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JAMES0. SCHRECK University of Nonhem Colorado
filtrates ac residues
Greeley, CO 80639
Atomic Spectroscopy with a Compact Disc Roben C. Mebane and Thomas R. Rybolt University of Tennessee at Chattanooga, Chattanooga, TN 37403
There have been a varietv of articles in this Journal in recent years pertaining to &Fraction gratings and simple atomic s~ectrosco~v ex~eriments( 1 4 .The com~actdisc ICI), useb by the &sicindustry provides a convenient and ubiauitous dilfraction eratina that can bc uscd with a mcrcur$ street lamp to explore the electronic structure of atoms through simple atomic spectroscopy observations. In this approach we are using a common but widely unrecognized grating that is found in many students'homes. We are also emphasizing "doing science" with readily available materials in a nonlaboratory environment. The Diffraction Grating of a CD Disc The diffraction grating in a CD results from microscopic pits that contain the music and tracking information for the CD player. These microscopic pits are arranged in a spiral track that is almost 5 k m long (10).Each pit is approximately 0.5 pm wide, 0.8-3.0 pm in length, and 0.1 m deep (10,11). Remarkably, the width of a single human hair can cover 60 of these CD tracks (12). The reflective property of the CD is due to a thin layer of aluminum that is deposited on a layer of polycarbonate plastic, which protects the microscopic pits. For the interested reader, descriptions of compact discs and compactwith more disc technology are presented elsewhere (10,ll) detail. Theoretical Background When light waves impinge upon the series of closely spaced groves that characterize diffraction gratings, white light is broken into its component parts. This is the same effect as that seen when light passes through a prism (9). A visual comparison can be easily made between the effects seen when light from two different sources-a white incandescent source and a mercury vapor lamp-is diffracted off the surface of the CD. It quite clear that the white incandescent source is continuous because all the
colors of the spectrum are observed. However, the light from a mercury street lamp proves to he made of only selected portions of the visible spectrum. Mercury street lamps may contain an incandescent fdament in addition to the mercury bulb. The intensity of the mercury light, however, is much greater than the visible continuum from the incandescent filament. Street lamps are safe for this experiment because they are shielded and do not give off harmful ultraviolet light. The Experiment Caution: Students should be instructed not to stand in the street while making the observations using the street light. Caution: Lamps found in science laboratories may give off harmful ultrauwlet light. They should not be used by students as a light source in this experiment.
To carry out this experiment it is best to find a location where stray light is minimized. You should also be able to stand far enough away (20 to 60 meters) from a mercury street lamp that it approximates a point source. The CD should be turned print side down in its holder so that the writing on the disc does not interfere with the observations. Holding the plastic case with the CD about waist high, the CD should be tilted down until the reflected image of the street lamp can be seen on the CD. Starting from this angle, the CD is slowly tilted up toward the observer. As the angle is changed, the visible spectrum of the light source is scanned. Viewing should be done along the line from the light source to your body. Closing one eye insures that only one line of color is seen across the surface of the disc. One can scan back and forth through the visible region by tilting the CD up and down. At greater angles the entire spectrum may be repeated. Repeating this experiment in the same manner using an incandescent bulb for a light source produces a continuous spectrum. Comparing the Light Sources While light from an incandescent bulb and from a mercury lamp may not seem different to the eye, the diffraction patterns of the two light sources appear quite different. The diffraction pattern from a continuous white source yields the range of colors with no dark regions between any of them:
Visible Mercury Spectral Lines
red, orange, yellow, green, blue, indigo, and violet However the pattern from a mercury lamp yields regions with fewer colors: yellow, green, and indigdviolet 300
400
500 600 WmelengU~(nm)
700
SO0
The relative intensity of the known mercury emission lines plotted against wavelength.
It is easy to see dark regions, and the blue region is gone completely. An extremely faint orange-red region was also seen on the disc. Volume 69 Number 5 May 1992
401
The Mercury Spectrum
I t is interesting to compare the observed spectral pattern from a mercury street light to the known spectral lines for mercury. Although there are dozens of mercury atomic emission lines in the visible region, the four most intense peaks appear a t 404.656nm, 435.833nm, 546.074nm,and 614.950 nm (13). The f i-w e shows a plot of the relative intensity of mercury emission lines versus the wavelenbflh. Simole CD s~ectrnscuoicobservations of violetlindilro. -. green, and yellow regions agree with the spectral intensities shown in the figure. These simple observations can be used to introduce students to the concept of discrete energy levels. These are the source of specific spectral lines that occur when electrons go from a higher energy level to a lower one, and when atoms emit electromagnetic radiation. 'Sodium vapor lamps may be high-pressure or low-pressure lamps. High-pressure sodium vapor lamps exhibit a more-continuous spectrum, but also have a more intense yellow region. Low-pressure sodium vapor lamps are monochromatic based on the intense lines in the yellow region.
402
Journal of Chemical Education
Additional Challenges Other activities in which students might be engaged include the following.
Make plots of intensity versus wavelength (see the figure) from available mercurv data (13). Compare their visual obsekatiuns 4 t h their plots, observing spectral l m w for other elements such as sodium.' Use n CD difinnion grating to determine if particular lamps generate a continuous or discrete spectrum Literature Cited
6. Juergens, F H.J. Ckam Educ 1988.65.1006. 7 . Casanova, J.:Arellano, M.; Lam, L.;Gdmez, H . J C h .Educ 1989,66,A201 8. HsnrahaqE. S. J Chern. Ed=. 1989,66,359. 9. Zumdahl, S. S. Ckmlsby, 2nd ed.; D. C. Heath: Lexington, MA, 1989; p 269. 10. Rosaing,T. D. Phyg Teoeh. 1981.25.556562. 11. Noldeke. C.Phvs. Teach. 1990.28.48b486.
