Spectrum
Objective:
 To observe the emission spectrum of hydrogen
 To measure the wavelength of the emission lines for hydrogen spectrum
 To compare the measured energies of the photons with Bohr’s prediction
Equipment:
 Spectrum tube power supply



Transmission diffraction grating
Helium, Mercury, and Neon discharge
tube
Wooden spectrometer apparatus
Theoretical Background:
Light is an electromagnetic wave. The wavelength of visible light ranges from about
400 nm for blue-violet to 700 nm for red. It is possible to separate the light into its
constituent components. The pattern of resulting colors after separation is called a
spectrum. There are three different types of spectra. These are continuous spectra,
bright-line (emission), and dark-line (absorption) spectra.
A hot dense gas object will produce a continuous spectrum whereas a hot transparent gas
will produce an emission spectrum. A continuous band of colors are visible in a
continuous spectrum. In an emission spectrum, a series of bright lines are visible against
a dark background. In contrast, a series of dark lines are visible against a continuous
spectral background for a absorption spectrum. The dark lines seen in the absorption
spectrum represents the wavelengths of the light which are absent. By observing the
spectrum, it is possible to determine the properties of the light source. This is the method
which is used to determine the features of the distant stars and galaxies as well as objects
in our solar system.
. When an electron undergoes a transition from a higher state with higher energy to a
lower state with lower energy, then the atom will emit a photon of energy
E  E i  E f
1
(4)
where
E  hf
or
E  h
c
(5)

In this laboratory, a transmission diffraction grating will be used to produce bright-line
spectra from hydrogen gas-discharge tubes. A transmission diffraction grating is a just a
piece of material having a large number of equally separated slits. Typical distance
between the slits – grating spacing – is on the order of the wavelength of the light and
varies from about 500 nm to 2000 nm .
Procedure:
1. Place the grating in the grating slot of the apparatus.
2. Measure and record the distance from the grating to the slit, L , in data table.
3. Record the number of lines per unit length, n, for the transmission grating.
4. Use extreme caution when using spectrum-tube power supply. Do not touch the
supply electrodes while the supply is turned on. Replace the discharge tubes only
when the power supply is turned off.
5. Place the helium, mercury, or neon discharge tube in the tube holder of the spectrumtube power supply.
6. Align the light source such that the slit, as seen through the grating, is as brightest as
possible as shown below.
meter stick
Balmer line
eye

grating
Hydrogen gas
L
7. Look straight through the grating.
8. A first-order Balmer lines should be visible on the left and the right sides of the
discharge tube.
9. Measure and record the position of Balmer lines on the left, x left , and the right, x right ,
of the slit in Data Table 1.
10. Replace the hydrogen source with another source.
11. Measure and record the location of the strongest visible line.
2
Data Sheet:
Data Table 1: Hydrogen Spectrum
Distance from the slit to the grating, L = ______________
Number of grating per unit length = ___________________
Data Table 1: Helium Spectrum
Color
xhigh
xlow
xaverage
xlow
xaverage
xlow
xaverage
Data Table 2: Mercury Spectrum
Color
xhigh
Data Table 3: Neon Spectrum
Color
xhigh
Calculations:
 From the number of lines per unit length on the grating, calculate the distance
between the slits on the grating.
 Show calculations for the wavelength and the frequency of each line.
 Using the frequency, calculate the experimental value of the energy of the emitted
photon corresponding to each line.
 Determine the theoretical value of the emitted photon energy.
3


Calculate the percent error between the expected and the experimental photon
energies.
Determine the wavelength of the unknown source. Using the spectrum chart, identify
the element in the second discharge tube.
Results:
Results Table 1: Helium Spectrum
wavelength
Color
average

Results Table 2: Mercury Spectrum
wavelength
Color
average

Results Table 1: Neon Spectrum
wavelength
Color
average

frequency
f
Eexperimental
Etheoretical
frequency
f
Eexperimental
Etheoretical
frequency
f
Eexperimental
Etheoretical
4