CBGAM0,FYGA03, VT11
ATOMIC LINE SPECTRA
You will need a calculator and Physics Handbook
Goal:
The purpose of this lab:
• To study the interference and diffraction of light using a spectrometer.
• To learn how the spectrometer can be used to investigate line spectra from a gas
discharge tube (Na, Hg and H).
• To demonstrate the atomic structure and quantisation of energy by constructing an
energy level diagram from experimental observation of line spectra and compare
with calculations using chapter 39 in Physics for Scientists and Engineers, Knight,
2nd ed.
Related topics:
Diffraction spectrometer, Double slit interference, Diffraction grating, Line spectra,
Bohr model of the Hydrogen atom and energy levels. See corresponding chapters in
your course book.
Principle and task:
When the low pressure gas in the glass tube is excited by the input of a large amount
of energy, the atoms that make up the gas become excited. This means that an electron
of the atom is raised from a lower to a higher energy level. The excited atom, when it
returns to the ground state, may emit light with specific wavelengths that are
characteristic for this atom, called the line spectrum.
You are to observe the emission spectra (bright line spectra) of sodium (Na), mercury
(Hg) and hydrogen (H) by using the spectrometer shown in Figure 1. A spectrometer
is a device for extracting quantitative information from a spectrum, such as the
wavelength of the lines in a spectrum. Observation of these line spectra played a
crucial role in the development of the atomic theory of matter and quantum
mechanics.
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Figure 1. Spectrometer.
Reading the angle on the spectrometers:
On the spectrometer there are two places to read the angle, placed 180° apart. Both are
otherwise identical. The scale is in degrees and minutes. The reading is done in a
similar way to the method used on a ruler where a scale mark on the rough and the
fine scale shall be in line.
There are two different versions of fine scales, one with 20 minutes (figure 2) and one
with 30 minutes (figure 3). On the corresponding rough scale there are for the 20
minutes version three sections between each degree and on the 30 minutes version
there are two sections. One degree is thus divided into 60 minutes (and 3600 seconds).
The degree mark closest outside the zero-minute mark gives you the degrees and in
some cases 1/3 or 1/2 degrees. To convert the minutes to degrees one can use for
example: 22°18 min = 22° + 18/60° = 22,3°.
Figure 2. Fine scale with 20 minutes
Figure 3. Fine scale with 30 min
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Experiment:
There are four tasks to perform:
1. Set up and calibrate the spectrometer
The first thing to do before the measurements is to adjust the spectrometer. This
must be done to ensure a proper measurement so be sure to do it careful.
•
Turn the eye-piece so that the hair-cross is vertical (and horizontal) and
sharp.
•
Aim the eye-piece section towards an object >25 m away (the further
the better) and adjust the focus.
•
Make sure that light can come in to the collimator through the slit and
adjust the focus on this one when viewed through the eye-piece. The
width of the slit can be adjusted to achieve a thin (less light but better
resolution) or a wide slit (lighter but poor resolution).
•
Read the angle that the telescope makes on the scale (see above). This
will be your “zero angle” for the future reference.
•
Do not make any change to the spectrometer except adjusting the
telescope arm or collimator slit.
2. Double Slit Interference (Na lamp)
•
Put the Na lamp into the special power supply and turn it on. Put the
lamp in front of the slit of the collimator.
•
Adjust the position of the light source until the vertical line you see is
as bright as it can get.
•
Put the double slit in the holder between the telescope and collimator
and observe the interference pattern through the objective.
•
Use the observed pattern to calculate the distance between the 2 slits.
Estimate the error.
•
The wavelength of the light of the Na lamp can be found in Physics
Handbook. Check that this is in the orange region of the visible
spectrum.
Caution! Do not change tubes when the power supply is turned on and don’t
touch the tube with your hand. Before touching the tube, wait for it to cool
down.
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3. Line spectrum of mercury
•
Use the Hg lamp and a diffraction grating with 300 lines/mm and
observe the line spectrum.
•
Determine the angles for the 2:nd order spectral lines of mercury. You
should be able to measure at least the strong lines according to Table 1.
•
Use the result to calculate the wavelength of each line that you observe
by using the diffraction grating formula.
•
Compare the values with Table 1 and Physics Handbook. Comment!
4. Line spectrum of hydrogen
•
Use the H lamp and a diffraction grating with 600 lines/mm and
observe the line spectrum.
•
Determine the angles for the 3 strongest spectral lines of hydrogen.
•
Use the result to calculate the wavelength of each line that you observe
by using the diffraction grating formula.
•
Compare the values with Table 1 and Physics Handbook. Comment!
•
Calculate the photon energy for each line you have identified.
•
Construct an energy level diagram and indicate which transition is
responsible for each of the experimentally observed lines. Compare
your derived energy levels with your calculated energies using chapter
39 in Physics for Scientists and Engineers, Knight, 2nd ed. Comment!
Home Assignment
Before you do the lab you must have answered the following (written answers):
1. What are the differences between the interference patterns obtained from a
doubled slit and from a diffraction grating?
2. What are the wavelengths of the visible light? To which series (Lyman or
Balmer or Paschen or….) do they belong?
3. In the Balmer series, what is the wavelength of the light from a hydrogen atom
when an electron drop from n=3?
4. What is the energy of the red light from hydrogen that has the wavelength
656,3 nm?
Table 1. Colours and wavelengths for Mercury and Hydrogen Spectra.
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Mercury
Colour
Hydrogen
Wavelength (nm)
Violet
404.7
Violet
407.8
Blue
434.1
(strong)
Blue
435.8
(strong)
Turkos
486.1
(strong)
Blue
491.6
Green
546.1
(strong)
Yellow
576.9
(strong)
Yellow
579.1
(strong)
Red
656.3
(strong)
Good Luck and don’t forget to hand in your report with all
group members listed on It´s Learning!
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