Physics307L F08:People/Joseph/Notebook/071024

=Balmer Series=

Experimentalists
Nik Joseph and Bradley Knockel

Objective
We are going to study the emission spectra for hydrogen and deuterium and from there attempt to determine their Rydberg constants. The emission spectra of both hydrogen and deuterium are generated by the transitions of each atoms' single electron from their principle quantum state to a higher state. Using Balmer's formula, $$\frac{1}{\lambda}=R\left(\frac{1}{2^2}-\frac{1}{n^2}\right)$$1, where $$R\,$$=Rydberg constant, and measuring where these emission spectra occur, we should be able to calculate the Rydberg constant. We'll measure the spectral lines for hydrogen1 at: and we should be able to measure the Rydberg constant from there.

Equipment
Our equipment consisted of:
 * Spectrum tubes filled with hydrogen, deuterium, mercury, and sodium gases
 * Power supply for spectrum tubes
 * "constant-deviation" spectrometer, made in-house at UNM

Setup
We began by sorting through the spectrum tubes, identifying which ones we needed. We familiarized ourselves with the spectrometer, playing the dial and focus on it; getting a general feel for the equipment. We inserted the mercury tube into the power supply and turned it on. We were looking through the eyepiece at the spectral lines, when Devon came over, removed the cover on the prism and moved the prism around. He did this so we'd have to readjust it on our own, which took a few moments. The spectrometer needed to be calibrated next. A simple way of doing this to was find a known line on the mercury spectrum, adjust the dial so you are looking at the correct wavelength, and then move the prism until the spectral line was in the cross hairs. We tried this several times, just to be sure. To take the data was a bit cumbersome. Because the dial that adjusted wavelength was old, you have to account for lash in the gears by turning only in one direction at a time. Whenever we reversed direction we'd turn to the end of the screw, and then reverse direction.

Data
We took two sets of data: one set when we were increasing in wavelength and one set as we decreased the wavelength. We felt this would be a good idea, because it gives us a simple way of calculating error: take the difference of the two.

Results and Analysis
Manipulating Rydberg's formula we wind up with: $$R=\frac{1}{\frac{\lambda}{4}-\frac{\lambda}{n^2}}$$ which can be used to find the Rydberg constant for each wavelength. Once we have calculated those for each element we can take the mean and come up with a reasonable value. We need to take our uncertainty into account as mentioned before. Our adjusted data is:

Using the formula for $$R\,$$, propagating the error, and averaging the values we wind up with:

$$R_{hydrogen}=1.09686(86)\times10^7 m^{-1}$$

and

$$R_{deuterium}=1.09730(142)\times10^7 m^{-1}$$

Compared to the actual value of $$R_{hydrogen}\,$$ which can be found here, the error comes out to $$error\,=8.206\times10^{-5}$$, which is well below our measured uncertainty.

Conclusions
We measured Rydberg's constant very closely to the accepted value, even with our ridiculous uncertainty. Still, this lab wasn't very difficult and we learned how to use the spectral lines to analyze a material. Bradley and I discussed in great detail the reasons why or why not the Rydberg constant for deuterium would be bigger than that of hydrogen, but in the end the experiment settled the argument. In the end, data defeats rhetoric. We continued to analyze all the other samples, and we identified the "unknown" element to be neon.