PURPOSE: Demonstrate diffraction spectrum of white light along with line spectra of mercury and cadmium.
DESCRIPTION: Three sources are permanently mounted on a roll-around cart, from top to bottom: (1)a clear glass long-filament incandescent light bulb which produces a continuous white light spectrum, (2) a mercury lamp which produces a line spectrum, and (3) a cadmium lamp which produces a line spectrum. The spectra are shown in the photograph at the right.
These spectra are seen using 1"x2" sections of a large roll of replica diffraction grating material with 13,200 lines per inch. The pieces of grating material are relatively cheap, and may be given to the students as a momento of their outstanding physics experience at Maryland. Tell your students to go away and look at the spectra of various lights.
The three lamps are mounted in a vertical line so the colors of the lines are the same as those in the adjacent white light spectrum. Point out that the spectra of mercury and cadmium are very different, and generalize that observation to suggest uniqueness of the spectra for each material.
SUGGESTIONS: Can you see what is wrong with the spectra in the photograph? It has to do with the nature of the film (or the video) used to produce the photograph. See Demonstration N3-51: SPECTRUM WITH MINICAM AND VIDEO PROJECTOR.
REFERENCES: (PIRA 7B10.10)
EQUIPMENT: Rolling cart with three sources, box of hand-held pieces of replica diffraction gratings.
SETUP TIME: None.
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