Light
- For the homonymous articles, to see Light (homonymy).
light indicate them electromagnetic waves visible by the human?il. In addition to visible lightsometimes, by extension, one calls "light" of others electromagnetic waves, such as those located in the fields infra-red and ultraviolet ray.
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Synopsis |
Theories on the light
light, like any phenomenon of displacement, can conceive itself like one wave or like a flow of particles (called in fact ).
, or on a more human scale laws ofgeometrical optics, describe well the behavior of these waves. This traditional description is done everything valid and very much used within the scientific community. However, it does not explain the quantification of the energy transported by the radiation, phenomenon observed and explained by as of by postulating the existence of .
However, modern physics consider that each one of these photons can itself be regarded as one wave (what is called duality wave-particle or wave-corpuscle in ).
- Perception of colors
Speed of the light
The speed of the light, 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 inducedexperiment of interferometry of Michelson and Morley and was clearly stated by in 1905.
So the speed of the light is exact, because it does not depend on one measure (vague and suitable for change with progress of measurement). Other units are defined starting from the speed of the light (cf will infra).
Addition of speeds and celerity
The law of addition speeds v' =V+v is about true for low speeds compared to the speed of the light.
- From the point of view of traditional physics, a traveller walking in a train has, compared to the ground, a speed equal to that of the train more (vectoriellement) his 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 on the ground, which implies the traditional law of addition speeds.
This is only one approximation, which becomes increasingly false as speed v considered increases.
One 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 restricted relativity ; the law of composition speeds resulting from the mathematical transformations of Lorentz give to the limit low speeds (compared to the speed c) the same results as the transformations of Galileo.
Speed of the light in materials
To note: the speed of the light is not always the same one in all the mediums and all the conditions. For example, the variations speed observed between two mediums are at the origin of the phenomenon of refraction who allows the operation of the lenses.
The variations are generally rather weak, which made it possible many people to speak about speed of the light instead of speed of the light in the vacuum. But 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.
Use in Si
Nowadays, majority of units of the international system are defined starting from the celerity of the light.
A speed being the quotient of one length by one duration, 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
second is defined in the international system by a luminous phenomenon: it is the 9 192 631 770 periods duration of radiation corresponding to the transition between the two hyperfine levels from the fundamental state from the cesium 133 atom.
Measure of distance
- , unit of the international system length. In 2005, it is defined as the distance covered by the light into 1/299.792.458 of second. It is a conventional definition, any evolution in the definition of the second would have a direct impact over the length of the meter. With the current definition of the second, the meter is thus equal to
- <maths> \frac{9.192.631.770}{299.792.458} </maths> time the wavelength of selected radiation.
One can also say that speed of the light in vacuum, is precisely 299.792.458 m.S-1 : there is not least uncertainty on this figure, uncertainty resides only in the definition of the second.
- The meter, with its multiples (millimetre, kilometer), is very practical to measure the distances on , on the other hand for the astronomers, it is a little short and little adapted (since the astronomer observe practically only light). Indeed, , the star nearest to us, is with 380 000 000 meters of us.
, it nearest is with 150 000 000 000 meters. It is not very practical!!
With the principle (distance describes previously = C X lasted), one defines the light-year as being the distance which the light traverses in 1 year. Thus the Sun is not any more but to 8,32 minute-light of us. And the Moon with only a little more than 1 second-light. light-year is worth 10 000 000 000 000 000 of meters (10 million billion meters).
Light in practice
Monochromatic lights and polychromatic lights
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. If in more all the waves have the same phase, then the light is coherent: it is what occurs in one laser.
Measure light
As regards measurement of the light, it is important to define well what one speaks
- the unit of luminous flow is it lumen = .steradian. One electric bulb current (15 Watts low consumption or 75 Watts with traditional incandescence) produced approximately 1500 lumens.
- The international unit of light intensity is .
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 stars produce more energy than they do not receive any
- The Moon and more generally small celestial bodies (them planets and theirs satellites, them , them , etc.), produce less energy than they do not receive any. Certain giant planets (like Jupiter or ) produces a little more energy than they do not receive any, but not sufficiently to be visible. In both cases, these bodies are luminous by reflexion of the sunlight.
- is overheated by friction with the air and end up burning there, the two phenomena being source of light
Chemical lights
- Certain living organisms:fish, molluscs, fireflies and towards luisants, are the seat of producing chemical reactions of lights
- Intense heatings, therefore them combustions in general, it fire, them fire-merry, produce light:
quantum lights
- fluorescence, them laser, mercury discharge lamps or of sodium, them plasmas such as those produced by flashes in storms produce light resulting from quantum phenomena in the heart of the atoms: the excitation of the electrons ("optical pumping", which can be obtained by simple), then the de-energizing of its electrons, which turns over to a more usual energy level, lower, by emitting one (i.e. of the light).
other lights
sparks produces of intense friction, etc.
electric
Electric lights, sources of light of standard lamps, spots, headlights, lamp-torches, etc., can use a phenomenon of heating or a quantum phenomenon.
Human vision
to see ?il
Optical phenomena
Imagery
See too
- | Doublet (optical) | Lens | Optics | | | Stigmatism | slow light | Reflector | Optical fibre
- and more especially the chapter devoted to
External bonds
History of the discoveries on the site toutsurlaphysic.Fr
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