When I started as a freshman (that's first year of university for those who are not familiar with US college terminology) physics major, I think I had the same grandiose (or jaundice) view of physics as most incoming students - that of theoreticians working on particle or nuclear physics. I thought that was what I wanted to do. So I went through the standard curriculum for a physics major. One of the major turning points came in my Junior year (3rd year) when I interned during the summer at Fermilab. Since I had aspirations of going into particle physics, this was a logical thing to do. It was an eye-opening experience. I discovered how LARGE the project was, and how, even if I were a full-blown physicist, I would be only one small player in the whole project. At the end of my internship, I knew that particle physics was NOT something I want to do.
One of the things that happened not by design was my discovery of solid state physics. During my Senior year (4th year), I had several graduate level classes that I could take to fill up the required number of credits. So I opted for a class in that subject. I will admit that it was god-awful boring. But it did opened my eyes to a wider view of physics in that many of the useful things we take for granted did originate out of an area in physics, and that physics and its applications wasn't just something esoteric and "pure knowlege" alone. I think this was the first time I decided to actually explore on my own all the various areas of physics.
One of the disconcerting feeling I had was when I received my undergraduate degree, looked at the piece of paper, and asked myself "Now what exactly do you know and can you do?" I had a nagging feeling that I really didn't know that much, and have very few skills. Other than teaching high schools (which I did for a year and a half), I had no other ability. It was when I made a decision to go to graduate school, and not only that, become an experimentalist. Strangely enough, I didn't have a clear idea in what area of physics I wanted to go into. I did know for certain that I wasn't going into particle physics. Probably at that point, getting a Ph.D was an end in itself for me. So after passing the qualifying exam, I literally "shopped around". I went to various professors and asked what research work they were doing. I also asked around various more senior graduate students asking for their perspective, especially with regards to the "intangibles", such as which professor was easy to work with, and which were more difficult. The one area that struck my fancy was the technique of using tunneling spectroscopy to study properties of superconductors. At that time, high-Tc superconductors was still a very hot area of study. Even I was fully aware of that. So that became my research project and my thesis research work. I studied tunneling spectroscopies of almost everything, from conventional superconductors (in which I reproduced all the "textbook" results) to high-Tc superconductors. Not only that, even the tunneling technique came in several different flavors. I did both planar junction technique and point-contact technique.
One of the most important aspect of experimental work that I discovered was that you seldom just do one thing. Although my primary activity centered around the actually tunneling measurments, I did not do just that. I was also involved sometime in the fabrication process, especially when we were doing planar tunnel junctions. I actually made thin films using sputtering technique, and then fabricated the insulating layer, made terminal contacts, etc. In the process, one also needs to know the quality of the thin films one has made, so I ended up learning how to use and analyze x-ray diffraction method to characterize the film I fabricated. Even the process of the tunneling measurement itself required me to learn about cryogenics, since I had to deal with liquid nitrogen and liquid helium. So one just simply do not learn just one thing - one ends up having to learn a series of methods, equipments, techniques, etc.
The other important discovery was, of course, publicizing one's work either by publishing, or via presenting it at a conference. This can be fun, nerve-wrecking, informative, frustrating, and joyful, all at the same time. This is something that isn't part of a formal education in becoming a physicist, but yet, it has to be one of the most important aspect of becoming a physicist. I felt lucky today that I had a supervisor who insisted that I had these experiences as part of my training. By the time I received my Ph.D, we managed to published several papers and I had given several presentations at various conferences.
I went on to do my postdoctoral fellowship in a slightly different experimental area - photoemission spectroscopy, but with a focus on the same type of material, high-Tc superconductors, and later on, expanding to strongly-correlated electron systems. In principle, there is an intimate relationship between tunneling spectroscopy, and photoemission spectroscopy, especially angle-resolved photoemission spectroscopy (ARPES). Tunneling spectroscopy measures the "momentum averaged" single-particle excitation spectroscopy. So it can give an energy resolved spectrum. ARPES, on the other hand, measures both energy and momentum resolved spectrum. So in principle, if you take a complete ARPES result, sum up all the momentum, you should end up with what tunneling experiement should get (if only life is THIS easy!).
In any case, I spent 2 1/2 years working in this area, and it was extremely productive professionally. It was, I think, my first taste of what it would be like to be a "professional" physicist, doing nothing by research work, and no classes, no tests, no exams to worry about. Alas, all good things must come to an end, and a postdoctoral position is only transitional. I made a major turn in my career by accepting a job at Argonne National Lab, and file this in the Irony Hall of Fame, I joined the High Energy Physics division! After swearing off particle physics, it was rather funny to be in this position, but I didn't sell my soul to the devil completely. I actually joined the accelerator group to work on their photocathode development. To produce electrons that they will accelerate using their novel technique, they shine high intensity lasers onto a cathode, creating photoelectrons. It was why they wanted someone who has worked in photoemission (me) to join the group. It did mean that I have to learn a whole new area of accelerator physics. This is where, if you have been equipted well during graduate school, you should be able to adapt to whatever situation you are in.
If there's any one impression that I wish someone can take from this story is to be "flexible". To be able to do this, one needs as wide of an experience as possible while in college. I did many things and many studies during my graduate years that never went anywhere, never became part of research work, or never ended up in my thesis. But in the process of doing those things, I developed many skills that I had to call upon later on during my professional career. Again, these are the stuff they don't teach you in school - you only acquire them by doing.
I look forward to your questions/comments, or even share your own journey to get to where you are in physics.