Line Spectrum Demonstration for the Large Lecture Hall 1The undergraduate's learning experience with quantum mechanics and Bohr's WCW theory can be greatly enhanced by the presentation of a line spectrum demonstration. s , , . h An instructor gains a certain degree of student confidence by actual performance of V the experiment in contrast to pictures, diagrams, or having the students recall seeing I ~ . a line spectrum. The mercury spectrum is clearly visible to a class of 350 students k,,and sodium is visible but unsatisfactory. Unfortunately other spectra were too weak ,smm,,-* for viewing in the large lecture situation. The following physical arrangements (see figure) of the apparatus can be easily assembled. A high intensity mercury vapor lamp has proven to be an excellent light source. Most laboratories and schools have this apparatus in the form of an ultraviolet lamp. Simply removing the visible light filter from the lamp allows hoth ultraviolet and visible radiation to be viewed. The lamp is covered with aluminum foil to contain the radiation inside the tube. The convex lens should have a focal length of 15-30 cm. Size is not critical. The first lens concentrates the radiation on the slit, and the second lens is for focusing. It is mounted on a ring stand for easy adjustment without enclosure. The width of the slit can vary 1-3 mm. A machined slit in a metal sheet can be replaced by essembling a slit from available materials such as cardboard, metal sheets or foil. The trammission diffraction grating is available from most scientific equipment companies (Education Absorption Spectra kit, Bausch and Lomb Cat. No. 33-80-10,$47.50). The grating has 600 lineslmm and concentrates the light in a 20' angle to make a first-order spectrum. One can substitute a diffraction grating from Edmund Scientific Co. 16'.40,, hut there will he l n i ~ o resaluuun f and intenstly. The Imes are clearly visible on an ordinary screen, and by using fluorescent rnateriali the ulrraviolrt lines can he viewed. ~Caurion:ultraviolet radiation ran damage the rprthel~umuf the cornea, but g~nerallydoes not penetrate the eye except in extremely high concentration.)' For classes less than 30 students the spectrum can be observed by mounting a narrow hydrogen discharge tube (spectrum tube) in a boa with Edmund's plastic sheet diffraction grating over the front. Other spectrum tubes can be substituted such as 12, Hg, 0rD2.
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1 Steere, Norman V., (Editor), Hsndhook of Laboratory Safety," 2nd Ed., The Chemical Rubber Ca., Cleveland, Ohio, 1971, p. 90.
J e r r y A. Driseoll University of Utah Salt L a k e c i t y , 84112
Volume 51, Number 2, February 1974
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