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 (plural of spectrum) of substances on Earth shows us what things are made of. Spectroscopy reveals constituent elements of materials, 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 in the Homeroom; you can do part C in the CAL (or on your own computer, if properly set up). D: Finally, read the poem, think about what you have learned, and turn in the survey before you leave. Have fun!

Goals    Equipment    Activities    Feedback   Poem

(A) To learn how to observe spectra from some common elements and objects.

(B) To understand what spectra can reveal about the nature of a distant light source.

(C) To learn to analyze spectra more quantitatively.

(D) Reflect on what you have learned, using Pattiann Rogers' poem, The Origin of Order, and your feedback form

• diffraction gratings (small squares in film mounts)
• handheld spectrometers (triangular plastic devices)
• incandescent light bulb (in the lamp on the back shelf)
• emission tubes + power supplies (H, He, Hg...)
• cool online software from (I) Teacher's Lab and (II) Kansas State University (written up in the Feb.2006 Physics Today, p.33)

(A) Diffraction gratings and spectrometers: A diffraction grating has many closely-spaced lines. These act like a prism to make light of different colors bend through different angles. The little plastic disk in the eyepiece is the heart of your spectrometer.

First, look through the square diffraction grating (if available) to see what it does to different sources of light:

• Rotate the square to see how the rainbow rotates. Get the spectrum oriented horizontally, for maximum spread in the colors.
• Look at an incandescent 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, get a handheld spectrometer:

• Point the opening on the right front of your spectrometer at the fluorescent lights in the room. (The light needs to come in the little slit on the far right of your spectrometer ).
• Find the rainbow spectrum, 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 (if possible), following the instructions on the top of your spectrometer.

Underlying physics: 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,...). 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) Understanding Spectra: Look at different ionized gas sources, in the glowing tubes and in the fluorescent lights. Agree with classmates when to turn out the room lights and shut the blinds. Do not touch the metal ends of the gas tubes - they are in high-voltage sockets.

• 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 with your naked eyes (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? (The pink color of Hydrogen comes from the dominant line in the visible series, the "H-alpha" line.)

Underlying physics: 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. Kirchhoff's laws describe the differences between spectra 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

(C) Analyzing Spectra: (refs: Universe Ch.3,4; Giancoli Ch.32.6, 38; Raff.11.2)

I. Calculations:

• Measure the wavelength (L=lambda) of the dominant (brightest) line in each source, using your spectrometer.
• Note how its color compares to what you see with naked eye.
• What is the corresponding energy (E) and frequency (f) of that light?
• Estimate each source's temperature (T).

II. Online explorations:

• Try to identify the composition of the "mystery star" in the "Stellar Spectra" workshop at the Teacher's Lab.
• Try to reproduce the Hydrogen spectrum by inputting energy levels in the "Hydrogen Spectroscopy" lab at KSU.
• If you're ambitious and have time, try this for other elements at the KSU site (Spectroscopy Lab Suite - Gas Lamps - Emission)

Underlying physics: Since each color has a different wavelength (L), each color has its own Energy (E) and frequency (f). Recall (below) the defining relationships between energy, wavelength, frequency, and temperature of light. [N.B.: Superscripts, subscripts, and symbols may not show up quite right on this web page, so consult your text to be sure.]

E= hc/L= hf, where Planck's constant h=6.63 x 10^-34 kg.m²/s, speed of light c=3x10⁸m/s, Energy has units of Joules = Newton.meter = kg.m²/s², or electron volts = eV=1.602x10^-19 J; frequency has units of (1/s = Hz), and temperature is in Kelvin.

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

Extra:

• How does the temperature of the gas in an emission tube compare to the temperatures of its several emission lines? What's the difference?
• How could one measure the spectra of distant stars?

***************************************************************************

D: Spectra reveal that stellar deaths produce the elements of life on Earth.

As stars burn, they transform light elements into heavier ones, through nuclear fusion. This process releases energy (the star's radiation) until iron is formed in the star's core - the heaviest stable element. 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.

A classmate with a fine voice may read Rogers' poem slowly and clearly.

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.

***************************************************************************