SPECTROSCOPY
Quantitative Analysis with Light
Instructor Resources
Learning Objectives
The objectives of this experiment are to:
•
identify band and line spectra, and relate the physical state of a light-emitting substances to the type of
spectrum observed.
•
determine the relationship between the colors of the visible spectrum and wavelength and frequency.
•
determine the relationship between the energy, frequency, and wavelength of light waves.
•
examine the fingerprint nature of spectra.
•
construct a spectrograph calibration chart and identify an unknown element by measurement of its
emission spectrum.
•
use an energy-state model to explain the atomic spectra of hydrogen gas.
Procedural Outline
•
Hot solid and gaseous sources of light are viewed through a diffraction grating spectroscope and
classified according to the type of spectrum they emit, a band or line spectrum.
•
A simple diffraction grating and the concept of wave interference are used to develop a relationship
between the color and wavelength of light.
•
Light-emitting diodes are used to develop a relationship between the energy and color, frequency, and
wavelength of light.
•
A gaseous element is "finger printed" by its emission spectrum.
•
An unknown element is identified using the program Atomic Spectrum by measurements made on a
colored photograph of its emission spectrum, together with a mercury spectrum.
•
Some evidence is collected to support Bohr's planetary orbit model for electrons.
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Name ______________________________ Section _______
Date ____________________
SPECTROSCOPY: Quantitative Analysis with Light
Report Sheet Questions
1.
Describe the differences in the spectra from the incandescent light bulb filament the gaseous discharge
tube and the fluorescent light fixture.
2.
Which of the above spectra does the hot nichrome wire most resemble? What does this tell you about
the incandescent light bulb filament?
3.
What is the difference between spectra from a heated solid and spectra from a highly heated gas?
4.
Did you observe a band or line spectrum in the yellow Bunsen burner flame? What does this tell you
about the composition of the yellow part of the flame?
5.
Which color of visible light has the longest wavelength? Which color has the shortest wavelength?
6.
From your data from the Energy of Light experiments, what is the relationship between voltage
(energy) and wavelength? How do you know this?
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Name ______________________________ Section _______
Date ____________________
SPECTROSCOPY: Quantitative Analysis with Light
Report Sheet Questions (page 2)
7.
If wavelength has the units “length/wave,” and frequency has the units “waves/time,” what units will
you get if you multiply these two?
8.
What physical measurement has these units?
9.
This quantity has a fixed value for electromagnetic radiation, and is called “c.” What is this value?
10. What is the value of the slope of the regression line for the EXCEL graph of the data you took with the
Energy of Light program?
11. How does this value compare with “Planck’s constant?”
12. Why does the Energy of Light program, with the current measured at 0.1 milliamperes give a better
value of Planck’s constant than the data taken at 0.5 milliamperes?
13. If energy is inversely related to wavelength, and frequency is directly related to energy, what must then
be the relationship between wavelength and frequency and, what would be the constant of
proportionality between the two?
14. What color do you observe in the flame for each of the sodium solutions? (Be sure to look at the flame
color above the band spectrum of the nichrome wire.)
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Name ______________________________ Section _______
Date ____________________
SPECTROSCOPY: Quantitative Analysis with Light
Report Sheet Questions (page 3)
15. What is the position of the color above the band spectrum of the nichrome wire?
16. After examining the spectra of a number of gaseous discharge tubes, can you make any correlation
between the complexity of the visible spectral lines with the element’s position in the periodic table?
17. Can you identify an element present in the fluorescent lamp?
18. After doing the calculations for Table 4, does Bohr’s model appear to accurately predict the visible
region spectral lines of hydrogen? Discuss your answer.
19. Why are some of the lines brighter than others? Can you develop an explanation that involves the
probability of each orbit having an electron heated enough to reside in it?
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SPECTROSCOPY: Quantitative Analysis with Light
Tips and Traps
1.
35 mm slide mounted diffraction gratings can be obtained from Edmunds Scientific, and possible other
sources. The 35 mm slides are easier to work than the spectroscope tubes, once they have mastered
where to look. The slide should be oriented so that the spectrum is horizontal, and then the student
should look at the flame with the slide angled at about 35 degrees to the eye-flame line. They should
be able to see at least three orders of diffraction by shifting the angle to higher values.
2.
You will need to help the students to understand that the shorter wavelengths are diffracted least, and
the longer wavelengths are diffracted most with the diffraction grating, and also that the shortest
wavelengths are the blue colors and the longer wavelengths are the red colors.
3.
The Energy of Light board can be purchased from MicroLAB, Inc., P.O. Box 7358, Bozeman, MT
59771, phone 1-888-586-3274, email at [email protected]
4.
Students will need some instruction in how to set up the Energy of Light program. If time is short, the
voltage can be set to higher values for the shorter wavelengths to decrease the scan time. A little
experimenting beforehand will help determine which voltage is best to start for each wavelength so that
a number of points are obtained before the 0.100 milliamp point is reached.
5.
Students will probably need some help in working out the unit analysis to understand the conversion
of volts to Joules, and wavelength to frequency.
6.
Be sure to caution the students to take turns looking at the flames through the diffraction gratings. It
is best to have one student manipulate the nichrome wire between the solution and the flame, while the
other student makes observations, then change off so both students see the results in the flame.
7.
Students have a tendency to concentrate on the nichrome wire band spectrum and miss the flame
spectrum, which appears Above the band spectrum. They must be helped, often individually to learn
where to look for the flame spectrum.
8.
Students will need to be coached to come up with the association of the nichrome wire and a band
spectrum, and the gaseous discharge tube and a line spectrum.
9.
Atomic Spectrum is a PC program that can be purchased from MicroLAB, Inc., address given above.
It is very helpful to the students in understanding the fingerprint nature of atomic spectra.
10. The unknown elements for the Atomic Spectrum program are: W = Helium, X = Neon, Y = Zinc,
Z = Cd.
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SPECTROSCOPY: Quantitative Analysis with Light
Report Sheet Questions
1.
Describe the differences in the spectra from the incandescent light bulb filament the gaseous discharge
tube and the fluorescent light fixture.
The incandescent bulb filament gives a band spectrum showing all of the colors of the rainbow. The
gaseous discharge tube gives a spectrum consisting of discrete lines, characteristic of the particular
element. The fluorescent bulb gives both a band with a line spectrum superimposed upon it. The
band spectrum is from the fluorescing coating on the glass tube, and the line spectrum is mercury,
the discharging element.
2.
Which of the above spectra does the hot nichrome wire most resemble? What does this tell you about
the incandescent light bulb filament?
The hot nichrome wire produces a band spectrum. A band spectrum formed by the incandescent
bulb arises from the hot filament of the bulb.
3.
What is the difference between spectra from a heated solid and spectra from a highly heated gas?
A hot solid always produces a band spectrum, and a highly heated gas always produces a line
spectrum.
4.
Did you observe a band or line spectrum in the yellow Bunsen burner flame? What does this tell you
about the composition of the yellow part of the flame?
A band spectrum was observed in the yellow Bunsen burner flame. It tells us that the yellow flame
consists of tiny incandescent solid particles, in this instance, carbon particles.
5.
Which color of visible light has the longest wavelength? Which color has the shortest wavelength?
Red has the longest wavelength and violet has the shortest wavelength of visible light.
6.
From your data from the Energy of Light experiments, what is the relationship between voltage
(energy) and wavelength? How do you know this?
Voltage (energy) and wavelength are inversely related to each other, because a plot of voltage versus
1/wavelength produced a linear relationship.
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SPECTROSCOPY: Quantitative Analysis with Light
Report Sheet Questions (page 2)
7.
If wavelength has the units “length/wave,” and frequency has the units “waves/time,” what units will
you get if you multiply these two?
The units will be length/time, i.e., length | waves = length
wave | time
time
8.
What physical measurement has these units?
This is a physical measurement of speed or velocity.
9.
This quantity has a fixed value for electromagnetic radiation, and is called “c.” What is this value?
“c” is a measure of the speed of light, 2.998 X 108 meters/second.
10. What is the value of the slope of the regression line for the EXCEL graph of the data you took with the
Energy of Light program?
The value should be in the neighborhood of 6.626 X 10-34 Js.
11. How does this value compare with “Plank’s constant?”
The value is within a few percent of Plank’s constant.
12. Why does the Energy of Light program, with the current measured at 0.1 milliamperes give a better
value of Plank’s constant than the data taken at 0.5 milliamperes?
When the voltage becomes large enough for electrons to begin jumping the energy gap of the LED, then
current begins to flow. Monitoring the voltage at the lower current allows a better evaluation of the
energy gap for each electron, which produces a better value for Plank’s constant.
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SPECTROSCOPY: Quantitative Analysis with Light
Report Sheet
13. If energy is inversely related to wavelength, and frequency is directly related to energy, what must then
be the relationship between wavelength and frequency and, what would be the constant of
proportionality between the two?
These two inverse relationships produce a direct relationship, i.e., energy is directly related to frequence,
with Plank’s constant as the constant of proportionality.
14. What color do you observe in the flame for each of the sodium solutions?
A bright yellow flame is observed for each of the sodium solutions.
15. What is the position of the color above the band spectrum of the nichrome wire?
The bright yellow flame is directly above the yellow portion of the band spectrum from the nichrome
wire.
16. After examining the spectra of a number of gaseous discharge tubes, can you make any correlation
between the complexity of the visible spectral lines with the element’s position in the periodic table?
It appears that the higher the atomic number of the element for a give group of elements, the more
complex the visible spectrum of the element.
17. Can you identify an element present in the fluorescent lamp?
If one observes closely, one can see the mercury lines superimposed upon the band spectrum of the
fluorescing material.
18. After doing the calculations for Table 4, does Bohr’s model appear to accurately predict the visible
region spectral lines of hydrogen? Discuss your answer.
Bohr’s model does very well in predicting the visible spectral lines for hydrogen. Unfortunately, it fails
for all other elements.
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SPECTROSCOPY: Quantitative Analysis with Light
Report Sheet Questions
19. Why are some of the lines brighter than others? Can you develop an explanation that involves the
probability of each orbit having an electron heated enough to reside in it?
It would seem that certain energy level jumps are more probable than others. The less probable an energy
jump is, the fewer number of electrons that will be making that jump, and the more faint will be the
intensity of the line. Conversely, the more probable the jump, the more electrons will be making the jump,
and the brighter will be the intensity of the line.
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SPECTROSCOPY: Quantitative Analysis with Light
Sample Data
C1 chart - Arrangement of colors in order of decreasing wavelength:
Red > orange > yellow > green > blue > violet
Energy of Light main screen, showing the six LEDs, their color and wavelengths, the table of
collected data, the top graph shows the six series plots, one for each wavelength, and the bottom
graph illustrates the energy in Joules versus the frequency. Note that this particular run exactly
reproduced Plank’s constant of 6.626 X 10-34 Js.
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SPECTROSCOPY: Quantitative Analysis with Light
Sample Data (page 2)
Each student should have a chart similar to this that lists
the salt solution in the left column, the flame color in the
middle column (They may have to make several tries and
look hard and quick to see some of the colors like Li or
Sr), and in the right column, where above the band
spectrum of the nichrome wire the flame color appears.
Solution
Flame Color
Spectrum
Table 2: Ionic solution Flame colors
Main screen of the Atomic Spectrum program showing the three tabs for Calibration, Curve
Fitting, and selecting and Unknown. The mercury and unknown spectra are shown in the
upper right graph, and the calibration graph in the lower right with regression line. The
wavelengths of the mercury spectrum are given in the table at left.
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SPECTROSCOPY: Quantitative Analysis with Light
Sample Data (page 2)
A table listing the wavelengths for the possible elements used as unknowns. Shown in this table
is the data for element W, showing that it is the element helium.
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SPECTROSCOPY: Quantitative Analysis with Light
Laboratory Preparation (per student station)
EQUIPMENT:
You will need to have the following equipment available per pair of students before beginning this
experiment.
Spectroscopes (or 35 mm diffraction grating slides), incandescent lamps, fluorescent lamps, gas
discharge tubes containing mercury, hydrogen, helium and neon and any others available.
1 - MicroLAB Model 402 interface and two MicroLAB programs, Energy of Light and Atomic
Spectrum.
1 - MicroLAB Model 210 Energy of Light Module (both items obtainable from MicroLAB, Inc., P.O.
Box 7358, Bozeman, MT 59771, phone 1-888-586-3274, email [email protected].
CHEMICALS:
Six sets of the following solutions, in 6 inch test tubes with a rubber stopper, a nichrome wire inserted
through it, with a small loop on the solution end for gathering solution to transfer to the flame. Both the
test tubes and the stoppers should be clearly labeled.
•
20 ml of 1.0 M NaCl solution
•
20 ml of 1.0 M Na2 CO3 solution
•
20 ml of 1.0 M Na2 SO4 solution
•
20 ml of 1.0 M SrCl2 solution
•
20 ml of 1.0 M LiCl solution
•
20 ml of 1.0 M BaCl2 solution
•
20 ml of 1.0 M CuSO4 solution
•
20 ml of conc. HCl solution (Used for cleaning of nichrome wires if they become contaminated.)
CAUTIONS OF CHEMICAL HAZARDS:
•
None of the solutions are dangerous, but normal laboratory precautions should be taken.
The chemicals are innocuous, however you should keep all chemicals away from eyes and mouth, wash
hands after use and before leaving the laboratory, and use prudent laboratory practices at all times.
DISPOSAL OF SOLUTIONS: If any of the solutions become contaminated, they should be
disposed of in accordance with current regulations.
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