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Spectroscopic Observations I
Purpose
To distinguish between a continuous spectrum and a line spectrum; to observe the spectrum of gaseous hydrogen and
mercury; to use spectroscopic observations to identify unknown gases
Equipment
Project STAR spectroscope, incandescent lamp, hydrogen and mercury discharge tubes, discharge tube power supply,
unmarked discharge tube, fluorescent bulb
Procedure
Study the spectroscope to find its viewing port and slit. To use the spectroscope, point its slit towards the light to be
studied and look straight ahead into the viewing port. Don't point the front of the spectroscope at the light source! We'll try to
keep the lab dimly lit so you can see the scale through the viewing port.
IMPORTANT NOTE: “PURPLE” IS NOT A SPECTRAL COLOR. “PURPLE” IS A MIXTURE OF RED AND
VIOLET. IT IS NOT POSSIBLE TO SEE A PURPLE LINE IN A SPECTRUM!
1.
Point the slit at an incandescent lamp. Describe the spectrum you see, telling whether it consists of a broad band of
colors or distinct lines, what the colors are, and what the corresponding approximate range of wavelengths is. Make sure your
wavelength limits match your description of the colors.
2.
Point the slit at the hydrogen discharge tube. Describe the spectrum you see, telling whether it consists of a broad band
of colors or distinct lines, what the colors are, and what the corresponding wavelengths are. Make sure your wavelengths match
your description of the colors.
3.
One of the spectra you observed is a continuous spectrum, and one is a line spectrum. Tell which is which and how you
are able to distinguish between them.
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4.
Point the slit at the mercury discharge tube. Describe the spectrum you see, giving the colors and the corresponding
wavelengths. Is this a continuous spectrum or a line spectrum? Describe clearly how you arrive at your conclusion.
5.
Every element produces a unique spectrum, so the spectrum can be used to identify an element. An uncoated fluorescent
light bulb will be turned on for you to observe with your spectroscope. Record the wavelengths you see and compare them with
the known spectra observed so far. Use these data to identify the gas in the bulb. If you can’t identify the gas in the bulb,
explain fully why identification isn’t possible.
6.
An unmarked discharge tube will be turned on for you to observe. See if you can identify the gas in the tube. Describe
your procedure, your observations, and your conclusion.
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