Emission Spectrum Lab
PSI Chemistry
Name _________________
Purpose
Use the Bragg equation to determine the wavelength of light of various spectral lines
produced by the hydrogen atom and correlate these to the specific transitions
undergone within the atom.
Materials
•
•
•
•
gas discharge box
hydrogen gas spectral tube
2 meter sticks
diffraction grating glasses or tube
Procedure
1. Arrange the meters sticks perpendicular to each other on the lab bench with the
discharge box containing the hydrogen spectral tube immediately behind the
horizontal meter stick with the tube positioned at the 50 cm mark. (see diagram
below)
gas discharge box and tube-->
diffraction grating -->
2. With spectral tube on, observe through
red spectral line closest to the
3. Communicating with your lab partner,
the meter stick and measure the
position from the center (50 cm) mark.
the diffraction grating and locate the
discharge tube.
locate the position of this red line on
distance along the meter stick to that
Analysis
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Chemistry
Atomic Structure
1. The Bragg equation can be used to determine the wavelength of the red spectral line.
d sin θ = m λ
where d = distance between slits
θ = angle of viewing the line (see diagram below)
m = order of maximum (1 for this experiment
Spectral Line
Discharge box
A
B
θ
Observer
2. Sin θ is measured by first determining the hypotenuse using pythagorean theorem.
The opposite distance is determined by the distance of the spectral line from the
discharge box while the adjacent distance is the length of the meter stick plus a
centimeter or two to the discharge box. Next, divide the opposite distance over the
hypotenuse to find Sin θ.
3. Record the values in the table below and find the wavelength of the spectral line.
Distance to
spectral line(A)
Distance to
discharge box
from observer(B)
Calculated
Hypotenuse
Calculated Sin
θ
Wavelength (in
nm)
4. The actual wavelength of the red spectral line produced by the hydrogen atom is 656
nm. What is your % error for this experiment?
5. Is this spectral line being produced from the 4-->2 transition or the 3-->2 transition?
Show all calculations.
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Chemistry
Atomic Structure
6. Would a transition from the 5-->2 produce light with a longer or shorter wavelength
than that produced in the red spectral line? Explain.
Teacher Guide and Helpful Tips
- Have students make measurements in low light to make it easy to see spectral lines
- The common diffraction grating available at most high schools contains roughly 15000
lines per inch. This translates to a “d” value of 1.693 x 10-6 meters between lines.
- Make sure to make clear to the students that they should be looking at the 1st set of
lines from the spectral tube.
- One can look at any spectral line you desire - sometimes the blue-green line is more
vivid and easier to see and use.
- Sample data can be found below:
Distance to
spectral line(A)
42.1 cm
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Distance to
discharge box
from observer(B)
101.2 cm
Calculated
Hypotenuse
109.6 cm
Chemistry
Calculated Sin
0.384
θ
Wavelength (in
nm)
650 nm
Atomic Structure