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Spectroscope Assignment (H)
1. What are the three types of spectra and how each is produced?
2. Why does each element have its own unique spectrum? (Be sure to refer to atomic structure
in your answer.)
3. Why might spectral analysis be a very valuable tool for astronomers to use when they view
stars and other heavenly bodies that emit light?
4. What colors are present in a continuous spectrum? Be sure to identify them in the correct
order.
5. What color of light has the longest wavelength? What color of light has the shortest
wavelength?
6. How does a spectroscope work?
Background Reading for Spectroscope Assignment
What Is A Spectrum?
(RESOURCE: http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectra.html)
Have you ever seen a spectrum before? Probably so! Mother Nature makes beautiful ones we
call rainbows. She takes sunlight, sends it through raindrops, and shows us the grand spectacle of
the rainbow. She spreads out sunlight to display its various colors (the different colors are just
the way our eyes perceive radiation with slightly different energies). A rainbow is a spectrum!
The word “spectrum” (the plural of which is “spectra”) comes from a Latin word, spectare,
which means “to make a display out of something.” In astronomy, the thing we often make a
display of is radiation. In particular, we spread out radiation into tiny increments of energy in
order to examine all of its pieces. On a "BIG" scale, we can think of the electromagnetic
spectrum, which refers to all the different energies of radiation from the very lowest energy radio
waves to the very highest energy gamma-rays. But since it is hard to examine the whole
electromagnetic spectrum at once, scientists often break it down into smaller regions for their
studies. In this laboratory we will be focusing on only the visible region of the spectrum.
What Can Scientists Learn From A Spectrum?
(RESOURCE: http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectra.html)
It took a long time for scientists to learn how to use a spectrum to gain insight into the
universe. Isaac Newton (1642-1727) saw the spectrum from sunlight as a continuous band of
colors. In 1802, William Wollaston (1766-1828) observed several dark lines in the spectrum of
the Sun and hypothsized they represented natural divisions between the colors. Later, Joseph von
Fraunhofer (1787-1826) looked at sunlight with an even better prism and saw 600 such dark
lines in the spectrum. Finally, Gustav Kirchhoff (1824-1887) figured out what was going on and
gave the world a set of rules describing what was making both the continuous spectrum that
Newton saw and the dark lines that Fraunhofer saw.
Rule #1:
A luminous solid or liquid emits a continuous spectrum of all wavelengths. It does
not have any lines in it.
Rule #2:
A rarefied luminous gas emits light whose spectrum shows bright lines. These lines
are called emission lines.
Rule #3:
If the light from a luminous source passes through a gas, the gas may extract certain
specific energies from the continuous spectrum. We then see dark lines where the
energy has been removed. These dark lines are called absorption lines.
Each element in the periodic table can appear in gaseous form and will produce a series of bright
lines unique to that element. Hydrogen will not look like helium, which will not look like carbon,
which will not look like iron... and so on. Thus, astronomers can identify what kinds of stuff are
in stars from the lines they find in the star's spectrum. This type of study is called spectroscopy.
The science of spectroscopy is quite sophisticated. From spectral lines astronomers can
determine not only the elements, but also the temperature and density of those elements which
make up a star. Furthermore, spectral lines can tell us about any magnetic field or winds a star
may have. The width of each spectral line can tell us how fast a material in a star is moving. A
shift back and forth in the position of spectral lines can indicate that the star may be orbiting
another star as well as be used to estimate the mass and size of a star. Lastly, an increase or
decrease in the strength/brightness of spectral lines can indicate a physical change occurring
within a star. Spectral information can also tell us about the material surrounding stars. This
material may be drawn by gravity into the star from a doughnut-shaped disk around the star
called an accretion disk. These disks often form around a neutron star or black hole. Light from
the stuff between the stars allows astronomers to study the interstellar medium (ISM). This tells
us what type of stuff fills the space between stars. Space is not completely empty! There’s lots of
gas and dust between stars. Spectroscopy is one of the fundamental tools with which scientists
use to study the growth and composition of the universe.
Review Of The Three Types Of Spectra
(RESOURCE: http://csep10.phys.utk.edu/astr162/lect/light/absorption.html)
An emission spectrum is produced when light passes through a hot, thin gas in which the atoms
do not experience many collisions (because of the low density). The emission lines correspond to
photons of discrete energies that are emitted when excited electrons make their transition back to
lower-lying levels.
A continuous spectrum results when the gas pressures are higher, so that lines are broadened by
collisions between the atoms until they are smeared into a continuum. We may view a continuum
spectrum as an emission spectrum in which the lines overlap with each other and can no longer
be distinguished as individual emission lines.
An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the gas
absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the
same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the
spectrum.
How Are Spectra Classified?
(RESOURCE: http://www.uccs.edu/~tchriste/courses/PES105/105lectures/ 105lecspectro.html)
Spectra are classified according to their appearance and nature of origin:
*
A continuous spectrum is said to be continuous because all wavelengths are present.
This type of spectrum is easily identified by the presence of all of the colors of the
rainbow (ROY G BIV), meaning there are not any breaks in color.
*
An emission spectrum consists of all of the radiations emitted by an atom or molecule. This
type of spectrum is easily identified by the presence of bright colored bands in an otherwise
dark background.
*
In an absorption spectrum, some wavelengths of light are missing because they have been
absorbed by the medium through which the light has passed. This type of spectra is easily
identified by the presence of dark bands in an otherwise continuous spectrum.
~ If the position of the dark bands of an absorption spectrum match the position of the
bright colored bands in an emission spectrum, then that element is present in the
medium through which the light has passed. Refer to the spectra pictured above.
This element is hydrogen.
What Do Spectra Have To Do With The Structure Of Atoms?
(RESOURCE: http://www.uccs.edu/~tchriste/courses/PES105/105lectures/ 105lecspectro.html)
*
Since each element has a different spacing between its electron orbital energy levels, each
element will have it’s own spectral “fingerprint.” The jumps that an electron makes can be
of different sizes, hence differing energies and wavelengths (colors) will be absorbed or
emitted creating the element’s “fingerprint.”
~ The “fingerprints” of the elements can be found on the following websites. You will
be expected to use these websites to either accept or reject you hypothesis for this
laboratory.
http://jersey.uoregon.edu/vlab/elements/Elements.html
http://astro.u-strasbg.fr/~koppen/discharge/
*
An emission spectrum is produced when an electron jumps down form a higher level to a
lower one. As a result of this jump, light with a specific energy and wavelength will be
emitted.
*
An absorption spectrum is produced when light with just the right amount of energy
(wavelength, color) hits the atom, and the electron jumps to the next higher energy level. As
a result of this jump, only this color of light will be absorbed.
How Does A Spectroscope Work?
(RESOURCE: http://www.windows.ucar.edu/tour/link=/teacher_resources/
space_astronomy/page35.html)
Unlike a prism, which disperses white light into the rainbow colors through refraction, the
diffraction grating used in this spectroscope disperses white light through a process called
interference. The grating used in this activity consists of a transparent piece of plastic with many
thousands of microscopic parallel grooves. Light passing between these grooves is dispersed into
its component wavelengths and appears as parallel bands of color on the retina of the eye of the
observer.
When light enters through the slit it strikes the transparent grating at an angle, passes through it,
is separated into its component wavelengths, and travels to your eye. As you look through the
spectroscope you will see a scale opposite the grating. The image your eye receives is
superimposed the lines of the spectrum on the scale so you can see them!