Notes on Atoms: part 1, atomic spectra

Today (Monday) we began our treatment of the quantum atom. I am hopeful that this will be the most interesting and important part of this conceptual physics course. Understanding the quantum nature of matter is a big thing.

This has two aspects. One is bizarre experimental results which were discovered in the 1900's. In particular spectroscopic measurement showed the presence or absence of sharp lines at particular frequencies (colors) in emission and absorption spectra respectively. This were quite a surprise and mystery, and they were completely unexplained until the wave concept for electrons was imagined and the wave equation for electrons* was discovered. *Also known as the "Schrodinger equation".

The other aspect focuses on the theoretical nature of atoms when the wave-like character of the electron is appropriately incorporated. In this class we will not do the wave calculations of the Schrodinger equation --that is done in physics 101 and 139-- but what we can and will do is look at the consequences of the wave theory: the phenomenology of atoms which emerges after the grungy math is done, which is really the most interesting part!

On Monday we discussed the nature of a spectrometer --how it involves a source, an aperture, a prism, a second aperture, and a detector. The 1st aperture "colimates" the beam, making it so that all the light the gets through it is going in the same direction. The prism splits the source beam into colors (frequencies). The second aperture slowly moves across the beam allowing only one color, or frequency range, to get through at a time. Then the detector measures the intensity of what gets through. By measuring what gets through (intensity) as a function of the position on the 2nd aperture one obtains what is called a "spectrum": a measured graph of Intensity as a function of frequency.

There are two types of spectra that are important to us. One is an emission spectrum, in which the light source is a heated atomic gas. In this case one will see zero intensity at many frequencies (or a very low non-zero background) punctuated by dramatic narrow spikes (peaks) at which there is a lot of intensity at a particular frequency. These are called "bright line spectra". "Line" refers to the narrowness of the peaks; they are so narrow they look like vertical lines in the graph. "Bright" means high intensity.

The other type of spectrum is an absorption spectrum. For these the source is essentially an ordinary light bulb, i.e., a hot piece of metal. This will emit a continuous spectrum with no sharp lines or anything dramatic. The drama begins when you put a cold gas of atoms in the beam. One then finds that the gas of atoms has absorbed almost all of the light at very specific frequencies, thereby creating lines in the spectrum where the intensity that reaches the detector is close to zero. These are called "dark line spectra".

We looked at both emission and absorption spectra for the example of a hydrogen atom where there were sharp lines at: 4.6 x 10^14 Hz, 6.2 x 10^14 Hz, 6.9 x 10^14 Hz and 7.3 x 10^14 Hz.
These correspond to red, green, blue and violet light respectively. On Wednesday we will begin the task on unravelling the mystery of where these sharp lines come from and why.

On Wednesday we will have a special visitor, Nina X. McCurdy, who has been working on explaining the origin of quantum spectra to a general audience. This will be a very important class. Please do not miss it!