the Universe" except as noted.
Slide 12: The Helix Nebula, NGC 7293
By this time in the activity, participants are used to looking through the filters, and tend to try them on the slides. I use this slide first so that everybody gets the same 'oh!' experience from the next slide at the same time. Talk a bit about planetary nebula and what they are, since they show up again in the NGC 6822 slide.
When the main fuel in a star (hydrogen) runs out, "the star may shed its outer layers to produce a beautiful planetary nebula like the one seen here." A planetary nebula doesn't have anything to do with a planet - it's a term we're stuck with historically. The gas is hot (heated by what's left of the star in the center), and so it glows. The chemicals in the nebula determine what the colors are.
Slide 8: The Cone Nebula
This is the real 'oh!' experience. What do you see through the red filter? Through the green? If you wanted to study stars, what might you do? What about this dark area - is it something or the absence of anything? How might you tell? (Hold your arm up to show where the dark region is, then have them look through the green filter.) Any difference in the density of stars? Have you ever been outside on a foggy day? Do distant objects disappear? Are they really gone? So what might be going on here?
The Cone Nebula contains both gas and dust. The red gas is hydrogen. The dark areas are generally dust blocking the light from the hydrogen gas and the stars beyond. There are stars hidden in the dust, and they tend to make the dust around them glow, so when you see a large, roundish glowing area, it's generally a hidden star lighting up the dust closest to it. If you were to see a wide-angle view of the area, you'd see that the dark cone shape is a finger of dust from a nearby dark (dusty) area.
Slide 5: Galaxy NGC 6822 in the Local Group
Last time the fuzzy stuff was red gas. Now the fuzzy stuff is blue stars, and you see stars through red. The blue stars are newer than the middle-aged yellow ones. If you wanted to study just new stars, what might you do? (For a lively audience: you're an astronomer. Your boss needs to know if this galaxy is just new stars, or if there are a bunch of old stars in the galaxy, too. How would you figure this out? ...note that whether people see the yellow stars that also form the galaxy depends on how bright your projector is - so respond to good reasoning rather than absolutes.) See anything else familiar? (Things that look like the planetary nebula we saw before -- actually these are called bubble nebulae, and they are much larger than planetary nebulae, and are caused by much bigger and brighter stars, sometimes a whole group of them.)
NGC 6822 is a galaxy that's relatively close to us (close to 2 million light years). The newer, blue stars form a slightly different pattern than the older, yellow stars. The latter form a fairly smooth ellipsoid (flattened ball-shape), and the new stars form a somewhat irregular shape.
Slide 10: Dust Cloud and Open Cluster NGC 6520
What's happening here? Is there a hole in space? Are stars absent in that direction out to infinity? Do you see the brownish stars near the edges of the dark region? Have you ever looked through polluted air? Where are the blue stars relative to the dark area? (This assumes the blue stars are clustered together in space; since some are visible in the dark area, they must be in front.)
Old stars tend to be yellowish, young stars bluish. Here is a cluster of young stars (NGC 6520). They're in front of most of the yellow stars. There's also a dark cloud (called Barnard 86) blocking the light from the distant stars (some of the blue stars are in front of it). Around the edges (where the cloud is thinner), it doesn't block all the light, and the stars behind seem faint and brownish. The cloud may be what was left after the stars formed.
Slide 14: The Horsehead Nebula in Orion
(Kids tend to say what this looks like - a dinosaur or dragon or horse.) Is this some cosmic symbol that humans are not the most important beings? The red is gas, but what is the horsehead? If you weren't sure, could you use your filters to check? Look through green and compare the density of stars in the two areas. What does this mean? Is the shape "something" or "nothing"? Right - there's something there (it turns out to be dust) blocking the starlight. See this blue at the bottom? There's a star inside that dust, lighting up the dust in this area.
The red part is glowing hydrogen gas. The darkness is a cloud of dust obscuring the gas, stars, and galaxies behind it. The horsehead is a tendril of this cloud of dark dust. (I use my hand to throw a similar shadow -- arc it while describing the motion of the tendril.) Because there's no light shining on it, you don't see the dust, only its effect on the other light. However, there is a star buried in the dust and lighting up an area of it - the blue, roughly round area (NGC 2023) is dust lit by an imbedded star.
JPL P-41491 Moon, false color
What object is this? This is not what it would look like to your eyes. It's called (surprise!) false color. Several images were taken through different colored filters, then assembled - but not to simulate the true colors. The colors were exaggerated. The different colors show different kinds of surfaces. Still don't know what it is? Maybe the next slide will help...
(ASP) The Planetary System, slide 13, the Moon
(I just flash this one up, then go back to the last one. Make sure they're in the same orientation!!)
Equipment, sources, hints, and details:
HOLOGRAPHIC GRATING: I got mine from the back of the LHS GEMS "Color Analyzer" booklet. They can also be purchased from Learning Technologies (see below). I left my 3" square in the plastic because skin oil can ruin the grating. I also made a cardboard frame for it so it wouldn't curl. The top and bottom of the frame are different widths for different projector heights. Alternatively, put the grating in one of those L-shaped picture holders (the cheap, clear kind). The bent part of the frame allows you to stand the grating in front of the projector or hang it on the lens, as needed.
SLIT SLIDE: You need a thin bar of light to make the rainbow. I took a piece of foil, cut a slit in it, and put it in a glass slide mount (that was before I discovered the glass slide mounts aren't good with the filters...they're fine for the slit). My slide has about a 1/4-inch wide slit. I haven't experimented with different widths yet, but I expect wider slits to give brighter but blurrier rainbows. Put the slit slide in the projector so it makes a vertical bar. Focus it. Put the holographic grating in front of the projector's lens (you may have to give it a quarter turn to get the colored spectra to the sides). Turn the projector and grating to the left or right so the spectrum is to the front of the room.
FILTER CARDS: These are just paper or cardboard pieces with holes cut in and colored filters (plastic) taped over the holes. The filters can be bought from a scientific supply house or theater supply house (see below). I haven't found colored cellophane or plastic report covers to be colored (discriminating) enough - you really do need good filter material.
The easiest thing is to fold index cards and tape red, green, blue, and yellow filter pieces in order across the top (it helps to have red next to green). You can do it with just a green and red filter pair. I've tried slide mounts, too, but am not happy with them (do NOT use glass-covered "newtonian" slide mounts - they blur the image).
SECRET CODE SHEETS: These are (large) pieces of paper with colored shapes on them. The colors are such that when you look through a filter, some of the shapes look dark and some look light. Through one particular filter (usually red), the dark shapes spell out a word or make a picture. I copied (enlarged, actually) mine out of "Color Analyzers" - I also like to make up one especially for the participants.
SLIDE PROJECTOR: Just an ordinary one. Put the slit slide in the projector, and prop (possibly using cardboard) or tape the grating in front of the projector's lens. I put the slit up-and-down, and make sure the grating is turned to make wide rainbows to either side. You'll have to aim the projector so it points to one side; this puts the rainbow on the front screen.
**If you're not standing near the projector, enlist a helper to remove the grating and re-position the projector when you move on to the astronomical slides.**
MIRROR: Mine is about 7 inches wide, and it's almost too small. It's also a bit hard to hold it steady - I now use a small vice intended to hold electronics parts (it has two alligator clips on universal joints). Hold the mirror relatively close to the projector, since the light then has to travel a shorter distance and will be stronger (but you have to balance this with getting one color at a time). Make blue appear above the red, then tilt the mirror to mix the colors. Mix green with blue, green with red. You can use the shadow of the mirror to figure out where you are in the spectrum.
COLORED PAPER OR OBJECTS: Red and green apples work well - the red apple looks truly unappetizing in green light. You can try a green apple without telling them it's green...but only if the room is VERY dark. Use volunteers if participants have on colorful clothes. Use big patterns - small patterns get lost for people sitting in the back. My enlarged secret code pages seemed to work pretty well.
I'm still experimenting, but I've tried to use slides from a single set - the Astronomical Society of the Pacific's "Splendors of the Universe" set 1 (David Malin, Anglo-Australian Telescope). When I do the activity, I add a few planetary slides; details above.
Where to get materials:
Lawrence Hall of
University of California
Berkeley, CA 94720
GEMS Color Analyzers Teacher's Guide (gives you the details and lots more. It's $15 + shipping, plus tax in CA). I recommend this very highly, whether you're doing a demo/activity or a classroom lesson. It gives the science background on light, class handouts, and ways of assessing the students' understanding.
Also available from Lawrence Hall are class sets of filters and grating (just red & green filters)
Technologies, Inc. 800-537-8703
59 Walden Street fax 617-547-2686
Cambridge MA 02140
diffraction grating (they and others say it's much brighter than
acetate) (prices are from 1994):
$ 8.00 PS-08A (9inx5in) $36.00 PS-08B (6ft x 5in)
$ 6.00 PS-08C (4.5in x 5 in, plus filters) $25.00 PS-08D (mounted grating for overhead, plus filters)
+shipping and handling
101 E. Gloucester Pike fax 609-573-6295
Barrington, NJ 08007-1380
from **1992** catalog: (credit cards okay, but call for up-to-date prices!)
$ 7.70 Y40,267 (8.5in x 11in sheet)
$17.35 Y50,180 (6ft x 8.5in roll)
$57.70 Y70,565 (24ft x 8.5 in roll)
$15.00 y39,140 (Optics Soc. of Am. kit - incl grating, 4 filters, lenses, mirror, polarizers, etc. and booklet)
Analyzers guide also lists
Flinn Scientific / PO Box 219 / Batavia, IL 60510 / 312-879-6900
as a source for diffraction gratings.
Society of the Pacific 415-337-2624
390 Ashton Ave
San Francisco, CA 94112
The set has been AS 298 "Splendors of the Universe, set 1", $19.95.
says the following two filters are good: Medium Red #27 Kelly
Edna DeVore recommends Roscolux # 27 "medium red", #90 "dark yellow green", #80 "primary blue", and #312 "canary". I looked up a theatrical supply house, and found it very easy to buy sheets of color filter material. I got 20"x24" sheets for $5.50 each. I went through a sample book from Rosco (that's one company that makes "gels") and I think the best are:
#27 "medium red", #90 "dark yellow green", #74 "night blue", and #10 "medium yellow".
I'm not completely happy with the yellow. If you have to substitute colors from another company, look at the plots in the sample book (shows how much light at each wavelength gets through the filter) and try to pick ones that are similar.
*This activity/demo is based on the GEMS Color Analyzers guide and Edna DeVore's (FOSTER Project) wonderful use of astronomy slides.*
Attributed, non-commercial distribution of these notes is encouraged. Comments and suggestions are appreciated. These notes are intended as a guide for those who have experienced the activity and wish to re-create it or us it with GEMS Color Analyzers in the classroom. These notes are not yet organized to stand alone.
Web version 26 June 1997
by Elizabeth E. Roettger.