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 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

 Spectrum tube power supply                         Helium, Mercury, and Neon discharge
   Transmission diffraction grating                 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                                   (4)

                                E  hf         or       E  h                              (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 .

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 spectrum-
   tube 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

                                                                      Hydrogen gas

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.

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

Data Table 2: Mercury Spectrum
      Color                xhigh               xlow            xaverage

Data Table 3: Neon Spectrum
      Color                xhigh               xlow            xaverage

 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.

   Calculate the percent error between the expected and the experimental photon
   Determine the wavelength of the unknown source. Using the spectrum chart, identify
    the element in the second discharge tube.


Results Table 1: Helium Spectrum
                           wavelength      frequency
   Color          average                              Eexperimental   Etheoretical
                                              f

Results Table 2: Mercury Spectrum
                           wavelength      frequency
   Color         average                               Eexperimental   Etheoretical
                                              f

Results Table 1: Neon Spectrum
                           wavelength      frequency
   Color          average                              Eexperimental   Etheoretical
                                              f


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