Polarization of light
Polarized light, polarizers and polarized lenses
The polarization of the light comes from the wave theory of light. Therefore, many applications like polarized glasses that are used to filter light, and see some movies in 3D, etc. Some insects and animals use the light polarization for orientation. Polarized light seems to also participate in the genesis of a phenomenon Visual entoptique called "Brush of Haidinger" ("Haidinger's brush). An entoptique phenomenon is induced by the eye itself (such as the floaters, which are related to the vitreous floaters). It teaches us that the human eye is also sensitive to the polarization of the light, even if this capacity is not being used any...
This page shows the main features that allow to understand the phenomenon of polarization of the light; the mathematical formalism is reduced to the maximum, in favor of a schematic representation for a graphic and intuitive understanding.
History: discovery and characterization of the polarization
Work on highlighting and the characterization of light waves and their polarization occupied many scientists over the 17e, 18e and 19e century.
Can be dated to 1669 the first mention of scientific evidence related to a phenomenon of polarization, which nature and the foundations were unknown at the time. Erasmus Bartholin, who was a mathematician and doctor Danish publishes a series of intriguing comments, which start from the observation that some of the images, seen through Crystal of Iceland spar, appear split: called a 'regular' image, and an image of "extraordinary." One of the images seems to turn when orients the Crystal in different positions, while the other remains fixed. When light passes through two pieces of crystals, one of the images can disappear according to certain directions of the second Crystal to the first Crystal, and rotation of a Crystal changes the intensity of one of the two images. A few years later, Christian Huygens is again studying this phenomenon, that it tries to explain the presence of 'secondary' wavelets, related to a double refraction (refractive bi), caused by the structure of the Crystal. It will collide with Newton, who was a supporter of a corpuscular theory of light.
Sir David Brewster, Scottish physicist, is credited with the discovery of the phenomenon of light polarization in early 19e century. 'S also more than a century after Newton's death, Etienne Louis Malus, officer under Bonaparte, it coincidentally on his return to Paris from the Egyptian campaign a discovery related to the observation of a reflection of the sunlight on the windows of the Palace of the Luxembourg, seen through the feldspar crystal. He notices that one of the two images disappears to a certain orientation of the Crystal, all 90 °. He understands that there is a link between the prior reflection of light on the windows ("glassy" reflection): the light of the Sun, after reflection on the glass of the window, could be in a condition similar to that seen after a first Crystal bi refrigent crossing. This phenomenon was related to the fact that reflection can cause the polarization of the light. The very nature of polarization remains unexplained.
Thomas Young will help clear up the mystery of the polarization: it suggests 1816 that the vibrations of light might be in a plan not only longitudinal with the spread (as for the sound), but at least partially perpendicular (transverse) to it. Thus, a particular crystal structure could "interfere" with the vibrations and select certain directions, explaining the appearance of double and offbeat images according to certain directions looking through some crystals. This theory, which puts the wavelike nature of light, there is strong opposition. James Clerk Maxwell, in establishing the theory of electromagnetic waves, will come to build forty more later this theory (1868).
Polarization of light.
Light is an electromagnetic wave and corresponds to the simultaneous spread of an electric field and a magnetic field, which oscillations are mutually perpendicular. The oscillations are in a plane perpendicular to the direction of propagation. Remember that in the case of the sound, the oscillations of the air molecules sought by changes in pressure caused by a sound wave are longitudinal, in the direction of propagation of the wave. Polarization match management adopt the oscillations of the electric field within the plane perpendicular to the direction of propagation. If this direction is more random (change constantly over very short time intervals), but is performed according to a management preferred, then the light is polarized.
If only one direction of oscillation remains, the polarization is said to be straight.
If the direction varies continuously by a circular loop in the clockwise (or counterclockwise), the wave is circularly polarized. When the direction and amplitude varies continuously, the polarization is elliptical.
Linearly polarized wave
The direction of the electric field is fixed, but the amplitude of the electric field is variable. The "tip" of the electric field vector sweeping space according to plan is parallel to the direction of propagation. We can break down a wave polarized linearly in a couple of components that are oriented orthogonally.
To understand certain phenomena related to the polarization, it is important to remember that we can consider that natural light, unpolarized, "behaves" like very quick succession in time (every 10-8 seconds) of both vertical and horizontal components in phase, but which the respective amplitudes change constantly, so as to describe each of the random directions taken by the successive wave trains.
Circularly polarized wave
The direction of oscillation is variable, but the oscillation amplitude is constant. The "tip" of the electric field vector is sweeping a circle in clockwise or counterclockwise. We can also break down a circular polarized wave into two components associated with linear phase has a particular value.
Elliptical polarized wave
The direction and amplitude are variable oscillation of the Visual field vary continuously, and the tip of the electric field vector sweeps an ellipse. We can also break down an elliptical polarized wave into two linear orthogonal components, which the phase shift is a value in a given interval depending on the direction of rotation of the tip of the vector field.
In all cases, an elliptical (or circular) polarized wave can be understood as the sum of two components that each corresponds to a linearly polarized wave.
As well, we show each other that polarized wave linear is the sum of two circularly polarized optical disturbances, but with an opposite direction (right and left).
The natural light, which the direction of the oscillating electric field varies randomly, can every moment being assimilated to the superposition of two waves orthogonal polarized linearly, but of different phase (one speaks of inconsistency as opposed to the) light coherence be observed with the generation of a laser light): this phase shift varies constantly over time, and this precisely explains the variations for the direction of the electric field.
Should retain information at this stage, it is that whatever the direction of oscillation of the electric field at a point (for a short period), We can always break it down into two orthogonal components (ex: vertical and horizontal). This vector design is convenient to capture some natural phenomena or giving rise to common applications. She can seem relatively abstract, but she still finds a match in Photonic optics with the States of spins (angular time) of the photons: a beam of polarized light circularly right carries only photons of 'spin right': a linearly polarized light beam carries photons which the probability of being in a State of spin 'right' is equal to that to be in a State of spin 'left '.
Natural light is unpolarized, but some 'natural' phenomena can induce polarization at least partially of light: the diffusion of sunlight in the atmosphere is accompanied by a partial polarization. The development of "filters" polarizers, able to polarize light goes back to the 1930s.
When the polarization is straight (one direction), the vector component according to the perpendicular direction (always in the plane of oscillation) is zero. You can imagine (and get in practice) 'extinction' total of this polarized light by placing a second polarizer whose axis is perpendicular to this direction. This is the "work" the so-called 'polarized' sunglasses, and those used for the visualization of films in relief (3D). If you put glasses polarized along different lines in front of the right eye and the left eye, we can make them see different pictures, making sure of course carefully align the polarization of these images with respect to that of polarizers of the right and left eye.
Remember that non polarized light is light whose electric field oscillates without preferred direction: it is emitted by sources such as the Sun, lamps, etc. The atoms of these sources emit trains of successive waves which are polarized, but on very short periods (on the order of 10-8 seconds): as a result, the average orientation of the vibration of the field has no preferred orientation: natural light, however, can be broken down into two linearly polarized orthogonal components (ex: vertical and horizontal), whose the phase shift varies over time.
It is therefore easy to understand that the polarized light can be obtained from unpolarized light: the polarization occurs when some components are "absorbed", and there are four physical phenomena able to produce light from unpolarized light polarized: the Dichroism (differential absorption by a filter), the broadcast (interaction of light with fine particles), the reflection (the light reflects off a surface and is polarised when reflecting), and the birefringence (the material has different properties according to the direction taken by the light, ex: feldspar Crystal).
We will especially emphasize the first three phenomena.
Polarization by reflection justifies the existence of so-called polarized filter glasses. Indeed, the reflection of light unpolarized on certain materials produces a polarization of the incident light.
Polarization by reflection
This type of bias is common, because it is generated by the simple reflection of light on a reflective surface: glass, water, snow... To understand its mechanism in an intuitive way, back to basic concepts about the interaction between an electric plane wave and a molecule. These concepts also enable to understand polarization by diffusion.
The vibration of an atom, that can be assimilated to an electric dipole (kernel: positive charge, electrons: negative charge) is in the direction of vibration electric field, i.e. in the direction perpendicular to the propagation of the light wave (a bit like a Cork on water, which rises and falls during the passage of a wave). The dipole creates a secondary variation of the electric field in the nearest directions from the spread, but not in the direction of vibration. The vibration direction is perpendicular to the direction of propagation.
Non polarized incident light reflected from a surface as a water body is unpolarized. It can therefore be seen as the sum of two parts (vertical and horizontal) whose amplitudes constantly fluctuate over time. L direction taken by the refracted Ray follows from the laws of refraction)law of Snell-Descartes). The angle of the reflected ray is equal to the angle of incidence.
When the direction of the light reflected by the surface is perpendicular to the one taken by the refracted Ray, the vertical component of the reflected wave is "off", and only the horizontal component is reflected. This may be deduced that the direction of vibration of the oscillating dipoles close to the surface at the point of reflection is exactly perpendicular to the direction of the reflected ray. The angle for which the thoughtful vertical component is zero is called Brewster angle. Sir David Brewster (1781-1868) was the first physicist to have discovered the phenomenon of polarization, and the inventor of the kaleidoscope.
In so-called impact of draw, the reflected light is completely polarized, the polarization direction is perpendicular to the plane of incidence. The Brewster angle depends on the refractive index of the material reflecting (n). In the air, the relationship that connects the angle of Brewster IB index n is:
Tangent (IB) = n
For a (n = 1, 50) air/glass interface, IB = 56.3 °
For a (n = 1, 33) air/water interface, IB = 53.1 °
The reflections of the light of the Sun on water, snow, are (for a close angle of angle of Brewster, 53 ° for water) are polarized parallel to the horizontal reflecting surface. A polarizer whose transmission is vertical (ex: sunglasses with so-called polarizing lenses) presents interest to remove a large part of the reflected light. The ability to 'cut' or 'transmit' selectively certain directions is specific to certain materials natural or artificial; they present a so-called "Dichroism" property, which allows them to absorb the electric induction of a light wave field more strongly in some directions than in any other.
Some natural materials, so-called anisotropic, to absorb a vector component of the electric field E. For example, if the material has a polymeric structure oriented to a particular direction, you can stipulate that the direction of the chains of the polymer behaves like a 'wire', that dissipates the vibration of the electric field of the wave in that direction.
The first films polaroids, which were the first artificial polarisateurs developed in 1938 by Edwin Land, who founded the company Polaroid corporation at the same time. Best known to the general public for his cameras to Flash development (which the update was later, to the post-war), the products designed by the Polaroid corporation were polarizers which fitted the viewfinders of arms and goggles of the American soldiers: Land had chosen to use quinine (in the form of crystals of iodosulfate of quinine), which was equipped with dichroic properties , but whose supply difficulties lead it to develop in the artificial synthesis, which constituted a step forward in pharmacology polaroids films consisted of a material on which a film of stretched polymers was filed, referred by mechanical stretching or the action of a magnetic field.
When a certain thickness of the anisotropic material is crossed by light, not polarized, he eventually "turn off" (by Joule effect) the vibration of the electric field when it occurs in the direction unique to the anisotropic organization of the material (this occurs randomly along the filaments -). In the opposite direction, there is no possibility of conduction, and the vector component of the electric field is unchanged. The direction of rectilinear oscillation of the electric field at the exit of a polarizer is called"management" feature of the polarizer.
The schematic representation of a polarizer does call for a "grid" in interstices, which takes up the orientation of the axes molecular filaments of the dichroic substance that dissipate the component of the electric field that is parallel to them. The light that comes out of such a polarizer is not polarized in the direction of the interstices, but perpendicular to these.
We can use a linear polarizer to determine the direction of polarization of polarized incident light.
Partial mitigation of vibration according to certain directions results in an overall reduction in light intensity, which goes hand in hand with the polarization of the light. In the case of a straight polarizer receiving unpolarized light, theoretical mitigation of the intensity of the incident light is equal to 50% (in practice mitigation is higher mainly due to the phenomena of reflection on the polarizer, etc.).
We understand intuitively that if an already polarized light of a straight line passes through a material with the property of Dichroism, the reduction will be total if the orientation of the "polaroid" is accomplished according to a particular direction. By rotating the polaroid, if one observes a total extinction of the transmitted light, so we can deduce that the light is polarised in a straight line, and that the direction of this polarization is perpendicular to the direction characteristic of the polarizer.
If a polarized light rectilinear way (by a first polarizer) and intensity Ip through a second straight polarizer (called parser), only the vector component of the electric field located in the axis of the second polarizer passes through: this component has standard Ip x cos A where A is the angle between the direction of oscillation after crossing the first polarizer and the second polarizer. The resulting intensity of the crossing of the second polarizer AI is proportional to the square of the amplitude of oscillation, so AI = Ip (cos A) x 2. When A = 90 °, the transmitted intensity is zero.
The "polarizing" so-called solar glasses act as "filters" that cut the light polarized by reflection (see below). The orientation of the polarizer (characteristic axis) must be done in a sensible axis of reflection. In the case of solar glasses, the transmission axis is vertical, because the light from the glare on the ground (snow, water) is horizontally polarized by reflection. Polarized lenses reduce the light intensity of a no polarized for at least 50% incident light.
A polarizer can be used to modulate a beam of light energy femtosecond laser infrared (modulation can usually reduce the energy of each impact when we reduced the distance between consecutive impacts and vice versa).
Polarization by diffusion.
It is a more complex phenomenon that can highlight the sky light is partially polarized. through a polarizer oriented horizontally, the light of heaven observed 90 ° of a sunset is very muted: indeed, the gathered light is then polarized preferentially in a vertical direction.
The sun light scattering in the atmosphere devoid of clouds is related to dissemination by the air molecules whose dimensions are far below those of the wavelength of the light (the molecules are at least ten times smaller than the light wavelength: these conditions of the so-called Rayleigh scattering). The molecule behaves like a dipole oscillating, vibrating under the action (in the direction) of the incident electric field: it transmits the vibration in all directions, except one located in the vibration of the dipole axis. Is the spread in all directions around the particle.
The intensity of the light scattered in the conditions of Rayleigh depends on its wavelength; It is all the more important that the wavelength is short (ʎ effect4). Blue (430 nm) is 1.6 times shorter than red (700 nm): thus blue is broadcast more than 7 times more than red. This scattering in all directions of the short wavelengths explains the blue color of the sky. The sky is blue for reasons related to the greater dissemination of blue radiation.
Clouds are white because they broadcast in the same way the different wavelengths: the water droplets that constitute were much larger than the wavelength of the radiation of the visible spectrum: broadcasting is then made in the conditions say crumb, which is not sensitive to the wave length, and mainly forward, in the direction of initial propagation. The color of the clouds is composed of a homogeneous diffusion of the colourful radiation in the visible spectrum, and these clouds are so white.
At sunset, the light passes through a layer of thick atmosphere, which depleted even more blue light reaching the eye of an observer. Light takes an orange tint, and the clouds also adopt a reddish tint.
Bias related to broadcasting can be understood as the result of the vibration of the electronic clouds of the atoms involved in the broadcast. Sinusoidal plane wave oscillates at the frequency and in the direction of vibration of the incident electric field. The atom or molecule is polarised and behaves like an oscillating dipole which will radiate a variable power according to the direction: maximum in the direction perpendicular to the dipole, it sucks in the axis of it (in the axis of the vibration).
Take back the principle of decomposition into orthogonal components of the incident light, and observe what happens to each of the components during the broadcast by a molecule (example: air molecule). This will allow to understand the effect of the angle of diffusion on the direction of polarization.
Relatively complex diagrams to understand the effect of polarization of solar light by diffusion into the upper atmosphere; at sunset, the polarization is vertical to the horizon, in particular at 90 ° from the position of the Sun, because unlike this one (ex: to the East when the sun goes down), the polarization is horizontal.
The use of polarization filters in photography is based on some components of the atmospheric light filtering: This allows to increase the contrast between clouds and the sky (clouded by "break" of a polarized component). The orientation of the filter allows to modulate this type of effect, since the light from the sky is partially polarized.
This entoptique (induced by the eye) is the only Visual phenomena induced by the polarization of light 'in situ', this at the level of the retina. It was discovered by an Austrian gemologist (Wilhelm K. von Haidinger's: 1795-1871). It is related to a particular orientation of certain molecules (lutein, pigment xanthophylls) which is a substance whose molecular chains have dichroic property. Under certain conditions, which require the receipt by the eye of an at least partially polarized light, we can see a very discreet pattern resembling a yellowish "Hourglass", whose direction depends on the direction of polarization of the incident light wave (it is perpendicular). The use of an LCD screen (for example, the computer on which you are looking at this page?) is convenient because this type of screen (containing a matrix of LCD) polarizes light, usually in an oblique direction. Must watch the sky, toward the horizon, at 90 ° from the Sun if possible, since we have seen that light adopted a vertical partial polarization direction. Thanks to this phenomenon, we can say that the human eye is sensitive (slightly!) to the polarization of the light!
These molecules must have a layout-oriented, at least relative to create polarization, and thus produce a spatial variation of the light intensity received. An explanation is represented diagrammatically below:
-Houard S. optics, experimental and practical approach. Oxford University Press, 2011 - Chapter 9: polarization of light
-Hecht E. Optics, Fourth Edition. New Pearson international edition. 2013 Chapter 8: Polarization