SPECTROSCOPY

 

 

The process of science:

 

The scientific method may sometimes involve deduction, where we can reach a specific logical conclusion based on known, observable evidence.  You might deduce after observing the amount of change in your pocket that you have just enough for a candy bar, but not enough for a latte, based on the known evidence of a menu listing prices for each.  For the conclusions to be valid, though, the observations you make must be equally valid.  Had you mis-read a price, or mis-counted the change in your pocket, the conclusion of what you could afford would be wrong.  And in science, making valid observations is not trivial.

 

 

Introduction:

 

All of the information we learn about stars comes to us by electromagnetic radiation.  Types of

radiation are distinguished by wavelength;  our eyes are sensitive to visible light with wavelengths from 400 nanometers (blue light) to 700 nanometers (red light).  One nanometer is one billionth of a meter.

 

Wavelength of light is mathematically related to its energy.  Blue light, with the shortest wavelength, has

more energy than red light, with longer wavelengths.  The energy of each wavelength of visible light is

usually expressed in electron Volts (eV). One electron Volt is a very small amount of energy; to raise an

apple one meter on Earth would take more than 6 million trillion electron Volts!

 

When light from a source is passed through a prism or fine grating, the light is spread out into its component colors, and absorption or emission lines can be seen. These lines provide information about the chemical composition, temperature, density, and motion of the light source.  Spectroscopy is the science of spreading and analyzing light.  A spectroscope is used to separate the component wavelengths of visible light.  In this lab, you will use a convenient hand-held calibrated spectroscope called a spectrometer to analyze light from lamps and hot gaseous emission tubes.

 

Objectives:

      1.  Recognize and measure the spectral lines of hydrogen.

      2.  Identify unknown elements by observing emission (bright line) spectra.

 

Materials needed:

      1.  Diffraction gratings and plastic Project STAR hand spectrometers

      2.  Spectral discharge tubes - Hydrogen, and at least 4 -5 other elements:

               Helium, Neon, Mercury, Argon, Krypton, Oxygen, Sodium, Carbon Dioxide

               (The discharge tubes will be masked, or unlabeled, to serve as unknowns.)

      3.  Incandescent light bulb - a 25 or 40 watt

      4.  Fluorescent lights - Usually available in the ceiling of a classroom.

      5.  Spectral Line Positions for Various Elements Chart

      6.  Colored Pencils of Pens (These will significantly improve the quality of your data!)

 

Background

 

Using the spectrometer you will see the following scales:

 

Your job will be to examine various gases and record as carefully and accurately as possible the spectral lines you see.  To use the spectrometer, you must align its right edge at the light source:

 

 

                                   

 

Hints to do well in this activity!

 

1) Much of science depends upon careful observation and recording of data.  To use the spectrometers, look first for the overall pattern of emission lines and or continuous colors of the rainbow.  Then focus on three or four of the brightest lines, and record their approximate energies and wavelengths.

 

2) While one group member is using the spectrometer, others can use the diffraction grating slides (which are not calibrated) to help identify the colors and sequences of emission lines. 

 

3) Collaborate with your partners.  Share the responsibility of observing the data and recording it accurately.  Take frequent breaks away from the spectrometer to relieve eye strain.

 

4) At the end of the activity, the instructor will reveal the correct names of the unknown gases.  Do NOT erase your answers!  Instead, record the correct name in the appropriate location, and complete the final questions about what you learned.

 

Activity 1: Observing Lights and Lamps

 

1.  Using a diffraction grating observe the light produced by an incandescent light bulb.  Sketch the appearance of the spectrum in the appropriate space on the Spectral Record Sheet.   Use colored pencils to show the individual colors produced.  If you do not have colored pencils, then use different shading patterns to show the individual colors, and label them appropriately.

 

Name:                                                                Date:                                      Spectroscopy

 

Lab Partners:                                                                           

 

 

 

Step 1: INCANDESCENT  LIGHT  BULB SPECTRUM: Sketch what you see with the diffraction grating, and indicate different colors with colored pencils or different shading styles.

 

 

2.   Now use the plastic spectrometer to observe the light bulb.  Be careful to aim the slit (on the right side of the instrument) at the light bulb, but look straight ahead at the energy and wavelength scales.  You may have to move the instrument sideways slightly until the light spectrum comes clearly through the slit onto the scales. 

 

      Record on the following chart the ranges of colors you detect; be careful to note where the colors begin and end according to wavelengths.  Use the same shading system or colored pencils that you used in Step 1.

 

3.   Record the number on the wavelength scale corresponding to the reddest light you can detect (on the left of the scale), and similarly for the bluest light you can see (at the right on the scale).  Notice that the light is continuous, without any missing colors.  A light bulb with a solid filament produces this kind of continuous spectrum.

 

            Reddest light wavelength detected:         ___________ nanometers (nm)

 

            Bluest light wavelength detected:            ___________ nm

 

4.  Now look at a fluorescent light bulb with your spectrometer.  Sketch what you see on the spectral record, and describe any similarities or differences from the light bulb spectrum.  Carefully record any bright lines you might see according to the wavelengths and energies shown in the spectrometer.  And look for any suggestion or evidence of a continuous spectrum as well.

5.   Record the three or four brightest lines you detect. Most fluorescent lights have bright emission lines from hot Mercury gas superimposed on a continuous spectrum.  Mercury emits a green emission line at a wavelength around 546 nm.  Do you see evidence of Mercury gas in your observations?

 

            Line                  Approximate     Approximate                

            Color                Wavelength         Energy (eV)

           

            _______________________________                      Evidence of Mercury Gas? 

 

            _______________________________                      _______Yes or No?

 

            _______________________________                      Evidence of Faint Background

                                                                                    Continuous Spectrum?

            _______________________________

                                                                                    _______Yes or No?

            _______________________________                       

           

 

Activity 2:  Observing Emission Lines from Hot Gases

 

6.   Observe the spectrum of Hydrogen Gas with the diffraction grating; notice the bright colored emission lines that gas produces.  Using the spectrometer, observe Hydrogen's spectrum and carefully sketch the emission lines below.  Be sure to note the approximate wavelengths and energies of the lines you detect.

 

     Line                  Approximate     Approximate                

            Color                Wavelength         Energy (eV)

           

            _______________________________

 

            _______________________________

 

            _______________________________

 

 

Name:                         

7.   Observe the unknown gases #1-5, and similarly record their bright lines in the appropriate records below.  Look at the spectral line position table for various gases, and determine as best you can which gas produced the spectra you observed.  Note your deductions on the data sheet that helped you identify the element.

 

Unknown #1

     Line                  Approximate     Approximate                

            Color                Wavelength         Energy (eV)

                                                                                                            Identification:

            _______________________________                      This Gas is believed to be:

            _______________________________

            _______________________________                      ___________________

            _______________________________

 

            Reasons for your choice:                                                                                                        

 

                                                                                                                                                           

 

            Actual Name of this Gas (Revealed by the Instructor):                                        Correct?     

 

Unknown #2

     Line                  Approximate     Approximate                

            Color                Wavelength         Energy (eV)

                                                                                                            Identification:

            _______________________________                      This Gas is believed to be:

            _______________________________

            _______________________________                      ___________________

            _______________________________

 

            Reasons for your choice:                                                                                                        

 

                                                                                                                                                           

 

            Actual Name of this Gas (Revealed by the Instructor):                                        Correct?     

 

Unknown #3

     Line                  Approximate     Approximate                

            Color                Wavelength         Energy (eV)

                                                                                                            Identification:

            _______________________________                      This Gas is believed to be:

            _______________________________

            _______________________________                      ___________________

            _______________________________

 

            Reasons for your choice:                                                                                                        

 

                                                                                                                                                           

 

            Actual Name of this Gas (Revealed by the Instructor):                                        Correct?     

 

Unknown #4

     Line                  Approximate     Approximate                

            Color                Wavelength         Energy (eV)

                                                                                                            Identification:

            _______________________________                      This Gas is believed to be:

            _______________________________

            _______________________________                      ___________________

            _______________________________

 

            Reasons for your choice:                                                                                                        

 

                                                                                                                                                           

 

            Actual Name of this Gas (Revealed by the Instructor):                                        Correct?     


Name:                         

Unknown #5

            Line                  Approximate     Approximate                

            Color                Wavelength         Energy (eV)

                                                                                                            Identification:

            _______________________________                      This Gas is believed to be:

            _______________________________

            _______________________________                      ___________________

            _______________________________

 

            Reasons for your choice:                                                                                                        

 

                                                                                                                                                           

 

      Actual Name of this Gas (Revealed by the Instructor):                                  Correct?     

 

Analysis of Spectral Identification

 

How many gases were you able to correctly identify?                             

 

What was the most difficult part of this activity for you and your colleagues?

     

                                                                                                                                               

 

                                                                                                                                               

 

Often in astronomy, scientists must not only identify what gas is involved, but also how dense it is, how fast it is moving, and its temperature.  All of those observations depend upon very detailed observations of each spectral line.

 

If two spectral lines were within 50 nanometers of each other, do you think you could detect that they were separate lines?                             (Yes or No)  Look over your data, and notice those cases when you saw multiple lines that were within 50 nm of each other.

 

What if the lines were within 5 nm of each other?                      

 

Optional Activity:  Observing Sunlight and/or Moonlight with the Spectrometer

 

8.   Depending on the time of day, weather, and phase of the moon, use your spectrometer to observe INDIRECT sunlight (off a cloud or the sky near the sun) or a bright full Moon.  If you observe sunlight, be careful NOT TO AIM THE SPECTROMETER AT THE SUN, because you will injure your eyes by staring at the Sun.  Record any dark lines that you see.