Midterm information

The midterm is Thursday, May 10.  
Closed book. 
One 3"x5" note card. 
Bring a _law_ school blue book. 
Bring a calculator. 

Special office hour: Wednesday 4-6pm, 416 Physics

There is an old midterm in SmartSite Resources.

You may not use your fancy calculator to store information relevant
to the exam. If any physical constants are needed, I will supply them.

There will be problem solving and some questions asking you to explain
the physics at work in some situation, e.g. "Why"  or "Explain your
answer" questions. Many of the conceptual questions in the back of the
chapters are good.

The exam will cover all material up through and including the reading for 
lecture 10.

An important theme is interference in both the classical and quantum contexts.

The midterm will cover all the assigned readings and all the documents
on our website. (Fourier analysis will not be required on the midterm, but a
familiarity of the ideas can be helpful in thinking about problems that do
not specifically require Fourier transforms or series.) 

I think that the chapter summaries are a good place to look for an
outline of the material we have covered. In addition, my list of main points 
for the QM we have covered so far is on the web. For the earlier classical
waves part, here is my list of main topics:

wave basics
  superposition principle
  transverse and longitudinal waves
  harmonic waves
  wave equation
  general solution to the 1-dimensional wave equation 
  standing waves
  energy and power
  doppler shift
  beats
interference and diffraction
  intensity pattern for 2 and N-slit interference
  intensity pattern for 1 slit diffraction

The behavior of quanta in the 2-slit experiments illustrates many of the
fundamental and strange aspects of quantum mechanics. Here are a few comments.

1) Resist the very strong temptation to try to reduce the behavior of quanta to
some classical model the you already know. That does not work. We have
something truly new and fundamental here that must understood itself and not
in relation to something else already known.

2) Resist the temptation to think about the detailed time evolution of a
process and about what "really" happened between the beginning and ending
specification of preparation and detection. The amplitude is for the process.
The ways or steps by which it happened are not specified and are indeterminate.
The amplitude receives contributions from all the possible ways. The quanton
does not follow this way or that way; there is an amplitude for all
the ways, and they are added together.

3) It is best to think of the interference as interference in the *amplitudes*
not as some way in which the *quanton* is literally interfering with itself. We
simply do not know what it has done between the preparation and the detection.

4) Each quanton has *both* classical wavelike and particlelike aspects. In some
experiments one aspect may be more prevalent. The quanton is neither a
classical wave nor a particle. Simple experiments refute either assumption.

5) When a quanton goes through the 2-slits to the detection screen it is not
possible to predict where it will hit the screen. Thus there is some randomness
to its behavior. But it is not *completely* random. It is more likely to hit
the screen in some places than in others. There is a probability for it to hit
in different places. It is that probability that can be calculated in quantum
mechanics. To calculate the probability, we first calculate the amplitude for
the process and then take the square of its magnitude to get the probability.

General comment: In preparing for physics exams, it is *not* a good
strategy to try to have a different ready formula for every problem that
might appear. This is not the way that physicists think about physics,
and it is a physicist writing the exam. Physicists like the idea of
using a very small number of general principles, concepts, and laws to
solve a large number of problems. When the fundamentals are understood,
many different kinds of problems can be solved. So you can expect
problems that use the physical principles you have learned, but do not
expect that the problems will be just like ones that you have already
done. It also follows that you should not put much weight on the old 
midterm that is posted. I put it up just to please those who ask for it. 
The idea is not to know a lot of little things, but rather
to know a few important things very well so that you can get everything
else from those. That is the "way" of physics.

"It's the superposition principle."