Today We Learn About:

Through the years, the nature of light has been a question pondered by some of the greatest minds the world has ever known. With each passing generation of scientists, it seems that some new, seemingly impossible quality is discovered. These breakthroughs have at times come in bunches, but hundreds of years have separated new discoveries. One thing has always been kept constant however, and that is the idea that there will always be something new to learn, and some great mind to shine through the darkness to reveal another unforeseen quality of light.

In the earliest models, light was a ray. It could be seen coming through clouds, and was thought to be responsible for a variety of phenomena. This model explained mirrors, and the effect of lenses to a point that was sufficient for the scientists involved in the research. This model was so sufficient, in fact that it lasted for one hundred years as the go to explanation for the behavior of light in the given instances.

Galileo was responsible for the improvement of the popular “spy glass” into a much finer lenses that could be used to view the heavens (Obly, et al. 214). This led Galileo to construct the first refracting telescope used to study space. The principle at work here is the refraction of light. The light “enters the objective lens and is focused through several more lenses, then passes into an eyepiece,” (Welton). This refraction gives a magnified, focused point of light through which the viewer can observe far away objects, such as the mountains of the moon, as Galileo did (Obly, et al. 214). The historical context of this implementation and manipulation of light is one that is quite intriguing, albeit not at all related to the progression of our models and theories.

While Galileo experimented and perfected refraction for daily use, and Descartes published the mathematical explanation (Asimov 1964 78), it was Willebrord Snell who actually discovered the mathematical reasoning behind refraction (Asimov 1989 137). As Galileo displayed, “when a beam of light passed from air to a denser medium, such as water or glass, and struck the surface of the denser medium at an oblique angle, it was bent towards the vertical,” (Asimov 1989 137). This theory of this phenomenon, until this point believed to have been caused by a relationship between the actual angles of the source and medium, was proven once and for all to be incorrect. Descartes, although a brilliant mind and namesake for the Cartesian system of plotting points on a graph, did not accurately cite Snell as responsible for this computation (Asimov 1964 82). While it is true that Descartes brought attention to the explanation, it should be noted that he had little to nothing to do with the actual computation of refraction.

Isaac Newton himself in his famed book, “Opticks,” used this ray theory of light in describing the actual light itself (Newton Part I, Defin. I). This was for descriptive purposes only, as Newton was a firm believer in the particle nature of light. He used this model to explain all features of light known in his day (Blaylock 10 Feb. 2008). These features included shadows, reflection, refraction, and intensity. His reasoning behind these claims was that light moved in straight lines, and shadows were cast with sharp edges. He compared light to sound, and reasoned that we can hear around corners, and you cannot see around a corner without a mirror (Asimov 1964 107). This was somewhat of a stubborn and silly approach taken by Newton, as many of his contemporaries had very solid evidence of a wave model of light. Both Grimaldi and Bartholin had performed experiments to contradict Newton. Grimaldi showed that light did in fact bend around obstacles, while Bartholin showed evidence of the double refraction of light (Asimov 1964 107). Newton countered with an example of refraction that “would occur if the velocity of light increased with the density of the medium through which it traveled,” (“Light”). It should be noted in this case, that Bartholin’s experiment also contradicted that of Grimaldi, but this was largely ignored (Asimov 1989 166).

Given this evidence, the wave model of light survived for another hundred years based solely on the reputation of Newton and the loyalty of his followers (Blaylock 10 Feb. 2008). It should be noted that one scientist working at the time had especially solid data that supported a wave model of light. Christiaan Huygens argued that light behaved just as sound did, moving through a medium in a so-called longitudinal fashion (Asimov 1989 166). He used this model to explain reflection, and refraction, stating, “every point on a luminous body experiences agitated motion and thus becomes a centre for disturbance in the ubiquitous ether. If the progress of a disturbance is considered sometime after it has departed its source, each point on its surface becomes the center of a secondary wave front (Obly, et al. 633). Essentially Huygens postulated that light moved through ether, and these effects could be seen very far from the source, and thus given its own set of points from which more waves may depart. For the next one hundred years, this was ignored.

In the early 1800’s, a gentleman by the name of Thomas Young turned the particle model of light upside down when he performed his famed “Double-Slit Experiment.” This experiment demonstrated the interference pattern when a solitary light source shined through two tiny slits that separated the light source from a projection screen. The result were startling, and devastating to the particle theory. The alternating light and dark bands that appeared when two beams of the same frequency cross paths could not be explained by any means other than that of a wave model for light (Blaylock 15 Feb. 2008). In addition to his Double-Slit Experiment, Young added additional slits to his model, and the same interference patterns were formed.

Augusten Fresnel was a French Physicist who was introduced to the work of Thomas Young by his lab partner at a time when Fresnel was completing some of the same sort of experiments (Asimov 1964 220). Upon this introduction Fresnel set out to give an explanation for the behavior of light waves. He knew from the previous work of others that there was a longitudinal wave theory on the board but this did not explain double refraction. Fresnel explained things differently. He felt that light must act as a group or cluster of transverse waves, because light can refract at one set plane, and also simultaneously at a perpendicular plane (Asimov 1964 221). With this new theory of the nature of light explaining reflection, refraction, and diffraction just as well as longitudinal waves did, while also explaining polarized light, Newtonian concepts of light were on their way out, and the debate as to the nature of light seemed to have been ended (Asimov 1989 273).

Science was about to enter into a very uncomfortable period (“Light”). There was experimental evidence that indicated that light could be a particle or a wave. There was experimental evidence that light was strictly a wave, and with the introduction of Einstein’s Photoelectric Effect and Planck’s Black Body Radiation, evidence pointed towards light behaving as a particle (Asimov 1989 431).

Max Planck discovered that if he inserted an infinitesimally small number (6.63×10-34) into the equation used to figure out the relationship between energy and frequency, he could mathematically solve the equation. His use of Planck’s constant, as it came to be called, was thought even by Planck to just be a mathematical trick (Asimov 1989 415). This proved to be false, and his work with Black Body Radiation would lay the foundation for all of Quantum Theory. Planck’s explanation for the UV Catastrophe had Planck himself describing his work as an “act of desperation,” but the truth is, his idea of radiation coming off in “lumps” was quite a good one (Blaylock 5 Mar. 2008). Planck obviously did not understand this at the time, but until he introduced the theory, the calculations predicted a constant rise in intensity at higher frequencies. This school of thought was no more, and it would take the later work of a well-known individual to help the world to fully realize the implication Max Planck had on physics (Blaylock 5 Mar. 2008).

Einstein had set up a photographic plate, and aimed a very weak beam of light towards it. If light were a wave, we would expect to see a uniform gray spot of exposed film. This was not the case. Instead, he saw a variety of black spots where photons, as he called them, had knocked away an electron (Marshall & Zohar 386). This experiment as performed Einstein combined the prior work of a variety of physicists and had quite a few consequences including: Once and for all explaining the Photoelectric Effect, proving that Planck’s quantum theory was not just a convenient mathematical trick used to explain his Black Body Radiation, bringing the idea to the forefront that light can act as a wave and a particle.

New discoveries are being made to this day in relation to the nature of light, and like in the past, with each new discovery comes a new set of questions that may or may not ever be answered. One thing has remained constant, and that is the fact that there is much to be learned about the field of Quantum Mechanics, and its implications on the universe. It is amazing that for how unimaginably large this universe is, we cannot begin to grasp the behavior of the smallest of particles. For all intents and purposes, many aspects of light are still a mystery to us, and this may be the case forever. As Professor Blaylock said, “The Universe—is weird.” (Blaylock 4 Feb. 2008)

Works Cited

Asimov, Isaac. Asimov’s Biographical Encyclopedia of Science and Technology. Garden City, NJ: Doubleday & Company, Inc., 1964.

Asimov, Isaac. Asimov’s Chronology of Science and Discovery. New York: Harper & Row, 1989.

Blaylock, Guy. Lecture. Hasbrouck 134, Amherst, MA. 10 Feb. 2008.

Marshall, Ian, and Danah Zohar. Who’s Afraid of Schrondinger’s Cat? New York: William Morrow and Company, Inc., 1997. 386.

Newton, Sir Isaac. Opticks: or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light. London, 1704. Rare Physics Book by Newton. 13 Mar. 2008 <http://www.rarebookroom.org/Control/nwtopt/index.html&gt;.

Olby, R C, G N Cantor, J R R Christie, and M J S Hodge, eds. Companion to the History of Modern Science. London and New York: Routledge, 1990.

Stroke, George W. “Light.” McGraw-Hill Encyclopedia of Science & Technology. 5th ed. 7 vols. New York: McGraw-Hill, 1982.

Welton, Sean. “Reflectors Versus Refractors.” Visual Astronomy. 27 Feb. 2008. 13 Mar. 2008 <http://www.visualastronomy.com/2008/02/reflectors-versus-refractors.html&gt;.

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