See also: Procedure-Background section from the "Graphing a Rainbow" activity.

When photons pass through a gas, they encounter atoms and molecules. The photons hit the atoms and molecules, and produce new photons. When we see this light (these photons) through a spectrograph, we see a pattern of light and dark bands called a spectrum. If the gas that we are looking at is a hot gas, we see an emission spectrum (bright bands on a dark background). If the gas is cool, we see an absorption spectrum (dark bands on a bright background). If two gasses contain the same material and one is hot and the other cool, the hot gas produces bright lines in the exact same places that the cool gas produces dark lines. Where the lines are, whether they are dark or bright, only depends on the material inside the gas. This makes it relatively easy to tell what material is inside of a gas.

The light sources that the students look at in this activity are emission spectra EXCEPT for the Sun's spectrum. The solar spectrum is an absorption spectrum because the "atmosphere" is cooler than the source of the Sun's light. Also, a few absorption lines in the solar spectrum are actually from Earth's atmosphere, which you cannot avoid while on Earth.

1. Ask students to bring in oatmeal containers from home.
2. A day before conducting the activity, spray paint (or have students spray paint) the insides of the oatmeal containers black.
3. (optional) Complete the "Patterns and Fingerprints," "Using Spectral Data to Explore Saturn and Titan," and "Graphing the Rainbow" activities to help build the students' conceptual knowledge of patterns produced by light and representations of spectral data.
4. If holographic diffraction gratings are in sheets, cut into 2-inch x 2-inch (5-cm x 5-cm) squares so that every student gets one square.
5. If needed, build a spectrograph to demonstrate to students what the final product should look like and discover any problems they may encounter.
6. Print copies of the Build a Spectrograph Student Instructions.
7. Gather materials.
8. Set up light bulb so students have a continuous spectrum light source to test their spectrographs.

1. Tell students that their job as engineers today is to design and build a prototype spectrograph for studying a variety of light sources.
2. Hand out student instructions that include directions and diagrams for building a spectrograph.
3. Cut a small rectangular hole, off-center in the lid of the oatmeal container, approximately 2.5 cm wide by 3.5 cm high.

Cut a hole in the lid of the container. click for copyright

Cut a hole in the bottom of the container. click for copyright

4. Cut a hole the same size roughly opposite the hole in the lid on the opposite side of the container.
5. Be careful not to cover any portion of the hole with tape. Gently tape the diffraction grating to the inside of the lid.
6. Cut 2.5 cm off each short end of the index cards. Keep the two pieces you cut.
7. Trim 1.5 cm off the length of the index card pieces.
8. Color the index card pieces black using black marker.

Half way done! click for copyright

Tape the index cards to the bottom of the container with a small gap between them. click for copyright

9. Using the straightest edge of the index card pieces, tape over the hole in the bottom of the container so that they are almost touching, but not quite. Make a gap of less than 1 mm, as straight as possible. Use masking tape so that no light leaks into the box, aside from the light coming through the gap (slit).
10. Put the lid back on, look through the lid, and point your spectrograph at the light source. You should see a continuous spectrum (a rainbow) very clearly. If you do not see a continuous spectrum clearly, rotate just the lid until you do.
11. Once your spectrograph is finished, decorate the outside of your container.

The finished product! Look through the lid, making sure that you see a complete, fairly-wide rainbow when you look through the light source. click for copyright

Brainstorming: Ask students what they believe causes rainbows to form.

Class Discussion: Ask students if they are familiar with any light that cannot be seen with the naked eye. Accept many answers. Ask if they know of any animals that see differently than humans (for example, snakes, bats, dogs).

Engineering Communication Homework: Take students outside with their spectrographs, colored pencils, and homework page to look at the spectrum of the Sun. They can fill it in as their first light source. Never have students look directly at the Sun! Follow the instructions on the homework page to see the Sun's spectrum. Make sure students do not block the paper with their shadows.

Upon returning to the class, emphasize that engineers share information through dialogue, conferences and written papers. Tell students that the way engineers get new ideas is through sharing with peers. Advise students to open their notebooks or engineering journals and write down something they did, discovered or wondered about during the lab. They can use the questions as a guide, or come up with their own statements. Give them a few minutes to write, and then ask for volunteers to share. If no one volunteers, choose a student to share. After the student finishes talking, allow other students to comment (respectfully). When the students finish the discussion, highlight key ideas on the board. Some ideas may be incorrect, but are still worthy of discussion. Be wary of labeling an answer "right" or "wrong" but rather have students demonstrate or discuss why something does or does not make sense. Ask students to write about the activity and what they learned in their journals.

Note: After students complete the homework, call attention to the fact that the same light sources produce the same spectrum. Tie this to the "Patterns and Fingerprints" and "Graphing the Rainbow" activities. Also, note that streetlights that appear blue or whitish-blue to the naked eye are filled with mercury vapor, and streetlights that appear yellow are filled with sodium vapor, thus they have emission bands unique to the substance with which they are filled.