overhead projector Spectroscopy on the Overhead Projector Sally solomon,' Chinhyu Hur, Alan ~ e e ? and Kurt Smith Drexel University Philadelphia, PA 19104 Any overhead projector can be converted easily into a simple spectrometer by placing a piece of diffraction grating over the projecting lens (1).A detailed description of the apparatus and suggested spectroscopy experiments follow. Apparatus Two types of diffraction gratings can be used effectively with the overhead projector in a partially darkened room. Both are examples of "holographic" diffraction gratings onlv because thev are created bv the same ootical arrangem e i t used to hologra& (2,3).u s k g the gratrng available from Project Star3 nearly all the light intensity appears in the first order producing a visible spectrum that is easily seen i n a large lecture hall. With the Flinn-Cspectra4 m;ny spectral orders appear. The first order, which is most intense, produces a narrow spectral image; the second-order image is similar to that of the Project Star grating, but much less intense. The overhead projector is placed approximately 3 m from the screen. The stage of the oroiector is covered with a cardboard housing &at has two sfits cut out, one to project a reference beam and one for a sample beam. Each slit is 4 cm wide and 1.5 cm high. They are 2 cm apart and positioned near the center of the projector surface. A 7-cm x 7-cm square of plastic grating film is taped over the projecting lens of the overhead. The Project Star grating bends easily and must be stiffened with a masking tape border. Care must be taken when handling the grating because it scratches easily. The grating is turned to produce a vertical spectrum (measuring about 66 cm) in which the red appears a t the top and violet a t the bottom. Performing the Demonstration The spectra of colored solutions can be seen easily by eye using this simple spectrometer. Absorptions appear as black bands. Beakers (100 mL) may be used a s c u v e t ~ . ~ Spectral peaks also can be traced. A second overhead projector isplaced about 15 cm to the side of the first projector. Ablank transparency slide is taped to a second cardboard cover that has a 2- x 10-cm rectangular cutout. The rectangle is centered in the bottom half of the cardboard cover. To sketch the spectrum, position the light rectangle 'Author to whom correspondence should be directed. 'Supported by an Academy of Applied Science REAP grant. 3~earningTechnologies Inc., 59 Walden St., Cambridge. M A 02140. Catalog No: PS-08 (9 in. x 5 in.) or PS-OBB (6 ft. x 5 in.) 750 linesimm. 4Flinn Scientific lnc., P. 0 .Box 219, 131 Flinn St, Batavia, IL60510. Catalog No: AP1714. 5For slightly better light transmission cuvets can be made by fusing a Pyrex disk onto a 5-cm section of 4.8-cm diameter glass tubing. '~ldrich Chemical Comoanv. Inc. 1001 West Saint Paul Avenue. Milwaulkee, WI 53233; thb cai& number is 28,918-3(neodymium chloride) and 20,514-1 (praseodymium chloride).
250
Journal of Chemical Education
sdited by DORISKOLB Bradley Univeniw Peoria, lL61625
alongside the visible spectrum. Mark the beginning, maximum, and end of the peak, then sketch in the spectrum. When you are not tracing a spectrum, cover the rectanm131. hol-c to eliminate str$ light. 'The solutions below can be used to illustrate mectral features such as peak position and width and the effect of slit size and path length. Cobalt Chloride The spectrum of cobalt chloride has a broad neak with a maximim a t 510 nm. The cnvet is filled to a le;el of 0.5 cm with 0.5 M cobalt chloride (12 g CoC1~6H90in 100 mL \irater,.The absorption appears a's a wid; hl3L.k band blockinc out most of thc preen. Transmitted light includes red. a little green, and violet. ~ r i ~ l i n g t h path e length by adding solution to a level of 1.5 cm creates a more intense color and a n absorption band that appears wider. Viewing the two solutions a t the same time makes the difference obvious. Potassium Permanganate The characteristic purple color of potassium permanganate is so intense that dilute solutions must be used. To prepare 6.0 x lo" M KMnO+ mix 0.95 g KMn04 with 100 mL of 1 M sulfuric acid, then remove 1mL and dilute by 100. Add 1 mL of the diluted solution to 100 mL of 1 M M solution. Use a sulfuric acid, giving 100 mL of 6.0 x oath leneth of 0.5 cm.The s ~ e c t r u mof ~ o t a s s i u moermanganate
