Discussion:

Refer to the discussion on Analytical Spectroscopes for information on how diffraction gratings produce spectra.

Glass prisms are heavy. The more separation one wants for the wavelengths, the thicker the glass needs to be. Grating spectroscopes can do the same job but are much lighter. A diffraction grating can spread out the spectrum more than a prism can. This ability is called dispersion. Because gratings are smaller and lighter, they are well suited for spacecraft where size and weight are important considerations. Most research telescopes have some kind of grating spectrograph attached. Spectrographs are spectroscopes that provide a record, photographic or digital, of the spectrum observed. Notes:

• Most science supply houses sell diffraction grating material in sheets or rolls. One sheet is usually enough for every student in a class to have a piece of grating to build his or her own spectroscope. Holographic diffraction gratings work best for this activity. Refer to the note on sources in the previous activity.
• Many light sources can be used for this activity, including fluorescent and incandescent lights and spectra tubes with power supplies. Spectra tubes and the power supplies to run them are expensive. It may be possible to borrow tubes and supplies from another school if your school does not have them. The advantage of spectrum tubes is that they provide spectra from different gases such as hydrogen and helium.

For Further Research:

• Using colored pencils or crayons, make sketches of the spectrum emitted by different light sources. Try incandescent and fluorescent lamps, bug lights, street lights (mercury, low-pressure sodium, and high-pressure sodium), neon signs, and candle flames. How do these spectra differ?
• How do astronomers measure the spectra of objects in space? What do those spectra tell us about these objects?
• Relate this activity to the Analytical Spectroscope activity that follows (page 34).