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!

Goals    Equipment    Overview    Details    Poem   Survey



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.


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:

Then, build a spectrometer from a kit, following the directions provided by Learning Technologies, and your prof's instructions in class. Hints: 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.

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.


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: 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?
  • ***************************************************************************

    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.



    "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