The light indicates the electromagnetic waves visible by the human eye, i.e. included/understood in wavelengths from 0,38 to 0,78 Micron (380 Nm (red) with 780 Nm (blue); the Symbole Nm indicates the Nanomètre). The light is closely related to the concept of color. It is Newton which proposes for the first time at the 17th century a circle of the colors chromatiquesz based on the decomposition of the white light.

Sometimes in addition to the visible Light, by extension, one calls “light” of others electromagnetic waves, such as those located in the fields Infrarouge and Ultraviolet.

Even if certain forms of life, at the bottom of the oceans can occur some, the light of the Sun is the first energy source of the terrestrial ecosystem S, via the Photosynthèse. It thus controls the cycles ecogeobiologic and the fossil storage of carbon such as they exist since 3,7 billion years. She plays also a big role by maintaining the Couche ozone and by limiting the pullulation of the Microbe S sensitive to the Ultra-violet S and/or the infra-red. This sensitivity is used by certain techniques of sterilization. It contributes to certain forms of pollution known as “ photochemical ” (tropospheric ozone, nitrogen oxides) and conversely to degrade (photodegradation) certain pollutants of the air, the surface ground or water (some Pesticide S present in the Air for example. It is still the light which via the duration of the day corrects the animal biological clocks, by the production of Mélatonine which is a only produced Hormone the night, at the majority of the animals. At the plants, the duration of the day also controls, with the temperature, the appearance of the buds, sheets, flowers, or the opening or the closing of flowers. This is why the presence of artificial light in the night Environnement can deteriorate the behavior or the functions of certain species or the ecosystems; phenomenon generally described under the name of “luminous Pollution”.

Historical: the light and the Observation S in Astronomy

The Observation S Astronomique S were carried out since the Homme exists : one saw lights in the Ciel: the Sun, the the Moon, of the star S with the Firmament, of the shooting stars… and one realized that controlled the Cycle Journée S (alternation Jour/Nuit), and the Cycle of the Saison S (lasted of the day throughout the Année). The fire also produced light.

In Antiquity, one observed the astronomical cycles, and one included/understood the cycles coming from the Moon (Semaine). In Ancient Greece, the philosophers of the Millet School still believed that the Earth was punt, but starting from Parménide, philosopher Présocratique, one admitted that the Ground was spherical. However, in certain books of Aristote, that one called later on, with 13th century approximately, Metaphysical ( méta = beyond, therefore what what is beyond the physis, which means old Nature in Greek), the astronomical theories divided the world in a world sublunary, imperfect, and a world supralunaire supposed to be Parfait. In this representation, the Feu (Sun) was one of the four basic elements, since one realized that the Combustion produced a light comparable with the Phénomène observed coming from the Sun or of stars. The Earth was in the center of the Univers (Géocentrisme), representation which Ptolémée at the 2nd century took again.

Thus, in the ancient Greece, one of the most advanced Civilization S of the Antiquity, one represented the world. It should obviously be recalled that the Greeks of Antiquity, not more Plato that Aristote which was posterior for him, since he was pupil of Plato, did not have the technical contributions of the 17th century, and a fortiori of our time. Only the astronomer Aristarque de Samos, a little after our two large Philosopher S Greek, understood that the Ground turned around the Sun (Héliocentrisme).

The modern Astronomie appears when Galileo, after the invention of the Telescope (probably at the beginning of the 17th century by the Dutchmen), improves the performances of them to use it in astronomy (see Telescope). He discovers phenomena which were not envisaged by the existing theories (Sunspots, mountains on the Moon, rings of Saturn,…). Enough quickly, one develops Télescope S, which confirm the movement respective of the Earth and the Sun, which could not be explained, mainly, that with mathematical formulations:

  • Trajectory: Laws of Kepler,
  • movement: Galileo and the uniformly accelerated movement,
  • force: Newton and the force of gravitation.

These heliocentric theories upset the representation of the world at the 18th century, because the concepts of movement and force were usually before employed with an ethical direction .

In 1670, Ole Christensen Rømer measurement indirectly speed of light by observing the shifts of the orbit of Io compared to the forecasts. Later in 1849, Hippolyte Fizeau measurement directly speed of light with a beam reflected by a reflective toothed wheel.

Undulatory behavior of the light

In 1801, Thomas Young tries out the Diffraction and the Interférence S of the light, but it will be necessary to wait nearly one century so that James Clerk Maxwell explains this phenomenon: it publishes in 1873 a treaty on the electromagnetic waves , defining the light as a Wave which is propagated in the form of a Rayonnement, the spectrum of this radiation being only part of the whole of the electromagnetic Rayonnement, much broader: infra-red, ultra-violet, radio waves, x-rays… the Maxwell's equations defining the electromagnetic Rayonnement will have many applications as of the 19th century, and even more at the 20th century (radio, Télévision, data processing, satellites…).

The Photographie also makes it possible to fix on the Papier the images obtained by the Télescope S, which makes easier from there the diffusion much.

Corpuscular behavior of the matter

In 1887, Heinrich Rudolf Hertz describes the photoelectric effect. In 1900, max Planck states the theory of the black Corps, then Albert Einstein in 1905 pushes the study further from the photoelectric effect and shows that the light with a behavior of quanta of energy. It received the Nobel Prize of physics in 1921 for its explanation of the photoelectric effect.

In 1927, Louis de Broglie prolongs this quantification while postulating that there is a duality wave-particle for any corpuscle: the concept of Photon emerges then. It is one of the first steps of modern science in the field of the quantum Mécanique.

A few years later, the Quantum theory of the fields appronfondit the concept of photon.

To the 20th century, one applies the electromagnetic theories to the astronomical observation: after the Second world war, one develops telescopes which collect the electromagnetic waves in the spectrum radio, the Radiotélescope S. That had the advantage of allowing to discover new celestial objects. Thus was born the Radioastronomie. One employs also now Space telescopes to avoid atmospheric pollution.

Nowadays, the astronomers make less “observations” with the naked eye. The astronomers amateurs continue nevertheless to observe the sky with telescopes, which are in their principle, equivalent to that employed Galileo starting from 1609.

Theories

The light , like any phenomenon of displacement, can be conceived like a wave or a flow of particle S (called in fact Photon S).

The laws of Maxwell, or on a more human scale the laws of the geometrical optical , describe well the behavior of these waves. This traditional description completely valid and is very much used within the scientific community. However, she does not explain the quantification of the energy transported by the radiation, phenomenon observed and explained by Albert Einstein as of 1913 by postulating the existence of the Photon S.

However, the modern Physique considers that each one of these photons can itself be regarded as a Onde (what is called the Dualité wave-particle or wave-corpuscle in quantum Mécanique).

Speed

Speed of light in the vacuum, C (like celerity ), is a constant of physics. It is the maximum speed allowed for any displacement of information or a material object by the Theory of relativity. This property was induced Expérience of interferometry of Michelson and Morley and was clearly stated by Albert Einstein in 1905.

So speed of light is exact , because it does not depend on a measurement (vague and suitable for change with progress of measurement). Other units are defined starting from speed of light (cf will infra). Thus, speed of light in the vacuum is of: 299792458 m/s.

Addition of speeds and celerity

The law of addition speeds v' = V+v is about true for low speeds compared to speed of light.

From the point of view of the traditional Physical , a traveller going in a train has, compared to the ground, a speed equal to that of the train more (vectoriellement) its own speed of walk in the train. And one writes D = (V+v) T = Vt +vt = the distance covered by the train + the distance covered in the train = the distance covered by the traveller compared to the ground in the time T which is classically the same one in the train and, which implies the traditional law of addition speeds.

This is only one approximation, which becomes less and less precise as speed v considered increases.

A Photon goes at the same speed C that it is compared to the ground or the train! The law V + C = it is thus false since C = it for V different from zero. The law of addition speeds is only one approximation of the law known as of transformation on speeds of Lorentz (called sometimes addition speeds, or more correctly law of composition speeds).

This result is one of the characteristics of the restricted Relativité; the law of composition speeds resulting from the mathematical transformations of Lorentz gives to the limit low speeds (compared to the speed c) the same results as the transformations of Galileo.

In materials

To note: speed of light is not always the same one in all the mediums and all the conditions. For example, the variations speed observed between two mediums can be connected to the phenomenon of refraction which allows the operation of the lenses.

The variations are generally rather weak, which made it possible many people to speak about speed of light instead of speed of light in the vacuum . However, in certain cases, a light wave can be slowed down considerably. The physicists managed to slow down the luminous propagation until a few meters a second in extreme cases.

In the International Système (IF)

Nowadays, the majority of the units of the international system are defined starting from the celerity of the light.

A speed being the quotient of a Length by a Lasted , one can thus define a distance as being the one duration old product by a speed (in fact C ), or a duration like the division of a distance by C .

; Measure time The second is defined in the international system by a luminous phenomenon: it is the duration of: 9192631770 periods of the Radiation corresponding to the transition enters the two hyperfine levels of the fundamental state of the atom of cesium 133.

; Measure of distance

  • the Mètre, unit of the international system length. Nowadays, it is defined as the distance covered by the light in: 1/299792458 of second. It is a conventional definition, because any evolution in the definition of the second would have a direct incidence over the length of the meter. With the current definition of the second, the meter is thus equal to
\ frac {9 \; 192 \; 631 \; 770}{299\;792 \; 458} time the wavelength of selected radiation. One can also say that the Speed of light in the Vide is precisely : 299792458 m·s-1: there is not least uncertainty on this value, uncertainty residing only in the definition of the second.

  • the meter, with its submultiples or multiples (millimetre, kilometer), is very practical to measure the distances on the Ground; on the other hand for the astronomers, it is too short and little adapted (since the astronomers observe practically only light). Indeed, the the Moon, the star nearest to us, is with 380  000  000  meters of us.

The Sun, the star nearest, is with 150  000  000  000  meters. It is not very practical!!

With the principle describes previously (distance = C X lasted), one defines the light-year as the distance which the light traverses in 1 year. Thus the Sun is only to 8,32 minute-light of us; and the Moon is only with a little more than 1 second-light. The Année-lumière is worth approximately: 10000000000000000 de  meters (10 million billion meters, is 1016 m).

In practice

Monochromatisme and polychromatism

The light consists of electromagnetic waves. In a general way, a wave is characterized by its Wavelength and its phase. The wavelength corresponds to the color of the light. Thus, a light made up of waves same wavelength, is known as monochromatic . So in more all the waves have the same phase, then the light is coherent: it is what occurs in a Laser.

Measure

As regards measurement of the light, it is important to define well what one speaks

The measurement of the light is complicated by the fact that one is interested, in practice, with the visible light, whereas human perception depends on the wavelength: Cf Brightness and Chrominance.

Celestial lights

  • the Sun and more generally the star S produce more radiation than they do not receive any,
  • the the Moon and more generally small the Celestial body S (the Planet S and their satellite S, the Astéroïde S, the Comet S, etc), produces less radiation than they do not receive any. Some Planet S giants (like Jupiter or Saturn) produce a little more radiation than they do not receive any, but not sufficiently to be easily visible with the naked eye since the ground. In both cases, these bodies are luminous by reflection of the light of the Sun.
  • the shooting stars are overheated by the Friction with the Air and end up burning there. This Phénomène is source of light.

Chemical lights

  • Certain living organisms: Poisson S, Mollusc S, Firefly S and towards luisants, are the seat of producing chemical reactions of lights.

  • intense heatings, therefore the Combustion S in general, the fire, the fire-merry S, produce light:
liquid: the Lamp S with Oil, Oil, or solid Gas,
: the Candle S, candle (Candlestick), candle.

Electric lights

The electric lights are the most current sources of light today: Standard lamp S, spots, Headlight S, lamp-torches, etc, they can use a phenomenon of heating or a quantum phenomenon.

The Electric bulb (“lamp with Incandescence”) revolutionized the daily life. The source of light comes from the Incandescence of a luminous filament. The fluorescent Tube, the Electroluminescent diode is electric lights, as well as the Cathode tube which employs the technique of a bombardment of electron S.

Quantum lights

  • the Fluorescence, the Laser S, the lamps with vapor of mercury or Sodium, the plasma S such as those produced by the flash S in the Storm S, produce light resulting from quantum phenomena in the middle of the Atome S: the excitation of the electron S (" pumping optique"), can be obtained by excitation, then de-energizing of these electrons, which while turning over to their usual energy level, emit Photon S (light).

Other lights

  • the Phosphorescence is a natural source of light, of low intensity.
  • the sparks are the product of intense a Friction on certain materials.
  • the emission of light due to friction, or triboluminescence, is not thermal origin and it occurs only with electrical insulators.
  • Certain animals and mushrooms are able to produce a cold light of biochemical origin: in particular night animals such as various species of Firefly S; or of the marine animals of the great depths; like, on the surface, certain species of Plankton.

Human vision

See the detailed article: Eye

Optical phenomena

Symbolism of the light

The light seems to have been the subject of a Interprétation symbolic system as soon as the men started to believe in beyond. Since the possible deification of the Fire, become Vital element for the prehistoric Man, then one of the four elements of the philosophy of ancient Greece, until the Christian Theology of God like " light of the lumières" , the illumination being present in many Religion S, one had of cease only to grant origins and to him Vertu S Surnaturel them.

Its Symbole S is Universel S and is declined in multiple forms:

  • the solar Circle (symbol of Aton, god of ancient Egypt, rosette " rayonnante" Gothic cathedrals),
  • the Arrow or the Triangle (with the image of the solar ray or bringing the man closer to the sky as for the Pyramid S Aztec S and Egyptian).

The Philosophe S used this symbolic system; Thus, Descartes affirmed it in the Principles of the philosophy (1644), which one could be based on the lights Naturelle S, without the lights of the Foi, this same Descartes being interested of close with optics (the Dioptric one, 1637). It was at the time where one admitted that the Ground turned around the Sun (see Révolution copernician, which constituted an upheaval in the Représentation S of the world, and consecutively in the social Représentations.

The expression “Lights” to indicate the Philosophical current European (in particular in France) which derived from this design rather Mécaniste of the world and of the Univers also gave its name to the Siècle during which this social Représentation was installation, commonly called the Age of Enlightenment, and that Michel Foucault analyzes in the words and the things (see also épistémè).

Georges Duby, in the time of the cathedrals (1975), also described, under the Artistic angle , the Theology of the Light, initiated with the Basilica Saint-Denis at the 12th century, which is at the origin of the architecture known as Gothic (word appeared at the 19th century and of the emergence of the Cathédrale S known as gothic scripts in all the Europe.
Selon the Theology of the light, the light of the Ciel passes through the Vitraux, where are represented Scène S of the Old Testament and New Testament, illustration Imagée of our modern Catéchisme, for Population S which were still little Cultivée S as a whole.

See too

Related articles

External bonds

light
  • History of discovered on the site toutsurlaphysique.fr
  • the measurement speed of light
  • luminous Phenomena, Explanations of the phenomena such as the arc in sky, iridescence, northern lights, etc
  • complete records on speed of light on the site Culturesciences-Physics of the ENS Lyon (a history, optical indices, relativity, the Cherenkov effect…)

Nds-nl: Locht Simple: Light Zh-min-nan: Kng Zh-yue: 光

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