(2)
You will now measure the wavelengths of the three
hydrogen atom emission lines. The
spectrometer to be used is a diffraction grating, which is a piece of plastic
having a large number of parallel grooves on its surface.
The gratings we use contain 5275 grooves per centimeter.
Light passing through the grating is dispersed into its component colors
much like how light is broken up as it passes through a prism.
The actual set-up for the spectrometer consists of the discharge tube
placed behind a meter stick with the diffraction grating at some distance x from
the tube on a line at a right angle to the meter stick (see diagram below).
[spectrometer diagram goes here]
When viewing a particular color of light through the
grating, the line will appear against the meter stick at a distance s from
the discharge tube. One set of
lines will be projected on the right side of the meter stick and another on the
left side. With x and s known,
the wavelength of each spectral line can be determined from equation (2) where g
is the number of grooves per centimeter on the grating.
Your task is to measure the wavelengths of these lines (red, blue-green,
and violet) and calculate the radii of the electron orbits to which those lines
correspond.
(2)
1. Examine the spectrometer, and become familiar with its use.
2. Observe and measure the diffraction distances (“s”) for each of the spectral lines and record in Table 1A.
3.
Calculate the wavelength and orbital radius of each of the three spectral
lines using Equations 1 (in the lab manual) and 2.
Record in Table 1B.
|
Line
Color |
Left
Position (cm) |
Right
Position (cm) |
Average
Position (cm) |
|
Red |
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Blue-Green |
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Violet |
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Line
Color |
Wavelength,
(cm) |
Wavelength,
(nm) |
Orbit
Radius, rn (nm) |
|
Red |
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|
Blue-Green |
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Violet |
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4.
On Fig. 1, indicate which of the transitions give rise to which of the
three spectral lines that you have measured.
Remember, too, that the shorter the wavelength, the greater is the energy
change involved in the transition.
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The Hydrogen discharge tube will now be replaced
with a “mystery” tube. Your
task is to identify the element contained in the tube by measuring its emission
spectrum.
1. Observe the emission spectrum through the spectrometer. Sketch on Fig. 2 the position of the spectral lines as you see them. Be sure to include meter stick and wavelength scales in your sketch. Also make a note of the apparent color of the unknown lamp:
Lamp Color _________________
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2.
Fill in Tables 2A and 2B.
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Line
Color |
Left
Position (cm) |
Right
Position (cm) |
Average
Position (cm) |
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Line
Color |
Wavelength,
(cm) |
Wavelength,
(nm) |
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3.
Compare your observed spectrum with those displayed on the wall chart.
Notice that on the chart the wavelengths are labeled in Ĺngstroms
(Ĺ)
rather than nanometers. [1
Ĺ = 0.1 nm.]
Using only the wall charts
as a reference, what element is in the discharge tube?
(Consider all types of evidence when determining the unknown)
___________________________
4. Examine another
discharge tube through the spectroscope. Describe
qualitatively the spectrum that you see. That
is, describe how many lines of each color are present, and whether some are
brighter than others, and how the lines are spaced apart from each other.
5. Astronomers use
spectral lines to identify elements in stars or elements in planetary
atmospheres. In this exercise, you
will play the part of the astronomer.
The
following data were transmitted to Earth but only a piece of the entire spectra
could be viewed. Your job is to
interpret the data provided by your instructor using the charts at your lab
table.
ELEMENTS FOUND
A. _______________
B. _______________
C. _______________
ELEMENTS FOUND
A. _______________
B. _______________
C. _______________
ELEMENTS FOUND
A. _______________
B. _______________
C. _______________
