The optical is the branch of the physics which treats Lumière and of its relationships to the vision.

Theories of optics

Introduction

Historically, the optics, appeared as of Antiquity, initially was geometrical.
The geometrical Optique proposes an analysis of the light propagation based on simple principles: rectilinear propagation, the reflection and the Refraction. It improved until the 18th century, where the discovery of new phenomena such as the deformation of the light in the vicinity of obstacles or the unfolding of the light at the time of the crossing of certain crystals led to the 19th century with the development of physical or undulatory optics.
The undulatory Optique regards the light as a Onde, it takes into account the phenomena of Interférence, Diffraction and polarization.

At the beginning of the 20th century the theories of Einstein on the corpuscular nature of the light will give rise to the photon and to the Quantum optic. The physicists are constrained to admit that the light presents at the same time the properties of a wave and a corpuscle. From there, Louis de Broglie considers, through the Wave mechanics , that if the photon can behave like a corpuscle, contrary, the corpuscles such as the electrons or the protons can behave like waves.

Geometrical optics

The geometrical Optique developed on the basis of simple observation and rests on two principles and of the empirical laws:

  • the rectilinear propagation in a homogeneous and isotropic medium

  • the principle of the return reverses which expresses the reciprocity of the luminous way between source and destination.
  • the Laws of Snell-Descartes for the reflection and the Refraction

The resolution of the problems is done using geometrical constructions (layouts of right-hand sides materializing the rays, calculations of angles), from where the geometrical name of optics. It gives good performances as long as one does not seek to model phenomena related on polarization or the interferences and that no dimension of the system is comparable or lower than the Wavelength of the light used.

Geometrical optics makes it possible to find the near total of the results concerning the mirrors, the Dioptre S and the lens S or their combinations in doublet and optical systems in particular constituting the instruments optical.

Moreover, within the framework of the Approximation of Gauss, geometrical optics gives mathematical relations Linéaire allowing the use of mathematical tools such as the matrix S and the systematization of calculations by computer.

Undulatory or optical optics physical

Whereas geometrical optics is a purely phenomenologic optics and does not make an assumption on the nature of the light, except possibly which it transports of energy, the undulatory Optique (sometimes called “physical optics”) models the light by a wave.

The model of the scalar wave (Principle of Huygens-Fresnel) makes it possible to interpret the phenomena of diffraction (at the time of the passage by a hole, a narrow slit, close to an edge…) and of interferences. Calculations rest then on the sum of the amplitudes of sinusoidal waves which are superimposed, nap which, according to dephasing, can lead to a null result. The superposition of two beams can thus give the darkness. It is what one observes on the level of the dark zones of the figures of Interférence or of Diffraction.

It should then be considered that it is about a transverse wave if one wants to interpret the phenomena of polarization. Lastly, Maxwell will allow to understand that the light waves are only electromagnetic Ondes characterized by a field wavelengths which makes them visible for the man.

Physical optics is the name of an approximation high frequency (small wavelength) usually used in optics, physics applied or electric engineering. In these contexts, it is an intermediate method between the geometrical optics, which is unaware of the undulatory effects and the undulatory optics which is an exact physical theory.

This approximation consists in using the rays of geometrical optics to estimate the fields on a surface then to integrate these fields on all surface lit to determine the transmitted fields and considered.

In the optical fields and radio frequencies, this approximation is used to calculate the effects of Interférence S, polarization and to estimate the effects of Diffraction. Like all the approximations high frequencies, the approximation of physical optics gains in relevance as one works with high frequencies.

The method generally consists in approaching the surface density of electric current \ vec {J} on the surface of an object by the density of current \ vec {J} _ {COp} induced by the incidental magnetic field \ vec {H} _i, as it is the case on an infinite metal level. Therefore the approximation of physical optics is sometimes called “assumption of the tangent plan”.

The density of electric current at the point Q of enlightened surface is then calculated by the relation:

\ vec {J} _ {COp} (Q) = 2 \ hat {N} (Q) \ times \ vec {H} _i (Q) where \ hat {N} (Q) corresponds to the unit normal vector external to enlightened surface. In the remote regions (not-enlightened surface according to the assumption of geometrical optics), the density of current regarded as null. The fields radiated by surface are then calculated by integrating the density of electric current on the surface lit with integral expressions of the Maxwell's equations, for example the integral equations of Stratton-Chu, Kottler or Franz.

Because of the assumption carried out on the density of electric current on the surface of an object, this approximation is all the more correct when the studied objects are large in front of the wavelength and smooth surface. For the same reason, this approximate density of current is inaccurate near discontinuities like edges or the borders between the enlightened zone and the remote regions. Moreover, this approximation does not give an account of the crawling Ondes.

Quantum optic

The problems involved in the radiation of the black Body, to the photoelectric Effet brought to consider that the light was made up of packages of energy ( licht quanta , in German, according to Einstein).
Later, the Effect Compton resulted in regarding the light as made up of particles with whole share: the Photons.

Those are characterized by a null Masse, a Speed equal to C (Célérité of the light), a energy E=h \ nu, where \ nu is its Fréquence, and a Quantité of movement p= \ hbar k with \ hbar=h/2 \ pi with H the Constante of Planck and K the Vecteur of wave.

The quantum theory of optics or Quantum optic was created to reconcile the two apparently incompatible aspects of the light, the undulatory aspect (phenomena of interference, of diffraction…) and the corpuscular aspect (photoelectric effect, spontaneous emission…). The quantum optic is primarily a reformulation of the undulatory optics in which the electromagnetic field is quantified.

With the quantum optic one gives up any certainty, one reasons only in term of probabilities:

  • probability that a photon is emitted or absorbed by an atom;
  • probability that a photon emitted by an atom has a given energy;
  • probability that a photon disintegrates;
  • ….

Fields of optics

See too

Related topics

External bonds

  • multiple Anamorphosis 3D animated optical Illusions.
  • Glossary of optics illustrated Glossary of the terms of optics.

Beats-smg: Optėka Simple: Optics

Random links:Abu Said Uthman | Pierre Dupuis | Charaxes solon | Small Koulaye | Animals of Miocène

© 2007-2008 speedlook.com; article text available under the terms of GFDL, from fr.wikipedia.org