Both continuous and line spectra emitted from different sources can be seen by students with small pieces of plastic diffraction grating replica. This demo can be used for freshman classes or at a more sophisticated level in p-chem or other advanced courses.

300 2x2 slide mounted pieces of diffraction grating replica (for CBS 204)
Four small cardboard boxes to collect slides after class
Mounted light sources: we have a tungesten lamp-neon-argon apparatus and a sodium vapor lamp apparatus.

This demonstration should accompany discussion of light, the visible spectrum, and emission of light by atoms. Begin by distributing the diffraction grating replica slides to the class and "walking the students through" their use. Put the four cardboard boxes by each exit to CBS 204. Ask the students to drop their slides in the boxes as they leave class.

Darken the room and turn on the incandescent light. Have the students hold up their pieces of diffraction grating in front of one eye. The students will see the light source and, less noticeably at first, a continuous visible spectrum on each side of the light source. (Some students may be holding the plastic so that the spectrum appears above and below the light source. Be sure to have them turn the plastic so that the spectra are visible on the left and right sides.) Explain that this is a continuous emission spectrum, which is typical of glowing objects.

The incandescent source is equipped with a dimmer switch. Explain to the class that the distribution of wavelengths emitted by a glowing object (blackbody) is a function of the object’s temperature (How does one measure the surface temperature of a star?) and that as temperature drops there is a shift to longer wavelengths. While the class is watching the glowing source, turn down the dimmer switch slowly. With a little imagination, the short wavelength end of the continuous spectrum becomes harder to see than the red end. (The human eye is also more sensitive to red under conditions of low illumination.)

Turn up the incandescent source again and turn on the neon tube. Students will see the continuous blackbody spectrum and the multiple red lines of neon emission. The spectra will be directly above each other as seen by the viewer.

Turn on the argon source. Students will see its line spectrum as well as the other two.

Turn on the sodium vapor source. This takes a minute or two until the lamp gets hot enough. Point out that this is the same sort of light source as used on the mall at night.

Class discussion should review the fundamental ideas of light emission by energetically excited atoms. You can also discuss why line sources like sodium or mercury lamps are cheaper to run than incandescent lamps. Ask the students to drop their slides in receptical boxes placed at each exit. You can collect the diffraction grating replica after class for use on another day. Actually, the stuff is pretty inexpensive, and you could give each student an unmounted piece on the promise that they go out at night and identify different types of light sources around town.

Don't electrocute yourself when you turn on the light sources. Don’t do it while taking a bath.