| Spectroscopes: To break
a beam of light into a spectrum, one separates the wavelengths of light with either a
grating or a prism. But, before the light
passes through the grating or prism, it should first pass through a narrow slit. Then the
light that passes through the grating or prism is an image of the slit and so an image of
the slit will be seen at each wavelength (the lines you may see in the spectrum). Since
the slit is narrow, there will be minimal overlap of the images at neighboring
wavelengths. The separate wavelengths can then be distinguished from each other. Imagine,
for example, if the light source was as large as this building and a slit was not used.
The red image of the building would overlap the blue image and all the colors in between
would be smeared together. You would not be able to see the individual colors and so you
would not have succeeded in breaking up light into a spectrum.
Lab Part I: Make a simple spectroscope and observe
various light sources
To
make you spectroscope, you will need a holographic grating, an eyepiece, a slit, and a
tube. Construct the spectroscope as follows:
align the piece of grating to the hole in the
eyepiece and tape it securely
tape the eyepiece to one end of the tube,
preferably with the grating to the inside of the tube
tape the slit to the other end of the tube
with the slit running in the same direction of the long side of the hole in the eyepiece
(the slit should run perpendicular to the direction that the grating spreads out light)
Your
spectroscope is ready to be used. Look through the eyepiece at a bright light source like
the overhead lights, and you will see a spectrum on either side of the slit.
Use
your spectroscope to complete questions 6, 7 and 8 below.
Lab Part II: Observe, sketch and
identify atomic spectra
With
your spectrometer, look at each of the hot gasses in the four Balmer light tubes. Note that
each gas emits a series of emission lines and that each has a completely different set of
emission lines. You will sketch each of the spectra, then identify
the element responsible by referring to the interactive periodic
table at the following website:
http://javalab.uoregon.edu/dcaley/elements/Elements.html
Consider the following:
Why does the light from the gasses appear as
‘lines’?
Draw a simple diagram of each of the spectra. Take care that you draw the correct pattern in the
relative separations of the lines. Also indicate the color of each line. These diagrams
represent the spectral fingerprints of these gasses.
Try looking at an ordinary fluorescent light
in the class or hall and see if you can identify what gas is responsible for the light.
Point your spectroscope at an ordinary
incandescent lamp. What does its spectrum look like?
How does the spectrum differ, in general
character, from the spectra of the light sources in the gas tube?
Sometime during the day (later in the week)
point your spectroscope at the sky, near (but not at!!) the Sun. What type of spectrum do
you see (emission line, continuum, continuum with emission lines, or continuum with
absorptions lines)?
Briefly explain why the Sun has the kind of
spectrum that it does (remember Kirchhoff’s laws).
Sometime later in the week point your
spectroscope at some lights around your home or place of work and determine which ones are
incandescent lights and which are not. Observe at least four
different light sources, with at least one having an emission
spectrum and at least one with an emission spectrum.
***** Submit the answers in a
basic lab report adhering to the following format *****
I. Introduction:
In this section you give some background information and
state the nature and purpose of the experiment. All of the
information you need to complete the introduction can be found in
the text book. This should be one to two paragraphs long.
II. Observations:
This section includes information from your observations. You
must include the following information:
*
reference to the sketches of the emission spectra you observed in
class (attach sketch to report)
*
element identification for each of the emission spectra you observed
in class
III. Results and
Discussion:
This is where you give the results of your experiment and
observations. You must include the following information:
*
answers to questions 1 through 8 above (you may need to refer to the
text for some of the information)
*
name the elements you observed in class and give the type of spectra
*
name the light sources you observed at home, and give the type of
spectra
IV. Conclusions:
In this section you summarize the experiment and your results.
A simple lab report such
as this one should be about one to two pages in length. Be sure to
attach your spectra drawings to your report.
This lab
report is due on Wednesday, February 9
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