Solar Absorption Spectrum II: Fraunhofer Lines

Now that you’ve learned a bit about what diffraction gratings do to monochromatic (single color) light, it’s time to use them to look at more complicated light sources.

In this lab, you will use a device called a spectrometer to perform precision measurements of the wavelengths of light emitted by or absorbed by different materials. These collections of light are called spectra, and every material has a unique spectrum, kind of like an optical fingerprint. By measuring the spectrum of an unknown material, you can identify what it is made of.

After some practice looking at different spectra, you will turn to our ultimate goal: looking at the spectrum of light that comes from the sun. We normally think that sunlight is made up of all the colors of the rainbow, but is that really true? Maybe there are clues in the solar spectrum that can help us understand what that big ball of gas is made of!

Objectives


By the end of this lab, you will have…

  • … learned to use an optical spectrometer;
  • … observed emission spectra from different isolated gases and identified the emission wavelengths;
  • … observed the absorption spectrum of holmium perchlorate and identified the absorption wavelengths;
  • … observed reflected solar light and identified the absorption lines present in the spectrum; and
  • … associated as many lines as possible in the solar spectrum with known emission lines from the individual discharge lamps.

Survey


Before we start today's lab, we are asking all students to complete a short (<5 minute) survey in which you will have a chance to provide feedback on your TA (and undergraduate LA, if applicable). Your answers are anonymous and will not affect your grade in any way. You may access the survey from your personal computer, a lab computer, or your phone.

At the end of the quarter, TAs (and LAs) will receive average scores and comments (without identifying information) from their lab section(s).

Do not include any identifying information in your responses. If you have any feedback to provide to which you would like a response, please send it to David McCowan (mccowan@uchicago.edu).

If you cannot or do not want to complete the survey now, you may complete it at home. The survey will remain open until Saturday, February 24 at 5:00 pm.

Procedure


The TA will introduce you to the spectrometer and how to use it. You will warm-up and practice using it by looking at the spectrum of an incandescent bulb and then measuring the emission spectrum of hydrogen.

When you’re ready, you can use your spectrometer to look at the solar spectrum to find as many absorption lines – the so-called Fraunhofer lines – as possible and investigate whether or not you see evidence for hydrogen in the Sun’s atmosphere. If you see other absorption lines, you can try to identify which other elements might be present.

Group lab notebook

Remember to write down everything you see and do in your group lab notebook.

  • Make sketches, do calculations, take notes, and record observations. Your TA may ask to see your notebook during lab, and will read over it afterwards to assign a grade to the group.
  • Use the digital group lab report to communicate the process and results from your experiment.

Outline of the lab


Getting used to the spectrometer

Consider: How do you measure angle? What do you see from the incandescent lamp? Do you see first and second order diffraction?

Hydrogen emission spectra

Consider: How does the hydrogen spectrum differ from the incandescent bulb? Can you identify all the hydrogen emission lines?

Absorption spectra (If time permits)

Consider: How do you need to arrange your light source and sample to measure an absorption spectrum? Do you see the faint absorption lines? Do your measured values match expectations?

Solar absorption spectrum

Consider: Can you see absorption dips? At what wavelengths? Do these wavelengths match any of the hydrogen emission lines you saw before? What does this tell us about the composition of the solar atmosphere?

Additional lines

Consider: What other absorption lines do you see? (Share them with the class on the board!) Do others in the class agree with what you observe? Can you find the same absorption lines as your classmates?

Additional emission spectra (if time permits)

Consider: Can you match the lines to emissions from gases available in the lab?

Conclusions


At the end of the lab, you will need to record your final conclusions (about 1 or 2 paragraphs) in your lab report summing up the important results and take-away points from your experiment. Remember that you should only draw conclusions which are supported by the data, so be ready to back up any statements you make!

When you're finished, save your file as a PDF and upload it to Canvas. (Only one student needs to submit the report, but make sure everyone's name is on it!) If you make a mistake, you can re-submit, but work done after the end of the lab period will not be accepted.

Remember to log out of all your accounts after you submit!