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Lab 11.2
Spectral Analysis of
Fluorescent Lights
Introduction
I
A marine ecologist ·notices that many bottom-dwelling organisms dying
in a local harbor. A high school junior with an intrest in boating and
ecology volunteers to help the ecologist find the cause. From mud
samples taken from the bottom of the harbor, the student chemically
extracts metal ions. Her quest is to see if there are any toxic metals in
the mud.
In the ecologist's laboratory, the student injects the metal ions into
the hot flame of the atomic emission spectrometer, an instrument that
can measure the wavelengths emitted by atoms. Energy from the flame
excites the electrons of the metal ions and boosts them to higher energy
levels from which they cascade downward in a distinctive pattern, losing
their energy in the form of light. As you know from the lecture,
· each type of atom produces a specific set of colors at wavelengths characteristic of that type of atom.
The light emitted from the metal ions is observed and recorded after
it passes through a diffraction grating. The diffraction grating spreads
the light into a line spectrum. In samples from the upper layers of mud,
the student observes a line spectrum with wavelengths of 563.1 nm and
615.0 nm, indicating the presence of tin.
The student traces the tin to a newly marketed paint used to coat
the bottoms of boats. The paint proves to be lethal to almost all bottomdwelling marine organisms and is banned from further use. So the mystery of the dying marine life is solved.
In this investigation you too will be a detective, identifying a gas by
its spectral lines. First, you will see how the spectrum of visible light
looks when viewed through a diffraction grating spectrometer. Then
will familiarize yourself with the line spectra of various elements, and
the relationship between color and light wavelength. Finally, you will
use this knowledge to identify the gas in a fluorescent light bulb.
Pre-Lab Discussion
Read the entire laboratory investigation and the relevant pages of your
textbook. Then answer the questions that follow.
1. What instrument will you use to separate colors of light and analyze
their wavel engt hs? _______________________
2. When atoms are heated so that they emit light, what characteristic
of their light allows you to tell one atom from another?
©Prentice-Hall, Inc.
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Spectral Analysis of Fluorescent Lights 61
Procedure
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1. View the incandescent light source with the spectrometer. In
Figure 11-1 in the Observations section, sketch what you see,
marking the wavelengths that define each colored region. Note
the regions where the colors are brightest and dimmest.
2. Use the spectrometer to observe the light from each of
the labeled gas spectrum tubes.
3. For each spectrum tube, carefully measure and list the observed
wavelengths of the set of lines produced by the gas. Record the
color of each line.
4. Now turn your spectrometer slit toward a fluorescent light
fixture. Look carefully at each colored region of the light spectrum. You should see a line spectrum ·standing out from the
full spectrum background. In addition, certain regions may be
abnormally dim or missing. Measure the wavelength of the line
spectrum and any dim or missing regions.
Observations
1. Sketch the spectrum of incandescent white light on the blank diagram
in Figure 11-1.
Gas Type:___________________
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Gas Type:___________________
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Gas Type:___________________
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Gas Type:___________________
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Gas Type:___________________
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Gas Type:___________________
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Gas Type:___________________
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Gas Type:___________________
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Critical Thinking: Analysis and Conclusions
1.
What is the gas inside the fluorescent light bulb? Support your conclusion with data from Table 11-1 and with your observations of the
line spectra produced by the spectrum-tube gases.
2. Why do you think that some of the reference lines given for the
various gases you observed were not visible to you?