Part III the wave theory and the wave-particle duality

(1) An early proposal of the wave theory of light, as opposed to the corpuscular,
was made by Christian Huygens in 1690. But it was not until 1803 that Thomas
Young’s experiments in light diffraction and interference provided strong support for
Huygen’s theory. Young’s research showed that light is a wave in motion. Other researchers added to the evidence; for example, Augustin Jean Fresnel, who showed that light is a transverse wave.

(2) In 1864 James Clerk Maxwell set forth the mathematical theory that led to the
view that light is of electromagnetic nature, propagated as a wave from the source to
the receiver. Each wave is made up of two components superimposed on one another: an oscillating electric field and a correspondingly fluctuating magnetic field. About twenty years later, Heinrich Hertz discovered experimentally the existence of electromagnetic waves at radio frequencies. He also demonstrated that electro­magnetic radiation had the classic properties of waves, including interference, refraction, reflection and polarization. Isaac Newton’s corpuscular view of light was shunted aside. In its place was a comprehensive wave theory which showed that all types of radiation, from candle light to radio signals, had the same nature.

(3) In 1895, William Conrad Rontgen discovered X-rays. He showed, among
other things, that like light, X-rays were propagating in straight lines but, in contrast
to light, penetrated through matter.

(4) At the turn of the twentieth century, a few puzzles still persisted which could not be explained by wave theory alone, for example the photoelectric effect. The research leading to an understanding of light and the emission and absorption processes which was conducted during the twentieth century has been of paramount importance. It began in 1900 with the development of quantum physics. The research reached a high peak in the 1920s and there was another high point in the mid-century years, which ended in the completion of the important Quantum ElectroDynamic (QED) theory.

(5) Discoveries relating to the particle nature of light also belong to this century. By 1910 a heated debate over the nature of X-rays occurred between two physicists; one holding that the rays are waves like light, the other that they consist of " streams of little bullets". In 1927, Arthur H. Compton found that there was a gradual change in the characteristics of X-rays in the extreme frequencies — a scattering of some parts of X-rays away from the beam direction, resulting in a longer wave length than the incoming radiation which could not be explained by wave theory. (This is now called the Compton effect). In 1938, Compton demonstrated that cosmic radiation consists of charged particles. He did so by using a spectrometer which showed that X-rays scatter as particles, a clear indication of the duality of light.

(6) At the beginning of the century, Max Planck formulated the quantum theory,
one of the most important discoveries of the twentieth century. The term " quantum" was coined to mean the minimal amount by which certain properties, such as energy or angular momentum, can change. In waves and fields the quantum can be regarded as an excitation, giving a particle-like interpretation to the wave or field.6 Thus the quantum of the electromagnetic field is the photon and of the gravitational field is the graviton. Quantum theory laid the groundwork for Einstein's theoretical resolution of the particle vs wave paradox. Einstein proposed his theory of the duality of light in 1905, but it was not generally accepted until Arthur Compton's experiments showed that X-rays were behaving like individual solid particles.

(7) The explosion of scientific investigation and the resulting understanding of the
photon's behavior and the nature of radiation provided insight into the way atoms are arranged in materials. Just as each human being has a unique set of fingerprints, each chemical element has a unique arrangement of its component parts. As a result, each is capable of absorbing and emitting only certain wave lengths of light.

(8) Although we have focused on visible light and X-rays in this section, scientists
have learned a great deal about other forms of electromagnetic radiation and have been able to apply that knowledge. For example, much of what has been observed and learned about the structure of the cosmos in the past forty years has been revealed by examining data taken in wavelengths other than visible light.

(9) Let us now turn to the subject of lasers. The subject is just one example of the many that illustrate the application of theory and of experimental data to the 60 development of new fields and technologies.