Master class

Here is a classic in new clothing. Some 37 years ago Adrian Melissinos wrote a guide for physics students taking a senior-year lab course. It was an instant success, partly because it was unique. It was not a laboratory manual and it was not a textbook in modern physics. Instead, it provided background material that students would need to know in order to perform a wide variety of experiments in modern physics.

Since that first edition of Experiments in Modern Physics appeared there have been several revolutions in our understanding of the micro and macro worlds. There remains, however, a permanent foundation of theoretical models and experimental techniques. A student must master these in order to take the next steps in research.

Although Melissinos’ book is not out of date and still provides a lucid summary of many of the foundations of physics, experimental techniques – even in undergraduate labs – have changed dramatically. Transistors have replaced vacuum tubes, and computers are now used to analyse data and plot graphs. This new version of the book, now by Melissinos and Jim Napolitano, brings the student laboratory up to date.

The authors assume that students can use calculus easily and that they are taking a concurrent course in modern physics. The first few chapters are relatively straightforward and might be used in the third year of an undergraduate physics degree. Topics include experiments on electrons in solids (resistivity, the Hall effect, superconductors), electronics and data acquisition (simple measurements, Johnson noise, chaos), lasers (basic properties, interferometers), optics (diffraction, Fourier series, the Faraday effect) and quantization.

Most of these lab exercises could be done as one session per week. The more difficult problems in the second part of the book would, however, take several weeks of class discussion and laboratory work with paired students. This section includes experiments with high-resolution spectroscopy, magnetic resonance, particle detectors, scattering and statistical analysis.

One of the more familiar exercises involves repeating Millikan’s oil-drop experiment to demonstrate quantization and to measure the charge on an electron. These days the trickiest part of the experiment can be avoided by using plastic spheres rather than oil droplets. Produced commercially for calibrating electron microscopes, the spheres are guaranteed to have a diameter of 1 µm ± 1%. Millikan, and thousands of physics students since, did not have such spheres available.

The challenge for students is how to measure the diameter – and hence the weight – of the droplet. But because the diameter must not be much larger than the wavelength of light, diffraction cannot be used to measure the dimension optically. Millikan resorted to measuring each droplet’s drift velocity, which, according to Stokes’ law, depends on the diameter. Melissinos and Napolitano do the same, providing the derivation for using Stokes’ law complete with error analysis.

The chapter on electronics and data acquisition starts at a fairly low level, designed to establish logical concepts, step by step. The reader is even reminded about the sum rules for resistors and capacitors. Basic electronic equipment is described, including oscilloscopes, digitizers and operational amplifiers. With the preliminaries established, the lab work involves the measurement of Johnson noise, the production and analysis of chaos, and the behaviour of computer interfaces.

The nature of the book can be best illustrated by describing the topics of the major section on particle detectors. Instead of starting with a description of the construction and uses of ionization or scintillation devices, the authors first summarize the theory of the several kinds of particle energy loss.

They first explain how charged particles ionize material through which they pass. The rate of energy loss is a function of the particle velocity, the density and atomic number of the material, and the square of the particle charge. Particles travelling slowly therefore ionize strongly, but when the particles are moving at speeds close to that of light, the rate of ionization passes through a minimum. Photons lose energy through stochastic processes – the photoelectric effect, Compton scattering and pair production. Particles with zero charge, such as the neutron or the neutrino, can be detected only by observing the charged particles that result from a collision of the neutral particle and a nucleus. In addition to ionization, electrons also lose energy through bremsstrahlung – the production of X-rays as an electron is accelerated by passing close to a nucleus.

These preliminary facts about particle behaviour serve as a review of nuclear physics. The authors then apply these facts to describe the operation of gas ionization chambers, including the Geiger counter, scintillation detectors and solid-state detectors. The lab exercises that follow the theory do not, however, include track devices such as cloud or bubble chambers, emulsions or wire chambers. In the last 40 years these have either become obsolete or have become part of giant arrays in the targets of large accelerators.

The book’s appendices provide useful warnings about the dangers of lasers and radioactivity. There is also a short introduction to MATLAB software, although the authors point out that the text procedures are compatible with many other computer programs. The book has no end-of-chapter problems, but the final appendix does contain a large assortment of exercises that can be used as homework problems or as take-home tests.

The level of maths used in Experiments in Modern Physics takes it out of the range of the casual reader. Even the background material assumes that sophistication at the level of a third-year undergraduate. Although this background material is very well written, it would be wasted if the reader were not also doing the experiments. But when used as the authors intend, the new version of this classic text continues to set the standard as an introduction to experimental methods in physics.