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Astronomy 3 Lab Manual Emission Spectrum 1
Emission Spectrum
Introduction
Astronomers have learned everything they know about the universe through observations
of electromagnetic radiation – light. Different elements emit light at different wavelengths;
these are referred to as emission lines. This allows astronomers to determine the composition
of various objects in the universe. In addition, the fact that light is also emitted at a specific
wavelength allows astronomers to determine the velocities of objects, through the use of the
Doppler shift. The purpose of this lab is to (a) examine the emission lines in hydrogen and
verify that the observed wavelengths agree with the theory, and (b) use your observations
of emission lines from an unknown gas to identify the gas.
Theory
Electrons orbiting a nucleus have an electric potential energy. An electron in an atom can
only have particular energies. That is, the energy of an electron in an atom can only take on
certain discrete (or quantized) values. The possible energy levels of the electron in hydrogen
are shown in Figure 1.
Figure 1: Energy levels for the electron in a hydrogen atom.
The lowest possible energy that an electron can have in an atom is called the ground state
and energies of the other allowed energy levels are given with respect to the ground state (ie.
level 2 in the hydrogen atom has an energy 10.2 eV higher than the ground state). When
an electron moves from a high energy state to a lower energy state, the energy goes into
emitting a photon of light. As the electron energy levels are quantized, so to is the energy
Astronomy 3 Lab Manual Emission Spectrum 2
of the emitted photon. The energy (E) of a photon is given by the formula
E =
hc
λ
(1)
where λ is the wavelength of the photon, h is Planck’s constant and c is the speed of light.
Thus, when an electron in an hydrogen atom goes from energy level 3 to level 2, it emits
a photon of light with an energy of 12.1 eV − 10.2 eV = 1.9 eV. Using equation (1), this
implies that the photon has a wavelength of 656.3 nm, which to our eyes appears red. This
line is refered to as the Hα line. What is the value of h (Planck’s constant)? Use the above
information, and equation 1 to answer this question. Look up the value of Planck’s constant
and verify that you did the calculation correctly.
Hydrogen is the simpliest possible atom, with only one proton. As you might guess, as
you change the number of protons, you change the electric potential energy of the electrons
surrounding the nucleus and so the allowed energy levels change. This leads to a change in
the wavelength of light which is emitted as electrons drop from one energy level to another.
Thus, each element has a unique set of emission lines. Observations of these emission lines
allows us to determine what elements are present in a object.
References
You should review sections 5.4, 7.1, 7.4 and 7.5 in your textbook (‘The Cosmic Perspective’)
before coming to the lab.
Procedure
The apparatus used in this laboratory consists of a gas discharge tube with power supply
and a grating spectrometer as shown in Figure 2. The gas atoms in the discharge tube are
excited by means of an electrical current and the light emitted is passed through a diffraction
grating of the spectrometer which disperses the light into its component wavelengths. The
individual lines can be viewed through the observation arm of the spectrometer. Each line
can be precisely located using the cross hairs in the eye piece of the observations arm. The
angle at which the lines appear is read off a scale on the base of the spectrometer.
The diffraction grating used is a multiple slit system one inch wide with approximately 6000
slits/cm. The exact value is given on each grating and you need to record this number. For
any multiple slit system, the angular location θ of maxima for light of wavelength λ is given
by the expression
λ = d sin θ (2)
where d is the distance between the slits1. Since d is known for the grating, once the angle
1For those of you who know some optics, this is actually the equation for the first order spectrum, which
is all that we are concerned with in this lab
Astronomy 3 Lab Manual Emission Spectrum 3
gas discharge tube/
scale
power supply
entrace slit
angle
cross hairs   grating
diffraction
observation arm/
Figure 2: The experimental apparatus. CAUTION: The power supply produces dangerous
voltages. Do NOT touch the metal connectors on the power supply when the power supply
is turned on.
θ for a specific emission line has been measured the wavelength of the line can be computed
from equation (2).
1. Make certain that the diffraction grating on the spectrometer is level (using three
leveling screws under the grating platform) and is in the plane which is perpendicular
(both horizontally and vertically) to the light exiting the arm containing the entrance
slits. Do not touch the optical surfaces of the grating. Always handle it by the edges.
Make sure that the hydrogen discharge tube is securely in the power supply. Do not
touch the thin, light emitting portion of the tube. Turn on the power supply and turn
off the lights in the room. Position the thin portion of the discharge tube so that it is
just in front of the entrance slit to the spectrometer. Shine the flashlight from behind
(at an angle) onto the lens where the light the the grating enters the observation arm.
This should illuminate cross hairs in the spectrometer. Position the observation arm
so that it is in line with the arm containing the entrance slit. You should see the bright
pink central maximum in the eyepiece.
The thickness of the line can e changed by opening or closing the entrance slit with
the knob next to the entrance slit. Adjust the slit opening until a thin line is obtained.
The spectrometer optics can be focused by moving the eyepiece in and out of its
holder. Focus the spectrometer so that both the observed line and the cross hairs can
be clearly seen.
2. Swing the observation arm to one side and set the cross hairs on the hydrogen red line
(the Hα line). There is a fine position adjustment on the lower part of the observation
arm which can be used to accurately position the cross hairs. [Note that the line has a
finite thickness and the cross hairs can be placed on either edge or in the middle of the
line. Where on the line the cross hairs are positioned does not matter as long as you
position the cross hairs in the same way each time you make a measurement (why?).]
Astronomy 3 Lab Manual Emission Spectrum 4
If the cross hairs can not be seen, play with the position of the flashlight until enough
light is let into the eyepiece so that both the line and the cross hairs become visible.
If this does not work, or if the line is washed out by the increased light, it may be
necessary to open the entrance slit a little more.
The angle scale on the base of the spectrometer is divided into two parts, a fixed
part containing the degree markings and a movable vernier scale. Each degree on the
fixed scale is divided into two parts each representing thirty minutes of a degree. The
vernier scale is divided into thirty equal divisions each representing one minute of a
degree. A magnifying glass is provided on the spectrometer to assist you in reading
the scales. The scales are read in the following way.
(a) Determine in which degree division on the fixed scale the zero line on the vernier
scale is falling. This gives you the whole degree of the angle being measured.
(b) Determine in which half of the degree the zero line on the vernier scale is falling.
(c) Determine which line on the vernier scale exactly lines up with a line on the fixed
scale. If the zero on the vernier scale is in the first half of the degree, then the
minute measurement is simply the value of the line on the vernier scale which
lined up with the line on the fixed scale. If the zero on the vernier scale is in the
second half of the degree, then the minute measurement is the value of the line
on the vernier scale which lined up with the line on the fixed scale plus thirty
minutes. Record the angle to the nearest minute.
Repeat this step for the red line on the other side of the central maximum.
Compute θ by finding the difference between the two angles and dividing by 2.
This method of measuring θ increases the accuracy the measurement (why?).
Estimate the error in θ
Record the data in a simple table:
Hydrogen
line θ1 θ2 θavg Error in θavg
red
green
violet
3. Repeat step 2 for the hydrogen green line (the Hβ line) and the hydrogen violet line
(the Hγline).
4. Compute λ and E for each line.
5. The Hα line corresponds to transitions from level 3 to level 2 (see figure 1), referred to
as the n = 3 to n = 2 transition. The Hβ to the n = 4 to n = 2 transition and the Hγ
to the n = 5 to n = 2 transition. Compute the theoretical wavelengths and energies
for the Hα and Hβ lines (from figure 1 and equation 1) and compare them to the
experimentally measured values. Compute the percent deviation of the experimental
Astronomy 3 Lab Manual Emission Spectrum 5
values from the theoretical values. From your measurements of the Hγ line, determine
the energy (in eV) of the n = 5 level in the hydrogen atom. Why do you not see the
transitions to the n = 1 level?
6. There are discharge tubes containing other gases available in the lab. Pick one of these
unknown gases. Mark the number in your lab book. Measure the wavelengths of 3
strong lines in the spectrum (in the same manner that you did for hydrogen). How does
the spectrum compare to the hydrogen spectrum? Record all of your measurements in
your lab book. The computers in the lab have a java applet running which shows the
emission line spectrum for all the elements. To use this applet, simply click the mouse
on an element, and the emission spectrum will for that element will be displayed in
the upper part of the screen. Clicking and holding the mouse in the emission spectrum
will display the wavelength scale (in A˚ngstroms; 1 A˚ngstrom = 10 nm). Using the
applet and your measurements, identify the unknown gas.
CAUTION: Do not attempt to change the discharge tube without first turning off
the high voltage power supply. After a short period of use, the discharge tube becomes
quite hot. Allow a few minutes for it to cool before changing it. Store the hydrogen
tube in its cylindrical storage container while you are looking at the other gases. When
you are done looking at your unknown gas, please make sure to put it back into its
own cylindrical storage container and return to the main storage box.
7. Put the hydrogen spectrum tube back into the power supply leaving the power supply
off when you have done so.