Spectra workshop

for Astronomy with Stars, Sky, and Culture, spring 1998

by E.J. Zita, April 1998, with excerpts from the Learning Technologies guide

This workshop is designed to let you see 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, has resulted in the discovery of 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

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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/draw what you see. What color is closest to the bulb?
Look at a fluorsecent light. Describe/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. Don't peel off the film. Clean any fingerprints off the grating gently before final installation, using a soft, clean cotton T-shirt.
Cut about 1/4 inch off the slit end of the long dark scaled film before threading it into your spectrometer
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 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.

Look at 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. Is there any correlation? 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.

What is the corresponding energy and frequency of that light? Also estimate each source's temperature. Refer to the lecture notes for Chapter 3 and Chapter 4, and look at the electromagnetic spectrum on p.55 of Kaufmann.

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DETAILS of underlying physics:

(A) Light is spread 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 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.

If you really want to know, 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 goes through the glass at slightly different speeds.

(B) Different elements have different spectral "fingerprints"

This lets us determine the chemical composition of stars. In addition, different types of sources have characteristic differences in their spectra. What differences do you find for:

the incandescent filament in an ordinary light bulb (such as in the overhead projector)
the hot rare gases in the glowing tubes
the cool rare gases in the absorption experiment, if available

Summarize your conclusions in your own words. These are also known as Kirchhoff's laws, but please don't look them up first in your text- that will only prejudice your observations.

Take your spectrometer home and use it for this week's 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.

C: EXTRA: After you measure wavelength, you can calculate the energy, frequency, and temperature of the light.

Remind yourself of 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^-19J

Stefan-Boltzmann law: Intensity = sigma T⁴ (Watts/m²), sigma = 5.67 x10^-8 (J/m².K⁴.s). Wien's law: TL=2.9 x 10^-3 m.K

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.

As stars burn, they transform light elements into heavier ones, through nuclear fusion. This process releases energy (the star's radiation) until iron is reached. To create elements heavier than iron, additional energy must be input. That happens primarily in supernovae, where the star 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

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Maintained by: E.J. Zita
E-mail:zita@evergreen.edu
Last modified: 22.April.1998