Physics Group: Raven Dean, Aime Torres, Cesar Cabato,
Kathryn Morales, Jonathan Soto
Faculty Advisors: Vladimir Gasparyan, Thomas Meyer
Objective: We present the spectra for various elements and molecules taken with an optical spectrometer.
Introduction
When a low density gas is excited by an electric current, it
glows, emitting light. When this light is observed through a
prism or diffraction grating, it is found to consist of only a
few colors, characteristic of the atoms or molecules making
up the gas.
Experimental Apparatus
Helium
1.
.
.
3.
2.
.
Hydrogen-Balmer Series
.
In 1885, a Swiss high school teacher, Johann Balmer, found
an empirical relationship for the wavelengths observed in
1
hydrogen :
where n=2, m=3,4,5… and the Rydberg constant RH
7
-1
=1.097*10 m . Niels Bohr gave a theoretical explanation
for this formula in 1913 based on the earlier discovery by
Max Planck that light is quantized. The Bohr model yields a
value for RH equal to the experimental result.
Carbon Dioxide
Water Vapor
The Noble Gases
.
4.
Fig. 1 - Optical Spectrometer
1. Telescope
2. Collimator
3. Light Source
4. Spectrum Power Supply
We use a standard optical spectroscope (Fig. 1) in which
light from various sources is passed through a prism or a
grating of either 300 or 600 lines per mm. We measure
the angular position of the observed lines with the
telescope. This allows the measurement of λ using the
grating formula d sin θ = m λ, where m=1,2,3… and d is
the spacing between the lines of the grating. The prism
was used to show the entire emission spectra for H, He,
O, N, Ne, Ar, CO2, and water vapor. All shown spectra
were photographed through the telescope with an
LGMS769 cell phone. For H and He we show the
measured wavelength of the observed lines at the top
2
with the calculated values at the bottom (all in nm).
Oxygen
Neon
Argon
Conclusion
For elements above hydrogen, many lines of characteristic
wavelengths are observed, each corresponding to a
specific transition of electrons between energy levels in
the atom. In fact, no two elements have the same spectrum,
which can be used to uniquely identify the elements
present in samples of unknown content.
Nitrogen
References
1. Anderson, Introduction to Modern Physics, Saunders (1982)
2. http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch6/bohr.html