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The nature of light: historical approach

The exact nature of the light remains a fascinating topic. The work on the study of light and the many debates that were born in yet occupy an important chapter in the history of science. We meet many illustrious scholars, whose work on the light is at the origin of many discoveries about vision and ocular optics.

Antiquity: light symbol

Since antiquity, of many scholars and scientists have studied the light to better understand its nature. Previously, the light seems to have been symbolic, contemporary interpretation of the appearance of "abstract" thought Lifeblood of prehistoric men, to whom they were certainly a supernatural virtue, the flame, 'apprehendable' source of light, was one of the four elements of the philosophy of ancient Greece. Enlightenment became a theological pillar of the symbolic "divine light". For the Greeks, the eye was not the receiver, but the "source" of the vision. Optics was then attached to the vision, understood science as an active mechanism of the eye, which "illuminated" the observed scene: for Euclid, the spokes that connect the eye and its target are issued by the eye.

The contribution of Kepler

It's at 17e century that theories of the enlightenment entered a new era, with the writings of Kepler, that equates the eye with an optical device, leading to the formation of a real image on the retina. He is convinced that the reception of the images is provided by the retina and not the lens as people thought at that time, the brain, giving the place the inverted image it receives. These advances make Kepler a founding father of physical and physiological optics. In parallel, Desmaps contributes to separate the light of the faith of the real light, which it seeks to understand the laws. At this shift in the representation of the world and the rise of a Cartesian thinking, the European philosophical current (especially in France) has borrowed the term 'Lights', and 18e century who carried him was "the age of enlightenment.


Light: pressure and corpuscles

Meanwhile, in the 17e century, the development of physical theories of light takes place in a framework of "mechanical models. For Desmaps, the light is a component of the material, which fills all space (the existence of the void is refuted by this scholar) and that the propagation is instantaneous: light is a pressure that is transmitted instantly in a 'ether', substance filling all the space. Desmaps contributions to optics are undeniable, not always positive. This is by analogy with the path of projectiles that Desmaps is going to build his law of Sines, assuming incorrectly that light travels better (faster) in the glass than in air. The prestige of Newton, strong supporter of corpuscular light design, will contribute to the adoption of this design or apply the principles of Newtonian mechanics. Light is made of particles that spread in a straight line, within a "ether". This ether fills space, since it transmits to us up to the light of the stars.

The light... a wave?

It is through this "ether" that the hypothesis of a wave theory of light will take body: rather than a movement of matter, the light may be a movement in the matter. Some observations as those of the Italian Jesuit Grimaldi on the diffraction of light suggest that, like the sound in the air or the waves at the surface of a medium liquid, light travels as a wave in a fluid (ether).

After 15 years in Paris to participate in the work of the Royal Academy of sciences, Huygens published in 1690 his "treatise of light", in which he wrote that the light "extends successively by spherical waves", and that "every little place of a luminous body (Sun, candle, burning coal...). generates these waves. The ether of Huygens is a fluid made up of microscopic particles elastic and capable of transmitting a vibration at high speed: Unlike the (incorrect) assumption of Desmaps, light moves more slowly in the glass or Crystal.

The tutelary shadow of Newton on the science of the 18e century certainly explains that few significant advances have been made in the field of optics, and that the wave of light character was the subject of little interest to the work of Thomas Young. Born in 1773 in England, this scholar was an encyclopedic mind, multiplying the study subjects in all areas: medicine, sound, optics, astronomy, electricity, etc. His contributions to the vision mechanisms were important.

Young; precursor of modern optics

Young speculated that feel colored dependent on the frequency of the light vibrations in the retina. accommodation, astigmatism). He explained that eye astigmatism was linked to a defect of the curvature of the cornea, and it was the lens that was responsible for the mechanism of accommodation. . His most famous experience, the slots (or holes) young, is a crucial step in the history of wave optics. This is in 1807 Young describes the experimental device to which his name is attached. By projecting a light beam on a perforated plate of two tiny holes, observed on a screen placed behind the plate the mixture resulting from the created beams: a succession of alternately dark and bright bands. Only the wave theory could explain this result. Depending on the amplitude of each wave in a point, it occurs a strengthening or a cancellation, with all intermediate situations (more explanation here). Baptizing this phenomenon of "interference", Young sees the ultimate proof that light is a wave. How do you explain the addition of particles to be able to produce the dark, on a periodic basis?  Despite this demonstration, Young will face the Newtonian aura, and can clearly explain some phenomena such as the polarizationAlthough he envisions a partial explanation of it linked to the plunge of the oscillations of the light wave nature.

Taking up the ideas of Huygens, is Augustine Fresnel who will complete the wave model of light assuming that the vibrations of the lights are in no longitudinal and transverse direction. It identifies the light to a "universal fluid stirred by the fast movements of the particles of the luminous body; These movements are carried out in the ether, and explain the phenomena of interference and diffraction.

These phenomena can be easily highlighted: Unlike what would predict a purely 'ballistic' theory of light, the observation of light intensity as a result of the spread of a flow of light waves beyond a perforated barrier highlights an area where light is not present in the "" light spot formed (a light sensor, sensitive to the intensity (, reveals a characteristic aspect where a central peak of intensity is surrounded by several concentric rings alternating absence and presence of light intensity).

light particle wave duality

A train of parallel light rays (left) and wave bright Parallels (right) meet an obstacle punctured. In the case of a "ballistic" model (light rays, embodying the path of "grains" of light), the theory provides that the image formed of the beam of incident rays spread beyond the obstacle is at any point can be stacked to the hole that allowed the light to spread: must be observed a bright disk of uniform intensity. The wave theory is more in line with reality: constructive and destructive interference phenomena explain the presence of light outside the area corresponding to the projection of the hole, and the concentric alternation between absence and presence of light.

The diffraction phenomena and their link with the principle of Huygens are explained here.

At the same time, the experiences of Foucault and Fizeau to measure the speed of light. Although the wave-like nature and the speed of the propagation of light starts to win, the nature of the light and its support (ether) are unknown. The unification of electricity and magnetism into a single theory (electromagnetic theory) of Maxwell led to realize that light is an electromagnetic wave.

At the beginning of the 20e century, the failure of experiments designed to characterize the properties of the ether and the theory of relativity by Einstein will lead to abandon the notion of support required for the propagation of light. The speed of light in a vacuum is maximum, cannot be exceeded and is independent of any repository.

Light: wave and corpuscle

The 20e century is the return of a corpuscular light design. From the work of Planck, Einstein introduced the notion of photon. A photon is a particle of light that transports a quantum of energy E = h n, where n is the frequency of the wave and Planck's constant h.  This work will give rise to the mechanics and quantum optics, or the light is both wave and particle. The quantum theory of light radiation will be at the origin of the effect LASER (Light Amplification by purpose broadcast Radiation), predicted by Einstein in 1917, and materialized in 1960 by Maiman, who secured the first LASER beam.

Quantum electrodynamics: unification

Corpuscular and undulatory mechanisms will be brilliantly brought together by a physicist American popularizer of genius Richard Feynman, in the second half of the 20th century, under the aegis of the theory of quantum electrodynamics. This theory allows to understand all the phenomena related to light and matter.

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