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Spectroscopic Observations II
Purpose
To observe absorption spectra; to observe the emission spectra of low pressure and high pressure sodium lamps; to
identify an element in pickles
Equipment
Project STAR spectroscope, low pressure sodium lamp, high pressure sodium yard light, dill pickles, Variac, nails, wires
Absorption Spectra
When a gas is excited as in the first part of the experiment, it produces light at wavelengths that are unique to that gas.
This emission spectrum is one of the many clues physicists used to arrive at the modern atomic theory. If white light is shined
through a cold gas, the gas will absorb light at the same wavelengths emitted by the hot gas. Because of the absorption, colors
will be missing from the light after it passes through the gas, and the resulting absorption spectrum seen in the spectroscope will
have dark lines where bright lines were seen in the emission spectrum.
Procedure
1.
A low pressure sodium lamp will be set up for you to observe. Point the slit at the lamp and describe the spectrum.
2.
Take the spectroscope to a place where you can see reflected sunlight. For example, point the spectroscope slit at the
sky. Never point the spectroscope directly at the sun!!!!!!! If you look carefully, you'll see many fine dark lines in the
spectrum. These are called Fraunhofer lines, and they are due mainly to absorption of the sun's light by gases in its own
atmosphere. (The element helium was discovered from its Fraunhofer lines and was originally believed not to exist on earth.)
Can you see the dark lines? Try to read the wavelengths of some of them to see if they match emission lines we’ve observed in
this lab. (All the emission lines we’ve observed should be visible as Fraunhofer lines.)
Spectroscopy 2
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3.
A high-pressure sodium vapor yard light will be set up for you to observe. Point the slit at this lamp and describe the
spectrum. Compare it with the spectrum of a low pressure sodium lamp. (Please give a fairly detailed comparison, including
wavelengths. Don’t write something vague like “It’s different” or “Both contain yellow light”. You should notice something
very special about the wavelength at which you saw a bright emission line in the low pressure sodium lamp.)
4.
You should observe a yellow band with a dark line in the middle of it. What is the wavelength corresponding to that
dark band?
5.
How does the wavelength corresponding to the dark band compare with the wavelength of the light emitted by the low
pressure sodium lamp?
6.
Please provide an explanation of the dark line in the spectrum from the high pressure sodium lamp. (Hint: Recall the
Fraunhofer lines in the solar spectrum. What causes them? Think about how a similar phenomenon could occur within the bulb
in the high pressure sodium lamp.)
Spectroscopy 2
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7.
A popular (but slightly dangerous) experiment is the "electric pickle". A dill pickle is impaled with nails at each end,
and the nails are attached to a lamp cord. When the cord is plugged in, something interesting happens. Describe that here.
8.
Look at the light from the pickle with your spectroscope. Based on observations made so far, what element do you
suppose is present in the pickle? How do you know that?
Spectroscopy 2
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