January 28, 2021

QNu Labs

If you haven't realized this already, your grade 12 chapters on optics and light is more like a history review than a physics course. I say this because one never reaches the interpretation of light and its duality until towards the end of the book. Nor does it address the absurdities of quantum mechanics. The understanding is known as the classical interpretation of light and until the early 19th century there weren't any other strong interpretations.

Pierre Gassendi and Newton were two pioneers who believed that light was in fact made of matter (or particles). This was aptly named the corpuscular theory of light. Though they were ahead of their time, their inability to elaborate on the theory and explain phenomenon such as refraction, diffraction and interference of light led to the subsequent decline in the adoption of this hypothesis by the physicists of the day. This was because it required the understanding of the wave nature of light to explain these phenomenon.

Huygen’s wave theory is one the earliest interpretation of light as a wavefront. He believed that every disturbance on a secondary wavefront can be treated as a source of spherical wave. Though he was able to explain phenomena such as reflection and refraction using this theory, he was unable to explain attributes such as why light travels in straight lines (rectilinear propagation) and why light bends effects on sharp corners (diffraction).

In the 18th century, Fresnel showed that, Huygen’s theory together with his own principle of interference could explain both rectilinear propagation and diffraction of light. The Huygens Fresnel principle provided a strong foundation for the wave theory.

Later Young's double slit method gave the most definitive evidence to the wave nature of light and cemented our understanding of light as a wave. During this time there were multiple theoretical interpretations of light from physics such as J.C. Maxwell who derived the iconic Maxwell’s equation to describe the polarization of light and other electromagnetic radiation.

JJ Thomson’s discovery of electrons was another step in the direction of quantum physics Though it was not intended to prove the quantum nature of particles, Davisson – Germer's double-slit experiment at Western Electric proved that electrons had a wave nature too. This helped to advance and confirm the hypothesis proposed by Louis de Broglie earlier. De Broglie’s hypothesized the dual nature of matter (not just light). De Broglie’s hypothesis constitutes the fact that any particle with a linear momentum can have wave like attributes. This has been since confirmed with many subatomic particles such as electrons, neutrons and even some macromolecules.

In 1900, just as the dust was able to settle in favor of the classical interpretation as the exclusive interpretation of light. Max Planck, a German physicist demonstrated that black body radiations can be explained by considering energy emitted as discrete quantized states or in other words, the energy can only be a multiple of an elementary unit. This postulate has been one of the foundations of quantum physics.

Einstein in his iconic paper in 1905 proposed that the photoelectric effect was a direct consequence of the quantum nature of light. Photoelectric effect is the emission of electrons from a metal when sufficiently energetic electromagnetic radiation hits the surface. It is observed that an increase in intensity of light does not add to the effect, a phenomenon that is exclusive only to the frequency of electromagnetic radiation. And when the energy (or frequency) was high enough, it would knock an electron off its orbit. Thus, Einstein proposed that a beam of light is not a wave propagating through space but a collection of discrete packets of definite energy. These packets of energy are what we call photons today. This paper is what won him that Noble Prize in physics in 1921 (and not for equation which he is most famous for in general relatively -> E=mc^2 )

Physicists such as Neils Bohr took this as the insight required to explain the Hydrogen and Balmer Series formed from by release of several distinct frequencies of light corresponding to the energy released by electrons which are moving to a lower energy orbit. Furthermore, atoms of individual elements emitted wavelengths in distinct lines of a spectrum rather than a continuous spectrum as seen in a black body radiation.

In 1927, Werner Heisenberg published a paper with the famous Heisenberg uncertainty principle. Heisenberg is widely credited for laying a mathematical foundation to the field quantum mechanics. Later, he aptly received the Nobel prize in physics in 1932 “for the creation of quantum mechanics” .On top of his efforts, other physicists such as Dirac, Born, Schrodinger and Pauli brought in more mathematical interpretations and evidence to quantum physics cementing our understanding of the quantum nature of light.

Today, the field of photonics and quantum mechanics is seeing a renewed interest and massive adoption as we are bring ideas from the realm of theory to reality. This extends to everything from the development of advanced sensors & radar to securing communication & processing information.

And all it took us was a few decades of continuous debating, questioning and rewriting of our fundamental understanding of physics and light.

Line Spectra and the Bohr Model