edited bv ROBERTREEVES Marlborough School 250 S. Rossrnore Avenue Los Angeies. CA 90004 ~p
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Easy Demonstration of the Visible Spectrum Using the Spectronic 20 Marie Sherman Ursuline Academy, 341 S. Sappington, St. Louis, MO 63122
T h e visible portion of t h e electromagnetic spectrum ext e n d s over t h e wavelengths of 400-700 nm. It is called t h e visible portion because it is t h e only range that t h e cones in t h e h u m a n retina can detect.' T h i s information is commonly found in textbook^^.^ a n d is utilized in spectrophotometers such a s t h e Spectronic 20 b u t is almost never demonstrated to s t u d a n k 4 T h e followinp demonstration can b e nerformed t o provide a relation between t h e wavelength measurements o n t h e Spectronic 20 and the actual colors of light being
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Materials Cut a strip 11 mm X 95 mm from a 3 x 5-in. file card. This strip will fit snugly inside the cuvette normally used with the Spectronic 20. See Figure 1. Tiny dabs of Elmer's Glue on the edges of the strip will hold it in place inside the cuvette, so that it will not be accidentally dislodged." In order tosee the colors easily without squinting, it is helpful to make s "viewing tube". Fold and tape a 15- X 30-em piece of black construction paper as shown in Figure 2. This cylinder of paper should he pinched at the ends into a lozenge-shaped "viewing tube" and will he placed around the cuvette holder far the demonstration.
Test tube (cuvettel Cardboard strip
Figure 1. Cardboard strip cut to fit snugly into a cwene that is normally used far the Spectronic 20.
Procedure
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The cuvette containing the cardboard strio is nlaeed in the instrumenr sothat rhe light pithrs at right nnglei to thecardbuard. (For the Spectronir 20, simply line up thr r a r d h w d strip with the mark n f tube nt the tront crf r h e c u w t t ~hdder.1 ~ P FP i g m ~ l ' c ~ r a v i e wthe from above. Leave the cuvette holder lid open. Note: Tape the test tube in place so that it cannot he turned during the demonstration. If the tube were to be rotated 90D,excessive light could damage the instrument. Set the viewing tube in place around the cuvette holder and secure it with two or three pieces of tape. Open the light control knoh to allow the maximum amount of light to strike the cardboard. The student can look through the viewing tube and down into the test tube with both eyes open.7 Start with the wavelength set a t 340 nm and ask the student if he or she can see any color. Then slowly turn the wavelength knoh while the student reports the color seen. The
' Langley. -. L : Tellord. I. R :Chr stensen. J 0 . Dynamic Analomy andPhyaology: McGraw-Hill: New York. 1969. pp 339-340. Masrerson. W L.; S ow nsri. E. J.: Walloro. E T Chemistry: hoit. Rlnehart. New York, 1980: p 535. Neoergal W. H.: hotzc aw. W H.. ,r., Robinson. W R College Chernrsbv. 6th eo.. death: Lexmaton. MA. 1980: DO 830-834. ~ e a c h e r sof anatomy and physiology might i i s o like to use this demonstration of coior vision in their study of the human eye. Riley, C. M. Laboratory Manual of Instrumental Analysis; McGraw-Hill: New York. 1959. Perhaps someone who is clever with optics can develop a "periscope-withln-a-cuvene" gadget that could project the colors onto a screen s o that an entire class could watch at the same time. S i n c e only 0.5% of women are coior blind, while 6 4 % of men are coior blind to some degree (Morrison. T. F.; Cornen, F. D.: Tether. J. E.; Gratz, P. Human Physiology; Holt. Rhinehart: New York, 1977; p 163). it is desirable to pair lab partners so that at least one of the partners has good color discrimination.
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Volume 64
Number 7
July 1967
627
Cuvette holder
Figure 2. The viewing tube is constructed by taping a black conmuclion paper.
15- X 30-crn piece
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student is provided with a printed sheet to record the color seen at each wavelength. Students are reminded that the observed colors of light are the complements of the absorbed colors. At about 400 nm, the student will report seeing a violet color. Recording each color as it is seen gives the student a vivid idea of the location of each band of color in the visible spectrum. When a waveleneth of about 700 nm is reached. the uoner . . limit of human color rlwm brcome~apparent as thr rrd rdor fndei from view. In rhe lab report, the student i s required to make n drawing rhowing thr e o l w i and thr curresponding uawlrng~hi.'
It has been found that taking the time for this demonstration is worthwhile for two reasons: (1)It enables the student
628
Journal of Chemical Education
Figure 3. A lopview of ma cuvelte dot of the Spectronic 20 wlm the cardboard strip insen& as shown in Figure 1. to make a transition from "concrete to formal" nnderstanding of the visible spectrum and the principles of spectroscopy. (2) It lays the groundwork for a number of later Spectronic 20 experiments that can then be accomplished with a less mechanical attitude and more mature comprehension. After the demonstration, me class is instructed in the theory of spectroscopy, Beer's Law, and the proper use of the instrument. Finally, each pair of students determines the visible spectrum of a colored solution such as 0.1 M CoCI, or a dilute solution of a dye. Readings of percent transmission and absorbance are made and plotted against wavelength. The resulting graphs thus emphasize the absorbance of some wavelengths and the transmission of others, as well as the inverse relationship between percent transmission and absorbance.
