edited b y
J. DUDLEY HERRON Purdue univenity west Lofayme, hdiana 47907
Demonstrations for High School Chemistry: Uses of He-Ne Laser
white or black light. At the appropriate time the relationship E = hclh is established by manipulating the two precedent formulas.
Joseph F. Castka Van Buren High School Queens, New York 11427 Demonstrations which provide students with experiences related to such topics as atomic structure, electron arrangements, and quantiled energy furnish the pedagogical weapons upon whirh the understanding and acceptance of these concents is based. The two laser demonstrations described in this ariirle may he performed using otherdrvices but the laser is a 'hot" topic these davs. Many of our science students have heard ah& lasers, t,h& very dramatic uses, and possibly of the potential use of lasers in a fusion reactor. The demonstrations of 1) surface photoelectric effect (the release of electrons by incident photons) and 2) measurement of the wavelength of the laser beam (the emission of photons by electron transfer) provide a measure of reality for student understanding of the relationship E = hu. A demonstration helium-neon laser such as the ML-610 model nroduced bv Metroloeic Instruments. 1nc.l or the equivalent which may he purcLased from ~ d m k Scientific d Co.2 costs approximately $100. This laser may he used for both physics and chemistry and two physics-oriented manuals by Gottlieb3 describe many of its uses. T h e facts which may he presented to students, as needed, to complement their observations are: 1) the laser light (beam) is very bright and has a high energy concentration, 2) the beam is monochromatic havina a single color, 3) the wavelength of the beam is 6328 A, 4) thiistrongest radiation is produced when orbital electrons fall from the 352 to the 2P4 level.
T h e resulting discussion helps students understand that the emiaaion of electrons is onlv bv of a definite ..-.---"nroduced . "ohotons . and restricted frequency and that the energy involved can be calculated ifthe wavelength associated with the photon(s) is
Surface Photoelectric Effect
A freshly polished Zn plate attached to a Braun (moving vane) electrosco~eis chareed neeativelv hv contact with a charged ehonite h . ~ x p o s u r eto; black liiht lamp or other source of a hiah ronrentmtion of the ultravi(det such as a HE lamp or a carbon arc produces a rapid discharge of the negatively charged electroscope. Exposing the charged electroscope to the incident laser beam does not produce the discharge. A positively charged electroscope is discharged by neither the laser beam nor the black light lamp. I t is probably strategic to precede these demonstrations with the following 11 Projection of the continuous visible spectrum using a filmstrip projector, a slit, and a replica diffraction grating. 2) Examination of a wall or textbook chart of the continuous white light spectrum and discussion of the relationship e = oh. 3)' The more or less standard demonstration series of surface photoelectric emission including (a) the discharge of the Zn-plate neg:rrively rhargrd rlcctroscop+ hy nmtlcr or n posnively charzed nd. 1 b, thr didlargc ofrhe negatively charged scope hy black 1,ghr ana t h e failure I., diwharer hv" uhiw lieht ltunesren lern~,,.1c1 u falure to discharge the positively ehargld eleectr&ope by kther
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Measuring the Wavelength of the Laser Beam This demonstration appears as part of Experiment 35, Diffraction and Interference Using a Helium-Neon Laser in the Gottlieb manual, "Laboratory Manual for the World of Physics." These topics need not he discussed with students but other information, described below, should be presented. The demonstration may he used in the classroom to precede and introduce the student experiment for measuring the wavelength - of the vellow light . of the sodium flame. This latter experiment appears in many physics manuals and some chemistrv manuals as the one hv Metcalfe, Williams, and ~astka.~. The laser is ~ositionedso that the distance from the laser aperture to a screen or the classroom wall is 1m. Operating the laser results in a bright spot on the screen. Placing a plastic renlica diffraction eratine over the laser aoerture results in aieries of bright dois inccuding the originai spot and a series of maxima to the right and left of the central spot. The distance, x , from the central spot to the first order maxima immediately to the left and right of the central spot is measured with a metric ruler or tape. These two measurements are then averaeed. T h e distance. L, from the first-order maxima-(left and rl'ght) to the replica diffraction grating is measured and the two measurements are also averaged. Students are told that the relationsh~pto he used in ralcditiny the wavelength is .4 = dx L. They are also told that d , the distance between adjacent groows of the plastic replica grating, is 1.9U X IU-' rm. The inexpensive grating used has lJ,40U gronws in. and may be purrhnsed (in class sets, i i desired, from Edmund -Scientific Co. The necessarv calculations are ~ e r f o r m e dbv students a t th;hoard and iitheir seats. ~ e ~ r e s k t a t i resuits ve (averaged) for this demonstration are: x = 0.350 m. L = 1.046 m. Usina m produces the calculatei the value for d = 1.90 X
'Metrologic Instruments, Inc., 143 Harding Avenue, Bellmawr, N.J. 08030. %EdmundScientific Co., Edsearp Bldg., Barrington, N.J. 08001. 3Gottlieb, H. H., "Experiments Using a Helium Neon Laser," 3rd ed., Metrologic Instruments,Inc., Bellmawr, N.J., 1974. "Laboratory Manual far the World of Physics," Addison Wesley Publishing Co., Menlc, Plrk. (.'nhfornin. 1973. '\lrtralfc. H. C., Wtllinmr. .I. E.. and ('aitka, d. F.,"l.sborarors Exnrrlmenta in (.'hrm~atr\.."Hdt. Rinehar~: i d Winston Inr.. U r w ~ o i k1974. ,
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Volume 53, Number 9. September 1976 / 573
wavelengtb of the laser beam as 6.35 X lo-" m. An alternate procedure is to measure the distance from the replica grating to the central spot instead of measuring the distance, L, from the first order maxima to the grating. The value of L is cald a t e d by using the Pythagorean theorem. Activities which may precede the demonstration include: 1)students view the line spectra of Ne and other gas tubes by individually looking through a replica diffraction grating a t a slit-activated cas tube assembly. 2) Student observations are hen compared u,ith a texthook or wall chart of these and other lines~ectra. The energy involved in the electron transition which produces the photon of the calculated wavelength may he calculated by interested students using the formula E = hclk. A representative calculation for the laser beam is
574 /
Journal of Chemical Education
E = 6.63 erg see X
X 3.00 X 1O1Oemlsec 6.33 X 10W crn 10-l3 erg
= +31.4 x The calculation for a mole of photons or electrons is calculated by multiplying the above value per electron by the Avogadro number. Dividing the result by 4.181 ergs gives the result in calories per mole. Interested students may be referred to the comprehensive article by Logan5 which deals with a variety of related observations and calculations based on the use of spectral color slides in studrnt experiments. Another sourre of related problems is Castka? 5Logan, K. R., J. CHEM. EDUC., 51(6), 411 (1974). 6Castka, J. F., "Chemistry Problems," Holt, Rinehart and Winston,
New York, 1962.
