Lab: Understanding the formation of spectra
Purpose: To be become familiar with the use of a spectrometer and able to distinguish
continuum, emission and absorption spectra.
Materials:
Diffraction grating, incandescent bulb, various low pressure tubes
Introduction:
There are three types of spectra, described below.
Continuous – every color is present. It resembles a rainbow.
Emission – only certain colors are present. Looks like individual colored lines.
Absorption – most colors are present, but certain ones are missing and appear as black lines in
the spectrum picture. It is unlikely you will observe any of these in a normal setting.
White light is a mixture of all the other colors of light. (This isn’t the same as when you mix all
colors of paint, which yields black or dark brown.)
!
A diffraction grating is a device that uses tiny
openings to separate the colors of light and sort
them out by wavelength. If you pass a beam of
sunlight through a diffraction grating, you would see
a rainbow on the other side because the grating
causes each color to bend a different amount.
White Light
Light Source
Diffraction
Grating
Light separated by
colors (rainbow or
spectrum)
Astronomers use diffraction gratings to analyze the
precise colors emitted by stars and other objects.
Since each element produces a unique spectrum,
spectroscopy is valuable for detecting the presence
of elements
Procedure:
1. Observing a continuum spectrum: Look at the incandescent bulb through your diffraction
grating. Hold the grating in front of you as shown in the diagram above. Draw the spectrum
you observe on your data table. Since the spectrum is continuous there should be no breaks
between the colors.
CAUTION: The power supply and spectrum tubes use several thousand volts. Do not touch the
spectrum-tube power supply or spectrum tubes when power is applied. Use thermal mitts to
handle the tubes.
2. Observing an emission spectrum: Use the diffraction grating to observe the gases supplied by
your teacher. Record the name of the gas and draw the spectra. Record, as carefully as you
can, which color is brightest and the positions of the colors. Try to draw a distinction between
sharp and fuzzy lines, bright and faint lines, and regions where continuum is bright and faint.
You will observe Hydrogen, Helium, Iodine, and Mercury.
Note: The
illustration shows
sample spectra
drawn carelessly
and with some
detail as an
example. (It is not
a real spectrum.
Don’t copy it.)
3. Mystery Gas – Now examine the mystery gas set up by your teacher. Draw the spectrum.
Compare the spectrum to those you drew earlier and identify the gas.
4. Observing an absorption spectrum: Go to
http://jersey.uoregon.edu/vlab/elements/Elements.html. The default view is an absorption
spectrum. Click on an element to see its absorption spectrum. Look at the spectra for several
elements. Draw the spectrum for Hydrogen and Helium.
5. Observing the sun: Look at the poster of the sun’s spectrum. Describe what kind of features
(emission, absorption, and continuum) you see.
Questions:
1. Compare the color of light emitted by the spectrum tubes to that observed through the
diffraction grating. Explain any difference.
2. Describe the difference in appearance between absorption and emission lines.
3. How did the emission spectrum you drew for hydrogen compare to the absorption
spectrum you drew? Were each of the emission lines present? Did they occur at the same
wavelength?
4. Describe the difference in the physical mechanisms responsible for absorption and emission
lines. Draw a diagram of an atom absorbing a photon, and a diagram of an atom emitting a
photon.
5. The absorption bands in the spectrum of the sun are produced when photons are absorbed
by atoms. When the photons are absorbed, the colors corresponding to the photon
energies are removed from the spectrum. Gaps or bands then appear in the spectrum
where the missing colors would have appeared. In which region of the sun do you think the
atoms which produced these lines are located? Explain. Hint: review your notes about the
conditions under which the three types of spectra are produced.