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Introduction to Optics

Is light a wave or a particle? Major discoveries in the physics of light.

How can light be described? For many, it acted as a wave, for others, simply a cluster of particles. From the geometry of Euclid in 300 BC, to the modern realizations of Einstein, there were five breakthroughs in history that ended this debate.

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History

Given the importance and cruciality of light in our lives we can in a sense say that humans are actually creatures of light. From the moment we are born to the moment we die, light is something we constantly meet with throughout the journey. This interaction however, develops only on the scale of our environment, which by itself is way bigger than the scale of the atom where the origins of light hide.

While we are still unable to reach the necessary depth to effortlessly observe the subatomic world, we have managed to gain some knowledge by carefully studying diverse phenomena involving light.

It took a very long time for the gradual formation and spread of engineering and philosophy in ancient Egypt, Mesopotamia and Greece to lead to the collective advancements in science, so that we can understand these basics. The first time movement and geometric qualities of light were vastly discussed was during the rise of the first known civilisations around 2600 BC.

Many years later, around 300 BC and near the end of the Classical Greece era, Euclid of Alexandria created a model of light that was quite ahead of its time. He described light as instantaneously fast and in constant motion. Phenomena such as shadows or reflections are just some of the examples easily described through the geometry of sun rays he presented.

Euclid's Optics was mostly dedicated to the geometry of vision and linear perspective. Unlike his predecessors, Euclid not only provided a theory behind the philosophy of his work, but also his mathematical evaluations to further strengthen the credibility.

Euclid’s geometry was later of great use to the thriving arabic algebra of late AD 10. It was also then that Hasan Ibn al-Haytham, a mathematician, astronomer, and physicist, took one of the first truly dedicated attempts to describe light.

He managed to successfully explain how, what we see, does not originate from our eyes, but rather occurs after light bounces off an object and then enters our eyes. The intromission theories of Aristotle and the calculations made by Euclid were the backbone of his research and in the end managed to get closer than ever before to the principles of optics and visual perception in particular.

After gaining fame and admiration in medieval Europe for his brilliant calculations, Alhazen became known as “the father of modern optics". His idea that light is made up of small particles which travel in a straight line dominated until the end of the Renaissance Era.

Wave properties of light

This particle theory was heavily argued by Rene Descartes in 1630, who for the first time managed to recreate the behavior of light by interpreting it as a wave, rather than a cluster of particles. Light is supposed to be an energy of the disturbance that is propagating in space.

And since any wave needs a medium to propagate, light was believed to move in a proclaimed universal medium i.e. luminiferous aether. The exact explanations of how aether functioned were always vague, but its existence was not questioned and was regarded as common sense in the conceptual solution of the transmitting medium.

This idea was later studied by Christiaan Huygens who just happened to be one of the groundbreaking contributors in the mechanics of oscillations. One of his most famous inventions is the pendulum clock. On the edge of Dutch Golden Age, Huygens for the first time described waves, with the brilliance of his mathematical calculations, which allowed him to re-discover and document most of lights’ properties.

Huygens’ “Traité de la lumière” was published in AD 1690 and was not immediately accepted. At the time Newton argued for the lack of proof regarding the aether theory and did not make things easier with the release of his own book “Opticks”, which plunged the scientific society into a constant debate regarding the nature of light.

With time the corpuscular theory failed to answer many of the questions regarding diffraction, interference and polarization of light and was abandoned for the wave theory of Huygens.

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The missing piece to resolving this whole debate was found in the Huygens-Fresnel principle. In 1801, Thomas Young performed a number of interference experiments, which seemed to be unexplainable by the existing particle theory of light.

This revived the not so old ideas of Huygens that waves and light are somehow connected. In 1821, Fresnel proved light to be a transverse type of wave to a longitudinal one. Thus the Huygens-Fresnel principle was born and served as the basis of many more advancements in optics and the theory of light.

As more experiments were performed, more data was collected, followed by more questions and even more paradoxal miscalculations regarding aether. This piece from Wikipedia summarises the situation very well: By this point the mechanical qualities of the aether had become more and more magical: it had to be a fluid in order to fill space, but one that was millions of times more rigid than steel in order to support the high frequencies of light waves. It also had to be massless and without viscosity, otherwise it would visibly affect the orbits of planets. Additionally it appeared it had to be completely transparent, non-dispersive, incompressible, and continuous at a very small scale.

Spectrum

This wave theory approach was expanded upon by James Clerk Maxwell, a Scottish scientist in the field of physics. His most notable achievement was the theory on electromagnetism, which lead to the realization that light is a transverse electromagnetic wave by bringing together the properties of electricity, magnetism and light together. The only difference between which is the energy, expressed in their frequency.

We see light, but not radio or infrared waves, only because of our perception, an outcome of the human evolution. Maxwell’s discovery is regarded as The Second Great Unification in Physics, with the first being Sir Isaac Newton’s law of universal gravitation documented in his own book “Principia”.

The debate went into a full circle when Albert Einstein came up with an explanation for the photoelectric effect in 1905. This time the wave theory of light failed to explain the situation. Einstein did so by postulating the existence of photons, quanta of light energy with particulate qualities. This event marked an end to the centuries old debate and a merger between the two opposing theories was reached. Or just the so called corpuscular-wave duality.

Today’s model of light is the result of a complex approach involving different domains of physics. In order to quantitatively define light, two methods have been accepted. To treat the light as wave, or rather consider light as a stream of particles. Although light is neither, it is just a fare and convenient way to describe its behaviour. By comparing these incompatible perspectives quantum mechanics gave optics a fundamental model easy to work with.

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