Spectroscopy: Hydrogen, Helium, and Neon
Introduction
Spectroscopy is one way to study the interaction between matter and radiated energy. Gases excited by
electrical discharge emit light at discrete wavelengths and frequencies. A spectrometer collimates light
with a thin slit and directs the resultant beam at a diffraction grating or prism to separate the
wavelengths, usually as lines of color. These so-called spectral lines are characteristic of the atoms that
make up the excited gas, and thus can be used to determine the composition of a substance.
Light transmitted through a diffraction grating generates narrow lines on a screen. Assuming that the
grating-screen distance is much further than the grating separation, these lines will appear at angles
given by:
(1)
where d is the separation of grating elements, theta is the diffraction angle, m is the order number, and
lamba is the wavelength of light. Figure 1 illustrates this concept.
Figure 1: The diffraction grating
If two wavelengths are incident on the grating, lines will appear at different locations on the screen. If
there is enough separation between lines, the two wavelengths can be distinguished. The diffraction
grating has a resolving power dependent on the spacing between slits. As the number of slits on the
grating increases, then the width of the spectral lines decreases, increasing resolution. Thus, the
resolving power is dependent on N, total number of slits illuminated on the grating.
(2)
where R is the resolving power of the grating,
is the smallest resolvable wavelength difference,
and λ is the wavelength of light. N is the number of lines utilized by the grating, not the number of lines
the grating has. For example, only a portion of the grating may be illuminated by the source.
To accurately determine the grating line spacing a hydrogen bulb can be used. Hydrogen produces 4
visible spectral lines (the Balmer series) between 400nm and 700nm. H-alpha appears red with a
wavelength of 656nm, followed by a light blue 486nm line, a blue 434nm line, and a dim purple 410nm
line. By accurately measuring the diffraction angles in the first and second order emissions, the grating
equation can be used to measure the grating line spacing. Once this is known, the grating equation can
be used to characterize the spectrum of other gasses such as neon and helium.
Part I – Using the grating spectrometer
The Pasco SP-9268A Spectrometer has 3 basic components (see Figure 2): a collimator, a diffracting
element, and a telescope. Light passes through a narrow slit of a collimator to create a thin, parallel
beam which ensures the light strikes the grating at a fixed angle of incidence. The diffracting element
diffracts the light, separating each wavelength at different angles. The telescope is then rotated to view
the diffracted light at angles measured by an angle vernier scale built into the spectrometer.
The adjustable collimator creates a thin beam ensuring the light incident on the diffraction grating strikes
perpendicular to the surface. If the collimator slit was too large, light would strike the grating at many
different angles diminishing the interference pattern.
Figure 2: Measuring an Angle of Diffraction
When viewing the un-diffracted beam through the telescope, the light source can be seen through the
collimator slit. As the telescope is rotated as shown in Figure 2, the angle of diffraction increases and
spectral lines of various colors can be seen through the telescope. By centering the vertical crosshair of
the telescope on the edge of the observed spectral line, an angle of diffraction can be measured. If the
number of slits on the diffraction grating is known, Equation 1 can be used to find the spectral
wavelength. If the wavelength is known (such as the spectral lines in a sodium doublet or hydrogen
Balmer series) then the exact value of d for the spacing of the grating elements can be calculated.
Part II – Spectrometer Calibration and Hydrogen Spectra (reference Appendix A)
1. Turn on the hydrogen light source and allow it 5-10 minutes to warm up while adjusting the
spectrometer.
2. Ensure the spectrometer is level. Adjust the eyepiece until the crosshairs are in sharp focus and
aligned vertically. This can be done by aligning the vertical crosshair with a door frame.
3. Focus the telescope at infinity by looking at a distant object. As shown in Figure 3, align the
telescope opposite the collimator and focus the collimator until the slit comes into sharp focus.
Do not change the focus of the telescope. Lock the telescope into place and use the fine
adjustment knob to align the vertical crosshair with the edge of the slit. Measurements will
always be made at the edge of the slit, so a narrow slit is not necessarily required. If the slit edge
is not vertical, loosen the collimator and adjust it so it aligns with the vertical crosshair.
4. Arrange the spectrometer so
that the hydrogen bulb is ~1cm
from the collimator slit. The
slit should be clear and bright
when viewed through the
telescope
5. Make an initial measurement
with the telescope opposite
the collimator, viewing the slit.
This is the reading for the
undiffracted beam as shown in
Figure 3. Diffracted beam
measurements minus this
value will yield the angle of diffraction (as shown in Figure 2).
6. When the hydrogen bulb is warmed up, view the zero order spectral lines by slowly increasing
the angle of diffraction while looking through the telescope. H-alpha appears red with a
wavelength of 656nm, followed by a light blue 486nm line, a blue 434nm line, and a dim purple
410nm line which is usually not visible.
7. Measure the red H-alpha line on one side of the undiffracted beam and rotate the telescope to
measure it again on the other side. If the diffraction grating is perfectly aligned, the diffraction
angles will be the same on both sides. If not, fine adjust the diffraction grating alignment to
compensate for the difference. Re-measure the H-alpha line until the angle of diffraction is the
same on both sides of the undiffracted beam. This ensures that the grating is perpendicular to
the collimator to generate precise spectral lines.
8. Use equation (1), both angle of diffraction measurements of the H-alpha line, and the known
values for the H-alpha wavelength to produce two values for d. The average of these two
measurements yields a more accurate value of d than is given on the grating itself. Use this value
of d in Part III.
Part III – Helium and Neon
Determine the wavelength of emission lines in helium and neon and compare them to known values.
Appendix A: Measuring Angles of Diffraction
Taken from “Instruction Manual and Experiment Guide for the PASCO scientific Model SP-9268”, ©1991
Pasco Scientific
The vernier scale is an analog reading and is subject to similar uncertainties as a meter stick or
equivalent. The PASCO SP-9268 is capable of arc-minute resolution when making measurements with
the vernier scale.