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PHYSICS 183 for Spring 2009
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Atomic Spectra Caution: Do not touch the metal ends of the discharge tubes while they are turned on. The tubes get quite hot, so turn them off when not in use.
In this experiment you will use an instrument called a spectrometer to measure the wavelengths of the light emitted by excited hydrogen atoms. The spectrometer allows light to pass through a small slit and then through a diffraction grating. The different wavelengths will form maximums at different angles. You can keep track of the different wavelengths by their color. The spectrometer allows us to very accurately measure the angular position of each maximum. You will use a vernier scale to measure all angles to the nearest tenth of a degree. You read the scale just like a vernier caliper. Set the telescope so that the central maximum is in the cross-hair. You may need to focus the telescope to see it clearly. You do this by moving the eyepiece in or out. Record this angle. Move the telescope to the left until you see the first spectral line. Carefully adjust the position of the telescope so that the cross-hair is centered on the line. Record the angle. Then calculate its angular distance from the central maximum. Continuing to move to the left, repeat the measurement for each first order line you can see. Then repeat the process on the right side. Average the right and left angular distance for the corresponding lines. How many complete orders can you see? Is it the same on the right and the left? Do any of the orders overlap? Measure the angle for the last line you can see on each side. Now use your data to calculate the wavelength of each line. The grating has 15,000 lines per inch. Compare your calculated wavelengths to the Balmer lines of hydrogen given in your book on page 1060. Does the wavelength of the last line you saw agree with the corresponding line in the first order?
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