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Of light refers to electromagnetic waves visible to the human eye, that is to say, included in the wavelengths of 0.38 to 0.78 micron (380 nm (violet) to 780 nm (red), the symbol means the nanometer nm. The light is closely related to the concept of color. Newton is proposing for the first time in the seventeenth century a circle of chromatic colors based on the decomposition of white light.
In addition to visible light, by extension, sometimes called "light" other electromagnetic waves, such as those in the infrared and ultraviolet.
Although some forms of life on the ocean floor can do without it, sunlight is the primary source of terrestrial energy through photosynthesis. It therefore controls the cycles and storage écogéobiologiques fossil carbon as they have existed for 3.7 billion years. It also plays an important role in maintaining the ozone layer and reducing the proliferation of microbes sensitive to ultraviolet and / or infrared. This sensitivity is used by some sterilization techniques. It contributes to some forms of pollution called "photochemical" (ozone, nitrogen oxides) and inversely to degrade (photodegradation) certain air pollutants, soil or surface water (some pesticides in the air ) for example. It is still the light through the duration of the fixes biological clocks of animals, the production of melatonin is a hormone produced only at night, in most animals. In plants, day length controls also with temperature, the appearance of buds, leaves, flowers, or the opening or closing of flowers. Therefore the presence of artificial light in the night environment can alter the behavior or functions of some species or ecosystems; phenomenon generally described as the "light pollution".
History: light and astronomical observations
Astronomical observations have been made since men are: they saw lights in the sky: the sun, moon, stars in the sky, shooting stars ... and we realized that this cycle of days governed the (day / night), and the seasonal cycle (day length throughout the year). The fire also produced light.
In antiquity, it was observed astronomical cycles, and we knew from the cycles of the moon (month). In ancient Greece, philosophers of the School of Miletus still believed that the Earth was flat, but from Parmenides, pre-Socratic philosopher, it was recognized that the Earth was spherical. However, in certain books of Aristotle, which was called later, about the thirteenth century, metaphysical, astronomical theories divided the world into a sublunary world, imperfect, and a world supralunar supposed to be perfect. In this representation, fire (sun) was one of four basic elements, since we realized that the burning produces a light similar to the phenomenon observed from the sun or stars. The Earth was the center of the universe (geocentrism), representation took Ptolemy in the second century.
Thus, in ancient Greece, one of the most advanced civilizations of antiquity, we represent the world. It must of course remember that the ancient Greeks, Plato nor Aristotle to him later, since he was a pupil of Plato, did not have the technical inputs of the seventeenth century, and even more of our time. Only the astronomer Aristarchus of Samos, just posterior to the two great Greek philosophers, knew that the earth revolved around the sun (heliocentrism).
Modern astronomy is when Galileo, who discovered the secret of the Dutch telescope (invented in the late sixteenth century), improves performance for use in astronomy. He discovered the phenomena that were not anticipated by existing theories (sunspots, mountains on the Moon, satellites of Jupiter, Saturn's mysterious coves (the rings of Saturn), ...). Fast enough, you develop telescopes, which confirm the respective movement of the Earth and the Sun, which check the laws:
* For movement: Kepler's laws;
* For their movement: the laws of uniformly accelerated motion;
* For the forces involved: Newton's laws and the force of gravity.
These theories upset the heliocentric view of the world in the seventeenth century, as the notions of motion and strength were usually previously used with a sense of ethics.
In 1670, Ole Christensen Romer indirectly measures the speed of light by observing shifts in the orbit of Io from the forecast. Later in 1849, Hippolyte Fizeau directly measures the speed of light with a beam reflected by a distant mirror and through a gear.
Wave behavior of light
In 1678, Christian Huygens proposed a wave theory of light, published in 1690 in his Treatise on Light. Thomas Young in 1801 experimented with diffraction and interference of light. In 1821, Augustin Fresnel states that the design wave of light is only able to convincingly explain all the phenomena of polarization by establishing cross-cutting nature of light waves and in 1850, Leon Foucault invoke the wave theory of the corpuscular theory Newton with his experience on the speed of light propagation. It was not until the work of James Clerk Maxwell to explain the wave phenomenon: in 1873 he published a treatise on electromagnetic waves, defining light as a wave that propagates in the form of radiation, the spectrum of this radiation n ' being a part of the overall electromagnetic radiation, much wider infrared, ultraviolet, radio waves, X-ray ... The Maxwell equations defining the electromagnetic radiation will have many applications in the nineteenth century, and even more in the twentieth century (radio, television, computers, satellites ...).
The photograph can also fix on paper the images obtained with telescopes, which makes distribution much easier.
Corpuscular behavior of light
In 1887, Heinrich Rudolf Hertz described the photoelectric effect. In 1900 Max Planck states the theory of black body, and Albert Einstein in 1905 furthers the study of the photoelectric effect, an effect in which light does not act as waves, and shows that the light also a behavior of quanta of light energy with particulate qualities. It reconciles the theory of Huygens and with that of Newton in explaining the photoelectric effect, for which he receives the 1921 Nobel Prize in Physics.
In 1927, Louis de Broglie extended this quantification by postulating that there is a wave-particle duality for any particle: the concept of photon emerges. This is one of the first steps of modern science in the field of quantum mechanics.
A few years later, Quantum field theory explores the concept of photon.
In the twentieth century, the theories are applied electromagnetic astronomical observation: after the Second World War, is developing telescopes that collect electromagnetic waves in the radio spectrum, radio telescopes. This had the advantage of allowing the discovery of new celestial objects. Thus was born the radio astronomy. It also now employs space telescopes to avoid air pollution.
Today, astronomers are less "observations" to the naked eye. Amateur astronomers, however, continue to observe the sky with telescopes, which are in principle equivalent to that used by Galileo from 1609.
Theories
Light, like any phenomenon of displacement, can be seen as a wave or as a stream of particles (called photons in this case).
Maxwell's laws, or a more human scale to the laws of geometrical optics, describe well the behavior of these waves. This classical description is quite valid and widely used within the scientific community. However, it does not explain the quantization of the energy carried by radiation, a phenomenon observed and explained by Albert Einstein in 1913 by postulating the existence of photons.
However, modern physics considers that each of these photons can itself be seen as a wave (the so-called wave-particle or wave-particle in quantum mechanics).
* Photons
* Colour Perception
Speed of light.
The speed of light in vacuum, denoted c (like speed), is a constant of physics.
This is the maximum speed allowed for any movement of a material object or signal by the theory of relativity. This property has been driven from the experience of Michelson and Morley interferometer and was clearly stated by Albert Einstein in 1905.
Other units are defined from the speed of light. In particular, the meter is set so that the speed of light in vacuum is worth 299 792 458 m / s. As a result, the speed of light is correct, because it no longer depends on a measure (imprecise and subject to change with advances in measurement).
Addition of speeds and speed
The law of addition of velocities v '= V + v is approximately true for low speeds compared to the speed of light.
From the perspective of classical physics, a traveler walking through a train, from the ground, a speed equal to that of the train over (vectorially) its own walking speed on the train. And we write d = (V + v) t = Vt + vt = the distance traveled by the train and the distance traveled on the train = the distance traveled by the passenger to the ground in time t which is conventionally the even in the train, which implies the classical law of addition of speeds.
This is only an approximation which becomes less and less accurate as the velocity v increases considered.
A photon is the same speed c either the ground or from the train! Law V = c + c 'is false when c = c' for V-zero. The law of addition of velocities is only an approximation of the law known processing speeds of Lorentz (sometimes called the addition of velocities, or more correctly composition law of velocities).
This result is a feature of special relativity, the law of composition of velocities after mathematical transformations of Lorentz gives the limit of low speeds (compared to the speed c) the same results as the Galilean transformation.
In materials
Note: the speed of light is not always the same in all environments and under all conditions. For example, the speed differences observed between two media can be linked to the phenomenon of refraction, which allows operation of the lenses.
The differences are generally quite low, which has allowed many people to talk about the speed of light rather than speed of light in vacuum. However, in some cases, a light wave can be slowed down considerably. Physicists have succeeded in slowing the spread light to a few meters per second in extreme cases.
In the International System (SI)
Today, most units of the international system are defined from the speed of light.
Speed is the quotient of a length by a length, we can define a distance as the product of a period with a speed (in this case c), or a period as the division of a distance c .
Time measurement
The second is defined in the international system by a luminous phenomenon: the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium 133 atom.
Distance measurement
* Meter, the international system unit of length. Today, it is defined as the distance light travels in 1 / 299792458 of a second. This is a conventional definition, because any change in the definition of the second would have a direct impact on the length of the meter. With the current definition of the second, the meter is equal to
\ Frac {9 \, 192 \, 631 \, 770} {299 \, 792 \, 458} times the wavelength of the radiation chosen.
One can also say that the speed of light in vacuum is exactly 299,792,458 m · s-1: there is absolutely no uncertainty about this value, the uncertainty lies only in the definition of the second.
* The meter, with its sub-multiple or multiple (mm, km), is very convenient to measure distances on Earth, for cons for astronomers, it is too short and poorly adapted (as the astronomers observe that almost of light). Indeed, the Moon, the star closest to us, is about 380 million yards of us and the Sun, the nearest star, is about 150 000 000 000 meters.
With the principle described above (distance = cx duration), is defined as a light year the distance light travels in one year. Thus the Sun is only 8.32 minutes of light us, and the Moon is only just over one second light. A light year is about 10 000 000 000 000 000 meters (10 million billion m or 1016 m).
In practice
Monochromatic and polychromatic
Light consists of electromagnetic waves. In general, a wave is characterized by its wavelength and phase. The wavelength corresponds to the color of light. Thus, light consists of waves of the same wavelength is called monochromatic. If in addition all the waves have the same phase, while the light is coherent: this happens in a laser.
Measure
In the measurement of light, it is important to define what we are talking
* The unit of luminous flux or luminous power is the lumen = candela.stéradian. A standard light bulb (15 watts or 75 watts low power conventional incandescent) produces about 1500 lumens.
* The international unit of luminous intensity is candela.
The measurement of light is complicated by the fact that we are interested in practice to visible light, while human perception depends on the wavelength: See luminance and chrominance.
Heavenly lights
* The Sun and the stars generally produce more radiation than they receive,
* The Moon and more generally small bodies (the planets and their satellites, asteroids, comets, etc..) Produce less radiation than they receive. Some giant planets (like Jupiter or Saturn) produce a little more radiation than they receive, but not enough to be easily visible to the naked eye from Earth. In both cases, these bodies are illuminated by reflected sunlight.
* Shooting stars are heated by friction with the air and eventually burn. This is a light source.
Chemical lights
* Some organisms: fish, molluscs, fireflies and glowworms, are the site of chemical reactions producing light.
* The intense heating, so the combustion in general, fire, fire-wisps, produce light:
liquid: oil lamps, oil, or gas
solid: candles, candle (candle) candle.
Electric lights
Electric lights are the most common sources of light today: street lights, spotlights, headlights, flashlights, etc.., They can use a process of heating a quantum phenomenon.
The light bulb ("incandescent lamp") has revolutionized everyday life. The light source comes from the incandescent filament of a light. The fluorescent tube, light emitting diode are electric lights, and the cathode ray tube employing the technique of electron bombardment.
Light quantum
* Fluorescence, lasers, lamps, mercury vapor or sodium plasmas such as those produced by lightning storms, produce light from quantum phenomena at the heart of the atoms: the excitation of electrons (" optical pumping "), can be obtained by excitation and de-excitation of these electrons, which by returning to their normal energy level, emit photons (light).
Other lights
* The phosphorescence is a natural source of light of low intensity.
* The sparks are the result of intense friction on certain materials.
* The light emission due to friction, or triboluminescence, is not of thermal origin and occurs only with electrical insulators.
* Some animals and fungi are capable of producing a cold light of biochemical origin: particularly nocturnal animals such as different species of fireflies, or marine animals of the deep, and, on the surface, some species of plankton.
Physiology
The various factors that determine the optical absorption of pigment molecules (blue, green - yellow, orange - red) in the three types of cones and rods are a central concern of photometry. Colour perception is made possible by the narrow absorption spectrum of pigments of the cones. Sticks in turn reflect the light intensity. They have a pigment called rhodopsin molecule (or retinal purple) with a broader absorption spectrum. They are more sensitive than cones.
From a physiological point of view the effect of color is due to the different degrees of absorption of the pigments of the cones. The perception of different colors correspond to different areas of the light spectrum. When certain wavelengths of light are absorbed areas that remain then give an impression of color. And a green leaf absorbs Does not the area of "green" wavelengths but others that are complementary: the "red" (680 nm) and "blue" (430 nm).
Light as we know the world is a mixture of different wavelengths. Through a diffraction grating or prism can break down the polychromatic light (colored) in different frequency monochromatic (United). Each of the monochromatic components of the light corresponds to a specific perception of the human eye: the colors of the spectrum or rainbow trout.
The transitions between the different colors are blurred. Their perception is subjective and depends on the tradition and language. The words used to describe the colors reflected.
Each area of particular color can be decomposed into different shades. Infill the area between the blue and green called cyan or turquoise. Other colors like brown, for example resulting from the superposition of several wavelengths (additive mixing of colors) or from a subtractive color mixture of light obtained by filtering white light (the sum of all colors).
Electromagnetic waves beyond the visible boundaries of the humans that are lower than the violet are referred to a certain extent by the term wave ultraviolet or UV. Those that are higher than red are called infrared. The bandwidth of visible light by animals can vary considerably from the visual capabilities of humans.
Main article: human eye.
Optical phenomena
* Diffraction
* Diffusion
* Interference
* Reflection
* Refraction
* Gravitational lens
Symbolism of light
The light seems to have been a symbolic interpretation as soon as men began to believe in an afterlife. Since the possible deification of fire, became vital for prehistoric man, then one of the four elements of the philosophy of ancient Greece to the Christian theology of God as "Light of lights", the illumination is present in many religions, it has continued to grant that the origins and supernatural virtues.
His symbols are universal and come in many forms:
* The solar circle (symbol of Aton, the god of ancient Egypt, rosette "radiant" of the Gothic cathedrals)
* The arrow or the triangle (like direct sunlight or close the man from heaven like the Egyptian and Aztec pyramids, or the pediments of Greek temples).
Philosophers have used this symbolic way, he argued Descartes in the Principles of Philosophy (1644), we could use the natural light without the light of faith, the same Descartes was interested close to the lens (the Optics, 1637). It was the time when it was recognized that the Earth revolved around the Sun (see Copernican Revolution, which was an upheaval in the representations of the world, and consecutively in social representations.
The term "Enlightenment" to describe the current European philosophy (especially in France) which is derived from this design rather mechanistic world and the universe also gave its name to the century in which the social representation began in place, commonly known as the Enlightenment, Michel Foucault and analysis in the Order of Things (see also episteme).
Georges Duby, in time of the cathedral (1975), also describes, in terms of art, theology of Light, initiated at the Basilica of St. Denis in the twelfth century, which is the origin of architecture called Gothic (word appeared in the nineteenth century and the emergence of Gothic cathedrals say throughout Europe.
According to the theology of light, sky light passes through the windows, which are represented scenes from the Old Testament and New Testament, pictorial illustration of our modern catechism, for people who were still not grown as a whole .
