Spectra workshop
for Astronomy and Cosmologies,
spring 2013
by E.J. Zita,with excerpts from the Learning Technologies
guide
This workshop is designed to show you how different light sources
produce different colors. Prisms and diffraction grating spread light into
its component colors, or spectrum. Observing spectra of substances
on Earth shows us what things are made of. This has revealed new elements on Earth, and is one of science's most powerful diagnostic
tools. Observing spectra of stars such as the Sun not only has revealed
new elements (such as Helium, named for Helios the Sun), but also helps
us understand the composition and life cycles of stars, and how stellar
deaths can create elements necessary for human life.
Do parts A and B. Leave C for the very end in case you have
extra time. Maybe design some investigations of your own... think about
the poem, and turn in the survey before you leave. Have fun!
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GOALS:
a) To build and use a spectrometer.
b) To become familiar with spectra from some common elements and objects.
To understand what spectra can reveal about the nature and material of
a distant light source.
c) To learn to analyze spectra more quantitatively.
Equipment:
-
spectrometer kits
-
incandescent light bulb
-
emission tubes + power supplies (H, He, Hg...)
-
absorption spectra materials?
OVERVIEW of activities:
A diffraction grating has many closely-spaced lines that make
light of different colors bend through different angles. The diffraction
grating on your little plastic disk is the heart of your spectrometer.
Don't peel off the film - that's the grating!
(A) Your Spectrometer:
First, look through the diffraction grating on your
plastic disk to see what it does to different sources of light:
-
Rotate the disk to see how the rainbow rotates. Get the spectrum
oriented horizontally.
-
Look at a light bulb. Describe and draw what you see. What color is
closest to the bulb?
-
Look at a fluorscent light. Describe and draw what you see. How is
it different from the bulb's spectrum?
Then, build a spectrometer from a kit, following the directions
provided by Learning Technologies, and your prof's instructions in class.
Hints:
-
Handle your little round diffraction grating carefully, by the edges. Don't
peel off the film. Clean any fingerprints off the grating gently
before final installation, using a soft, clean cotton T-shirt.
-
You might need to cut about 1/4 inch off the slit end of the long dark scaled film
before threading it into your spectrometer. Check the length first.
-
Cover the hole on the left side of the installed film strip with the piece
of black electrical tape that it came with.
Point the pointer on the right front of your completed spectrometer
at the fluorescent lights. (The light needs to come in the little slit
on the far right of your spectrometer ). Find the rainbow, and twist
your diffraction grating carefully so the rainbow lines up horizontally.
Look through teammates' spectrometers and help each other calibrate
your spectrometers by sliding the long film right or left, following
the instructions on the top of your spectrometer.
Write your name and email/phone number on your spectrometer, just in
case.
(B) Understanding
Spectra: Look at different ionized gas sources, in the
glowing tubes and in the fluorescent lights.
-
Sketch the different spectra carefully, noting colors or using colored
pens, and marking where they fall on the film scale. The scale measures
light wavelength in nanometers (10-9 m).
-
Compare the spectra from different sources, including an incandescent
light (with a burning filament - a "blackbody").
-
Summarize the tendencies you notice, in your own words.
-
Discuss observations and conclusions with your team members.
Also observe each source WITHOUT your spectrometer and write down its apparent
color. Then compare to the colors of the spectral lines that you see in
your spectrometer. Are there any correlations? Surprises?
(C) Analyzing Spectra:
Measure the dominant (brightest) wavelength L of each source,
and note how its color compares to the color of the source you see with
naked eye.
If you like (and if you have extra time), you can calculate the corresponding energy and frequency of each color of light. See equations in C below.
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DETAILS of underlying physics:
(A) Light spreads out in space when it
diffracts through a grating.
Longer wavelengths (from less energetic light) bend more as they go around
each stripe of the grating, splitting white light into a rainbow. A very fine diffraction
grating allows you to see details about the color of light from a source.
The film scale, once calibrated, shows you the wavelength of each stripe
of light.
In case you're curious, the angle Q through which light of wavelength
L will be diffracted by slits separated by a distance d is given by: d sin Q = mL (where m = 0, 1, 2,...) (HRW.1088) A prism
does something similar, but the spacing between colors changes, since different
wavelengths of light travel through glass at slightly different speeds.
(B) Different elements
have different spectral "fingerprints"
Spectra enable us to determine the chemical composition of stars. In addition,
different light sources can have characteristic differences in their spectra.
What differences do you find for:
-
the incandescent filament in an ordinary light bulb (e.g. in a reading lamp or an overhead
projector)
-
the hot rare gases in the glowing gas tubes
- the plasma in fluorescent lights
Summarize your conclusions in your own words. (Some of these are called Kirchhoff's laws, but please don't look them up until you have already articulated your own observations.
Take your spectrometer home and use it for further Observing
Projects: look at a bright white cloud, the lights on campus at night;
check out your lava lamp or a full Moon... but DO NOT LOOK DIRECTLY AT
THE SUN - it can blind you, quickly, painlessly, and irreversably. If you
want to look at the Sun's spectrum, point your spectrometer's slit toward
the bright sky NEAR the Sun, or at bright clouds.
C: EXTRA: After you measure wavelength, you can
calculate the energy, frequency, and temperature of the light.
If there is equipment available for observing burning samples, try that. It can be tricky, so be patient, or move on if it doesn't work. There is a fundamental difference between these spectra and those of glowing gases in a tube, even for the exact same elements. What is the big difference? Why? One is an emission spectrum, the other is an absorption spectrum. What does that mean?
Recall the defining relationships between energy (E in Joules),
wavelength (L in meters), frequency (f in 1/s or Hertz), and temperature
of light (T in Kelvin). Write down the constants you need, complete with
units. Estimate these four quantities for each of the sources you looked
at today. Since each color has a different wavelength, each color has a
its own E, f, and T.
E= hc/L= hf, where Planck's constant h=6.63 x 10-34
kg.m2/s, speed of light c=3x108m/s, and
Energy has units of Joules = Newton.meter = kg.m2/s2,
or electron volts: eV=1.602x10-19 J
Stefan-Boltzmann law: Intensity = sigma T4 (Watts/m2),
sigma = 5.67 x10-8 (J/m2.K4.s); Wien's
law: TxL=2.9 x 10-3 K.m
How does the temperature of the gas in an emission tube compare to the
temperatures of its several emission lines?
How could one measure the spectra of distant stars?
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Spectra from stars teach us that stellar deaths produce the elements of
life on Earth.
As stars burn, they transform light elements (such as Hydrogen) into heavier ones (such as Carbon), through
nuclear fusion. This process releases energy (mostly via the star's radiation) until
iron (Fe) is reached. To create elements heavier than iron, additional energy
input is required. That happens primarily in supernovae, when stars transforms
heavy elements such as iron into yet heavier metals and minerals, using
the kinetic energy of the exploding star in its death throes.
The Origin of Order
by Pattiann Rogers
in Splitting and Binding, Wesleyan University Press, 1989
Stellar dust has settled.
It is green underwater now in the leaves
Of the yellow crowfoot. Its potentialities
Are gathered together under pine litter
As emerging flower of the pink arbutus.
It has gained the power to make itself again
In the bone-filled egg of osprey and teal.
One could say this toothpick grasshopper
Is a cloud of decayed nebula congealed and perching
On his female mating. The tortoise beetle,
Leaving the stripped veins of morning-glory vines
Like licked bones, is a straw-colored swirl
Of clever gases.
At this moment there are dead stars seeing
Themselves as marsh and forest in the eyes
Of muskrat and shrew, disintegrated suns
Making songs all night long in the throats
Of crawfish frogs, in the rubbings and gratings
Of the red-legged locust. There are spirits of orbiting
Rock in the shells of pointed winkles
And apple snails, ghosts of extinct comets caught
In the leap of darting hare and bobcat, revolutions
Of rushing stone contained in the sound of these words.
Maybe the paths of the Pleiades and Coma clusters
Have been compelled to mathematics by the mind
Contemplating the nature of itself In the motions of stars.
The pattern Of the starry summer night might be identical
To the structure of the summer heavens circling
Inside the skull. I can feel time speeding now
In all directions deeper and deeper into the black oblivion
Of the electrons directly behind my eyes.
Child of the sky, ancestor of the sky, the mind
Has been obligated from the beginning
To create an ordered universe
As the only possible proof
Of its own inheritance.
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References
"The Spectrometer - measuring
the universe with color", by Learning Technologies, Project Star, Harvard
Smithsonian Institute
HRW = Halliday, Resnick, and Walker, Fundamentals
of Physics, 4th Ed., Wiley 1993
Kaufmann = Kaufmann and Comins, Discovering
the Universe, 4th Ed., Freeman 1996
Return to: Astronomy
home page
Maintained by: E.J.
Zita
Last modified: 8.April.2013