ABSTRACT
Spectroscopy is about light and matter and how they interact with one another. The fundamental properties of both light and matter evade our human senses, and so there is a long history to the questions: What is light and what is matter? Studies of the two have often been interwoven, with spectroscopy playing an important role in the emergence and validation of quantum theory in the early twentieth century. Max Planck’s analysis of the emission spectrum of a blackbody radiator established the value of his eponymous constant, setting in motion a revolution that drastically altered our picture of the microscopic world. The line spectra of atoms, though they had been employed for chemical analysis since the late 1800s, could not be explained by classical physics. Why should gases in flames and discharge tubes emit only certain spectral wavelengths, while the emission spectrum of a heated body is a continuous distribution? Niels Bohr’s theory of the spectrum of the hydrogen atom recognized the results of Rutherford’s experiments, which revealed previously unexpected details of the atom: a dense, positively charged nucleus surrounded by the diffuse negative charge of the electrons. In Bohr’s atom the electrons revolve around the nucleus in precise paths like the orbits of planets around the sun, a picture that continues to serve as a popular cartoon representation of the atom. Though the picture is conceptually wrong, the theory based on it is in complete agreement with the observed absorption and emission wavelengths of hydrogen! Modern quantum theory smeared the sharp orbits of Bohr’s hydrogen atom into probability distributions, and successfully reproduced the observed spectral transition frequencies. Observations of electron emission by irradiated metals led to Einstein’s theory of photons as packets of light energy, after many hundreds of years of debate on the wave-particle nature of light. Experiments (such as electron diffraction by crystals) and theory (the Schrödinger equation and the Heisenberg uncertainty principle) gave rise to the idea that matter, like light, has wave-like as well as particle-like properties. Quantum theory and Einstein’s concept of photons converge in our modern microscopic view of spectroscopy. Matter emits or absorbs light (photons) by undergoing transitions between quantized energy levels. This relatively recent idea rests on the foundation built by philosophers and scientists who considered the nature of light since ancient times. The technology of recording spectra is far older than the quantum mechanical theories for interpretation of spectra.
