ATOMIC SPECTRA FOR ASTRONOMY
You’ve seen that objects in the universe vary in color. An object’s spectrum is the pattern of different colors, and different amounts of light of each color, that comes from the object to your eye or detector. In this lab exercise, you will learn how to interpret and analyze spectroscopic measurements of objects that are hotter than their surroundings (known as “blackbodies”) and of atoms that emit light at specific colors.
Please read the instructions and explanations carefully. You will only need to turn in the Lab#6 Answer Sheet at the end of this document; please put all your answers there.
PROCEDURE
Step 1
(a) Go to the website phet.colorado.edu – this is the main portal of the PhET Interactive Simulations site at the University of Colorado. Survey. Among the physics simulations, Click on the “Blackbody Spectrum” link and read the brief “About” entry. Then, either click on the image to start running the simulator on your web browser or download the simulator and start it.
(b) You will see the spectrum (the curved line) plotted on the graph, and the temperature initially set to that of the surface of the Sun. Familiarize yourself with the simulator by clicking on the various buttons – the magnifying glasses on the X and Y axes, the boxes “Graph Values,” “Labels,” and “Intensity,” and the temperature slider on the right side. Spend some time, with your instructor as a guide, to get a sense of the meaning of the various terms and numbers on this simulation page.
Now, move the temperature slider to the number closest to the last four digits of your phone number. (For example, if your last four digits are “6678” then move the slider to 6700 K; if they’re “0765” then move the slider to 750 K.) Adjust the vertical and horizontal axes until you see the spectrum plotted on the graph with the spectral peak (the top of the big hump) near the middle of the page.
As accurately as you can, sketch this graph on the answer sheet. You must label the X and Y axes, the graph values of the spectral peak, and the intensity of the spectrum. Make sure the units are correct in all cases.
(c) Using the simulator, make 8 measurements of the graph values for the spectral peak at 8 different temperatures and fill out the table on the answer sheet.
(d) What mathematical relationship did you observe between the surface temperature and the spectral power density? Between the surface temperature and the wavelength of the spectral peak? Between the surface temperature and the intensity? Be as specific as you can – you don’t have to compute a mathematical formula, however; consult with your lab instructor.
Step 2
(a) https://www.physics.umd.edu/courses/Phys401/bedaque07/discrete_spectra.jpg is an idealized image of the spectra of what several kinds of gases might look like if they are viewed through a diffraction grating. Open this image in a separate browser window from the PhET Blackbody Spectrum simulator and look at them side by side. About what surface temperature should an object have to produce the spectrum at the bottom of the image? Explain in about 35-70 words how you figured that out using the simulator. (Your explanation is just as important as the temperature you derived.)
(b) Now look at https://www.umsl.edu/~physics/files/images/electricity/diffraction.jpg in yet another browser window. This is a photographic image the spectra of the gases hydrogen, mercury, and neon as shown in part (a) and some different ones as well. There are, however, no wavelength labels on the X axis of these spectra. Fortunately, the colors on the two images correspond pretty well. (If you have trouble distinguishing colors, speak with your instructor – there is an alternate viewing method.)
Using a ruler (or some digitally equivalent tool) and some math, calculate the wavelengths of 3 emission lines of different colors for hydrogen (H), mercury (Hg), and Neon (Ne) in the image in (a) – be sure to use the correct units, which in this case are nanometers (nm). On the answer sheet, begin by writing down the steps you will take as well as the measurements and calculations you will make to get your answer. Three hydrogen lines have their wavelengths filled on the table for your reference.
Lab#6 Answer Sheet for Atomic Spectra For Astronomy (p. 1 of 3)
Step 1
(b) Graph Values with units: ______________, ______________ Intensity: ______________
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(c) Write down the last 3 digits of your phone number: _______ this is N.
Temp (K) Peak Power Density Peak Wavelength Intensity
1000+N
2000+N
3000+N
4000+N
5000+N
6000+N
7000+N
8000+N
Lab#6 Answer Sheet for Atomic Spectra For Astronomy (p. 2 of 3)
(d) Mathematical relationship between surface temperature and:
Spectral Power Density _______________________________________________
Wavelength of Spectral Peak___________________________________________
Intensity _____________________________________________________________
Step 2
(a)
(b) Procedure:
Gas Line Color Calculations Wavelength
H (line 1) blue 434 nm
H (line 2) green 486 nm
H (line 3) red 656 nm
Hg (line 1)
Hg (line 2)
Hg (line 3)
Ne (line 1)
Ne (line 2)
Ne (line 3)
