Empirical Facts and Scientific Results - Part II Understanding Atom and Light

These Blogs are based on the lectures for a mini course on "Scientific Method for Non-Scientists".

Empirical Facts and Scientific Results - Part II

Understanding Atom and Light

Debi Prasad Choudhary

Los Angeles

During my student days, I listened to Professor Chandrasekhar saying, the triumph of 20^th century science is that we are able to understand the tiniest objects atoms and giant objects stars with the same set of physical laws. Yet, one hundred ago structure and composition of atoms were mostly unknown. Let us explore how we came this long way. The story of atoms and light is intertwined and gives an excellent insight of scientific processes leading to extraordinary outcome.

The rainbow is dispersed sunlight that contains all visible colors from violet to red continuously with out gap. When we observe similar dispersed light from a fluorescent lamp, we notice gap between the colors. Of course, a highly dispersed sunlight also show gap. The gap in the dispersed light was a mystery in early days of physics about one hundred ago.  In fact, those days famous astronomer Joseph Fraunhofer from Germany, who discovered gaps in solar spectrum conjectured that light does not exist at the colors that show gap!! The fluorescent light contains mercury atoms that get excited through discharge and produce light. Hydrogen atom with a single proton produced light at systematic set of discrete wavelengths (or roughly speaking color) that became key to develop atomic model.

There was another important result in the beginning of 20^th century. In 1909 Ernest Rutherford at the Physical Laboratories of the University of Manchester conducted an experiment to observe the scattering pattern of positively charged particles when bombarded on to a thin foil of gold of thickness 0.00004 cm. The result was that while most particles passed through the foil, one in 20,000 of the particles bounced back. This is possible only if most of the gold atom was empty with a concentrated mass at the center. This experiment played a crucial role in developing current model of atoms.

Niels Bohr, one of the finest minds of 20^th century physics, utilized discrete emission from an atom and results from Rutherford experiment to propose an atomic model that may illustrate commonly used scientific method. Atoms are neutral, so the positively charged nucleus must be accompanied by negatively charged electron. They cannot remain stationary due to the effect of Coulomb force that is attractive. If the electrons move around the nucleus, according to classical electromagnetism, the electrons would loose energy by radiation and eventually fall into the nucleus. Considering the known physics at the time, Bohr in 1913 proposed that electrons orbit the atomic nucleus in discrete stable orbits and do not radiate as long as they remain in the orbit. They gain or loose energy by jumping from one orbit to the other. This is the reason for emission of spectral lines in specific wavelengths (or color) and removal of light from solar spectrum in selected wavelengths due to gaining of energy in the orbit of selected atoms in solar atmosphere. This scientific model explained a number of observations known at the time but had several limitations. It reproduced the spectrum of hydrogen atom successfully, but failed to explain the spectra of larger atoms such as mercury, sodium or Argon and when they emit in a magnetized environment. It laid the foundation for the development of sophisticated version of quantum theory by Heisenberg and Schrodinger. Here, it must be pointed out that the scientific models explain the data for which they are developed and not the final word in the subject. They have scope for constant refinement and development, as more information becomes available.  

At this time, discrete nature of light was already known through photoelectric effect.  The violet light is of smaller wavelength compared to the red light. The violet to red is only a part of vast spectrum of electromagnetic radiation that is light. In photoelectric effect, it was observed that free electrons come out when the surface of a metal plate is illuminated by light. For a given material, the ejection of electrons did not depend on the intensity but wavelength of light. Einstein explained the phenomena by proposing quantum nature of light for which he received Noble Prize. If the wavelength is lower, the light quanta carried more energy, which resulted in electrons with higher velocity or kinetic energy. Quantum nature of light became a natural consequence of light in the Atomic model of Niels Bohr. He said, "Obviously, we get in this way the same expression for the kinetic energy of an electron ejected from an atom by photoelectric effect as that deduced by Einstein." This is another consequence of great scientific theory that encamps vast related phenomena to explain them in a unique fashion.

The fully-grown quantum theory (quantum mechanics) not only explained observed spectra and photoelectric effect, it was used to invent nuclear reaction leading to bomb and source of energy in the interior of the stars. In the center of a star, the pressure of overlying material become so huge that hydrogen atoms are heated to very high temperatures. They come very close and fuse to produce Helium and energy that is released as the light that we observe. The outflowing energy produce radiation pressure that balance inward material pressure leading to stability of stars. When, the internal nuclear combustion halts, the star collapses. For a sun like star this collapse is stopped by “electron degeneracy pressure”, which is also a consequence of quantum theory. One of the founding principles of quantum theory is known as “uncertainty principle”. According to this principle if electron is confined to a very small area, its velocity increases sharply (more precisely velocity x mass). The compressed star confines electron to such a small volume that the fast moving electrons generate enough pressure to stop further collapse for the stars whose mass does not exceed 1.2 times the mass of the sun. This is known as Chandresekhar limit. If the cores of the star exceed this limit they become a ball of neutron (or neutral star) or a blackhole that collapse endless. So the stability of the gigantic objects such as the stars and the stability of tiny particles atoms are understood using the same set of physical laws.

Each time, science is practiced to answer a well-defined precise question. Most basic assumptions are usually carefully designed such that they are not inconsistent with the experimental results. So, if a question is unanswerable with the contemporary science, it is not because the scientific method is incapable, it is because the tools to handle complex questions are not ready yet. Remember, one day we did not know the structure and composition of atoms, yet today we use them to understand the stars and obtain pretty pictures of our self and our loved ones!!