Electromagnetic wave

The electromagnetic wave is a model used to represent the electromagnetic radiations. It is associated with the concept of Photon.

It is advisable to distinguish well:

• the electromagnetic radiation, which are the studied phenomenon, and
• the electromagnetic wave, which is one of the representations of the phenomenon.

A light wave is an electromagnetic wave whose wavelength corresponds to the visible spectrum, either between 380 and 780 Nm or 1.5 to 3 eV.

Description

Like all the Wave S, an electromagnetic wave can be analyzed by using the spectral Analyze; one can break up the wave into waves known as “monochromatic” (see also Specter of plane waves ).

A monochromatic electromagnetic wave can be modelled by a vibrating Dipôle, this suitably reflecting model, for example, the oscillations of the electronic cloud of an atom intervening in the Diffusion Rayleigh (model of the electron elastically dependant).

The variations of the fields electric and magnetic are bound by the Maxwell's equations, one can thus represent the wave by only one of these fields, in general the electric field. One can then write the general equation of a monochromatic plane wave :

$\backslash \; vec\; \{E\}\; (\backslash \; vec\; \{R\},\; T)\; =\; \backslash \; cos\; (\backslash \; Omega\; T\; -\; \backslash \; vec\; \{K\}\; \backslash \; cdot\; \backslash \; vec\; \{R\}\; +\; \backslash \; varphi)\; \backslash \; cdot\; \backslash \; vec\; \{E\}\; \_0$

where

• $\backslash \; vec\; \{R\}$ is the vector position of the point considered,
• $\backslash \; vec\; \{K\}$ is the Vecteur of wave whose standard is worth 2π/λ, λ being the Wavelength;
• φ is the phase in the beginning.

One also frequently uses the complex form :

$\backslash \; vec\; \{E\}\; (\backslash \; vec\; \{R\},\; T)\; =\; e^\; \{I\; \backslash \; cdot\; (\backslash \; Omega\; T\; -\; \backslash \; vec\; \{K\}\; \backslash \; cdot\; \backslash \; vec\; \{R\}\; +\; \backslash \; varphi)\}\; \backslash \; cdot\; \backslash \; vec\; \{E\}\; \_0$

One will then obtain the sizes physical, real, by taking the real part of this complex form.

Properties

Polarization

Polarization corresponds to the direction and the amplitude of the electric field $\backslash \; vec\; \{E\}$. For a wave nonpolarized, or natural, $\backslash \; vec\; \{E\}$ turns around its axis in a random and unforeseeable way during time. To polarize a wave corresponds to give a trajectory defined in the electric field. There are several kinds of polarization:

• linear polarization when $\backslash \; vec\; \{E\}$ always remains in the same plan.
• circular polarization, the magnetic field turns around its axis by forming a circle.
• elliptic polarization, the magnetic field turns around its axis and changes amplitude to form an ellipse.

Undulatory behavior

; Propagation

In a homogeneous medium and Isotropic, the electromagnetic wave is propagated in straight line. At the time of the meeting with an obstacle, there is diffraction; during a change of medium, there is reflection and refraction, there is also refraction if the properties of the medium change according to the place (heterogeneity). See also Principle of Huygens-Fresnel .

; Reflection

During a change of propagation medium, part of the electromagnetic wave sets out again about the middle of origin, it is the reflection.

the most known case of the reflection is the Miroir, but this one also relates to x-rays (Miroir with x-rays) and the waves radio: reflection on the ionosphere of the waves megahertz, parabolic Aerial, reflection on the Moon…

; Refraction

During a change of propagation medium, if the second medium is transparent for the wave, this one proprage in the second medium but with a different direction. That of course relates to the light (optical Lentille, Mirage), but also the waves radio (refraction of the decametric waves in the ionosphere).

; Diffusion

When a wave meets an atom, it is diffused on this one, it changes direction. One distinguishes the Diffusion Rayleigh, known as “electronic diffusion”, during which the wave does not change a wavelength, the Raman Diffusion which is an electronic diffusion with reduction or increase wavelength, and the Diffusion Compton, in the case of x-rays diffusing on light atoms, during which the wavelength increases.

; Interference S

Like all the waves, the electromagnetic waves can interfere. In the case of the Radiocommunication S, that causes an interference of the signal (see also Rapport signal on noise ).

; Diffraction

the interference of diffused waves bears the name of diffraction:

* Theory of diffraction;

* Diffraction by a slit;

* Slits of Young;

* Diffraction pattern;

* Diffraction of x-rays;

* reciprocal Space.

; Flux of energy

the flow of energy through a surface is given by the flow of the Vecteur of Poynting.

Duality wave-corpuscle

The concept of wave electromagnetic is complementary to that of photon. In fact, the wave provides a more relevant description of radiation for the weak frequencies (i.e. big wavelengths) like the radio waves.

In fact, the electromagnetic wave represents two things:

• macroscopic variation of the Electric field and the Magnetic field;
• the Fonction of wave of the photon, i.e. the standard with the square of the wave is the probability of presence of a photon.

When the flow of energy is large in front of the energy of the photons, one can consider that one has a quasi-continuous flow of photons, and the two concepts are recovered. This is not true any more when the flow of energy is weak (one sends the photons one by one), the concept of “macroscopic variation” (average) then does not have any more a direction.

The Flux of energy is given by the Vecteur of Poynting. Each photon “carries” a quantity of determined energy, being worth E = H ·ν, H being the Constant of Planck and ν the frequency. One can thus calculate the flow of photons through a surface.

History

The undulatory theory of the light was mainly developed by Christiaan Huygens in the Années 1670, and by Augustin Fresnel. She was opposed at the time to the corpuscular theory, defended mainly by Rene Descartes. Huygens worked mainly on the laws of reflection and of Réfraction, Fresnel developed in particular the concepts of Interférence and Wavelength. The undulatory and corpuscular approaches were joined together by Albert Einstein when this one establishes the model of the Photon in 1905, in his work on the photoelectric Effet.

The large theoretical projection was the synthesis of the laws of electromagnetism by James Clerk Maxwell. The Maxwell's equations predicted the speed of the electromagnetic waves, and Speed of light measures it showed that the light was of nature electromagnetic.

The waves radio, low frequency and big wavelength, were discovered at the end of the 19th century with work in particular of Alexandre Popov, Heinrich Hertz, Edouard Branly and of Nikola Tesla. The X-rays, high frequency and low wavelength, were discovered by Wilhelm Röntgen in 1895.

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