Physics 128a: Experimental Physics
Fall 1998
Professor Jeffrey D. Richman
Broida Hall 5131, 893-8408





Welcome to Ph128!


The experiments in Ph 128a will give you the opportunity to observe some fascinating aspects of light, atomic spectra, sound, and other physical phenomena. This course will be very different from a typical course in theoretical physics, where you attend lectures and do homework problems. Ph128 is much more like doing actual research: you are working with real equipment, and you and your lab partner have to make things happen. Many students find this course to be very challenging. Don't feel bad if you do!

Ph 128 is hard for several reasons. To begin with, many of you have very little experience working in a laboratory. It's quite different from reading a textbook! Here is the basic method:

  1. Realize that you are totally confused about everything.
  2. Formulate a set of specific questions.
  3. Formulate a strategy for answering at least one of the questions.
  4. Carry out this strategy.
  5. Go back to 1.

Students who have the most trouble are usually the ones of have done little or no preparation before coming to the lab. Without preparation, you have a hard time even formulating specific questions (Step 2). In most of the experiments, your time in the lab must be used efficiently, so planning your strategy in advance is extremely important!

Second, there is a lot of theoretical physics involved that you have not learned in your classes. Before you can understand each experiment, you will need to do some research on its theoretical aspects. Here are just a few examples:

  1. If you add a neutron to the nucleus of the hydrogen atom (a single proton), what effect will this have on the energy levels of the atom? Will it have any effect at all?
  2. How does a gas laser work? What are the properties of a laser beam?
  3. What determines the speed of sound is a gas? What molecular properties are involved?
  4. How is the flow rate of a gas affected by the dimensions of the tube it is flowing through?
  5. How are diffraction patterns and Fourier transforms related?
  6. What is a hologram?

You will need to do substantial reading on your own just to understand the theoretical issues underlying your experiments. It is crucial that you move quickly in each experiment to do this.

To help you do this research, we have a small library of books that you may check out. In addition, most of the labs have documentation that provide theoretical background. If you check out a book, you must return it by your next lab period. Of course, going to the library is also a great idea. Far too few students make use of that powerful resource.

A third reason this course is difficult is that you will be presented with a substantial amount of equipment with which you may have very little familiarity. If you are struggling to figure out how it works, you can

  1. fiddle around with it,
  2. read the manuals in addition to fiddling around,
  3. ask for help.
Fiddling around is fine, as long as you are careful not to destroy anything. If a piece of equipment is not working, either in the state you found it or because of something you have done, you should report it immediately so that we can fix or replace it. You will not get in trouble if you break something, but you will be in trouble if you break something and don?t tell us. Although it may be quite difficult at first, you will gain a lot of self-confidence if you learn to open up a manual, find the relevant section, and figure out what?s going on. While some of the pieces of equipment can be regarded as "black boxes" without affecting your results, in other cases, you will not be able to successfully perform the experiment unless you really understand how the equipment works. This task may involve even more physics that you don?t already know! In addition, you may also need to learn some of the eccentricities of the equipment, especially if you want to get the best precision on your measurement.


The fourth reason this course is challenging is simply that it is always hard to get the right answer. I discuss this point in more detail below, under "Goals of the Course."


The Experiments


You will choose four experiments, spending two weeks on each one. The list of possible experiments for this quarter is:


  1. Hydrogen/deuterium isotope shift
  2. Acoustic resonance
  3. Properties of laser light
  4. Vacuum techniques
  5. Scattering of light by polystyrene crystals
  6. Spatial Filtering
  7. Holography

You will also do a small project in the machine shop to gain some experience with machining techniques. For your reference, the list of experiments for Ph 128b is:


  1. Mossbauer effect
  2. Molecular spectra
  3. Nuclear magnetic resonance
  4. Surface plasmons
  5. X-ray diffraction crystallography
  6. Zeeman effect
  7. Quantum Hall effect

Some of the experiments are challenging, and I urge you to do as much preparation as possible before starting each one. The experiments are also quite rewarding, however, and I hope that they make some difficult physics more tangible and immediate to you.




There are important safety issues in some experiments. This is particularly important in Ph 128b, where we use an X-ray machine and radioactive sources, but there are also safety issues in some of the Ph 128a experiments. I require that you start each experiment by doing an assessment of the safety issues. Please work like a professional and be actively involved in your own safety:


  1. Look for safety-related information in the lab manual.
  2. Look for safety-related information in the manuals for the equipment.
  3. Ask me, the T.A.?s, or Rich Harding about anything that concerns you.
  4. Record important safety-related information in your logbook.

Usually, it doesn?t take long to inform yourself about safety issues, and it is well worth the trouble. Some typical examples of safety hazards are:


  1. Intense light sources: lasers or gas discharge tubes. Never look directly into a laser beam. In addition, be careful not to look into a mirror or other polished surface that may reflect laser light directly into your eye. When working with lasers, it is always a good idea to keep the level of your eyes substantially above the level of the laser beam. Avoid looking directly at gas discharge tubes. In some experiments, we have provided eye goggles. If you would like a pair but they aren?t around for some reason, feel free to ask us and we will get some.
  2. Electrical hazards (high voltage);
  3. X-ray machines, radioactive sources (Ph 128b)
If any type of accident occurs, you must report it as soon as possible to the instructor, Rich Harding, or one of the T.A.s. If for some reason, there is an emergency and no one is around, you can go to the Physics Department Office on the third floor and ask someone to call 9-911.





You may work on an experiment after class hours, but there must always be at least two students together in the room.



Class Requirements


The requirements are:

  1. Do four experiments, and keep a good logbook record of each one. The logbooks should not just be spiral notebooks. Please keep two separate logbooks to facilitate grading; the T.A.s will grade one while you are using the other for your next experiment.
  2. Carry out a small project in the machine shop. You will also receive training in safe machine shop practices.
  3. Give one talk; this will be after the second experiment, and it can be on either of the first two experiments. You will give this talk jointly with your lab partner using transparencies and an overhead projector. We will provide blank write-on transparencies, but you should get your own pens. The best ones are Staedtler Lumocolor Fine-point permanent pens. You may also use a computer to generate your talk. You will then need to use a different kind of transparencies.
  4. Write one abstract. The abstract is about one paragraph long and will be due after the third experiment. It can be on either of the experiments that were not the subject of your talk.
  5. Write one full scientific research paper. The paper will be due after you complete all four experiments, but it may not be on the same experiment as the one your give your talk on. It may be on the one you have written your abstract for, but in general you will need to rewrite your abstract.

To determine your final course grades, I will use the following weighting:

  1. Experiments + logbooks: 75%
  2. Talk: 8%
  3. Abstract and Paper: 17%



Goals of the Class


There are several different goals for this class.


  Be tough-minded and skeptical.  
  1. Learn some modern physics. In your classical mechanics and electromagnetism classes, you are learning physics that was developed about 100 years ago; in quantum mechanics, you are learning physics from the 1920s and 1930s. In this class, you will learn some physics that is based on much more recent work. This will be challenging, since you may not have encountered much of this physics in your theoretical classes.

  3. Learn skills for giving talks and writing papers. These skill have a very wide applicability. They are extremely important in any scientific field and in many non-scientific jobs as well.

  5. Learn to obtain information from many sources, not just a single textbook.













Dates Remarks
Sept 28, 30 (29, Oct 1) Orientation, reading
Oct 5, 7 (6, 8) Expt 1
Oct 12, 14 (13, 15) Expt 1 
Oct 19, 21 (20, 22) Expt 2 
Oct 26, 28 (27, 29) Expt 2
Nov 2, 4 (3, 5) Expt 3; talks on Nov 4, 5
Nov 9, 11 (10, 12) Expt 3
Nov 16, 18 (17, 19) Expt 4; abstract due Nov 18, 19
Nov 23, 25 (24, 26x) Expt 4 (x=Nov 26 is holiday)
Nov 30, Dec 2 (1, 3) Wrap up
Dec 7-- Finals week: paper due Dec 10


Some important dates:


Grading of Logbooks


The logbooks will be due one week after you complete the experiment. Thus, for the M, W section, the logbooks will be due on the W following the two week period for the experiment; for the T, Th section, they will be due on the Th following the two week period for the experiment. The logbooks will graded on a scale from 1 to 20. Late logbooks will be accepted, but with 2 points deducted for each day late. If special circumstances arise, please discuss them with me as soon as possible.




Each group will give a 15 minute talk, which will be followed by 5 to 10 minutes of questions. The talk should contain

  1. A title slide, with title, authors, and main goals of the experiment.
  2. 2-4 of slides on theoretical background.
  3. 2-4 slides on the method.
  4. 2-4 slides on the results and conclusions.
All together, you should have 8-12 slides. Please label all plots and give the units for all quantities. The talks will be timed with a hard cutoff, so you should practice your talk to

make sure that it fits into 15 minutes. Sometimes, extra or "backup" slides are useful during the question period. The talk should be divided between the two lab partners, with roughly equal time for each student.




The paper should contain the following sections: 

  1. Abstract
  2. Theoretical Discussion
  3. Methods and Procedures
  4. Results
  5. Interpretation and Discussion
  6. References

The abstract


The abstract is meant to be a stand-alone description of the work-it should succinctly state what was done and what the results were. It should not refer to any other section of the paper, and it should not go into the theory, except perhaps very briefly. An abstract should not read like an introduction, which contains background information and is not a summary of the results. You should include the numerical values of important measurements, with their uncertainties. A person opening up the journal to your article will read the abstract first. If they are busy, this may be all that they will ever read!




You should write a section on the theory behind your experiment. Do not mix your results or interpretation into this section, which should be 2-4 pages. (If you want to write more, it?s fine with me.)


Experimental Methods and Procedures


Include at least one clear diagram of the experimental setup. Be sure to describe your experimental methods in detail. The length of this section will depend on the experiment that you did, but as a guideline, it should be between 3 to 5 pages. Detailed figures add a lot to the clarity of such descriptions.




Tables are great for summarizing results. In some cases, graphs are preferable to tables, since they make it much easier to see relationships in the data. In fact, a lot of students miss important observations because they don?t bother to graph their data. Be sure to include a discussion of how you estimated your experimental uncertainties in the raw data and how you propagated these uncertainties into your final results.


Discussion and Interpretation


I prefer to have the interpretation of the results in a separate section from the results themselves, since it explicitly separates what you measured from your interpretation. This rule may not apply to all situations, so if there is a strong reason to combine this section with that on Results, you can do that.




If there are important theoretical or experimental results that you wish to use in your paper, they should be referenced. In the body of the text, put a reference number, and

list the references in numerical order in this section.


Good luck!