Optics

(Optics includes study of dispersion of light. )

Optics is the branch of electromagnetism that describes the behavior and properties of light and its interaction with matter (photometry). The perspective deals with what are called the optical phenomena, on the one hand to explain them and on the other to obtain experimental results that allow it to grow as a phenomenological and modeling discipline. There are three branches of optics: geometric optics, physical optics and quantum optics.

The optics usually study the behavior of radiation with the frequencies of the visible, infrared and ultraviolet; however, analogous phenomena are encountered in the frequencies of X-rays, microwaves, radio waves (or radiofrequencies) and other ranges of electromagnetic radiation. Optics, primarily and in the classical sense, can therefore be considered as a part of electromagnetism. Then there are optical phenomena that depend on the quantum nature of light and which require tools and results of quantum mechanics (photonics, quantum optics and microphotonics).

The optics, however, is a sector rather separate from the communities of physics. He owns his associations, his lectures and his own identity. The most strictly scientific aspects of the sector are often dropped under the terms of Optical Science or Optical Physics, while the applied optical studies are related to optical engineering. In addition, the applications of optical engineering to lighting systems are assigned to lighting engineering. Each of these disciplinary sectors tends to differentiate itself from others in applications, technical skills and professional registers.

Given the extensive interventions of light science in the applications of the real world, the area of ​​Optical Science and Optical Engineering presents marked interdisciplinary characteristics. There are numerous disciplinary areas in which strong influences and crucial contributions of the Optical Science meet: electrical engineering, physics, psychology, medicine, earth sciences, etc.

Introduction

Historically, optics appeared in ancient times and was developed by Muslim scholars including Persians. It is first geometric. Ibn al-Haytham (965-1039), a Persian scientist known to Westerners as Alhazen, is considered the father of modern optics, experimental physics, and the scientific method. A Latin translation of part of his work, the Optical Treatise, has had a great influence on Western science.

Geometric optics offers an analysis of light propagation based on simple principles: rectilinear propagation and inverse return. It was able to explain the phenomena of reflection and refraction. It was perfected until the eighteenth century, when the discovery of new phenomena, such as the deformation of light in the vicinity of obstacles or the splitting of light during the crossing of certain crystals, led it in the nineteenth century to development of the physical or wave optics.

Physical optics consider light as a wave; it takes into account the phenomena of interference, diffraction and polarization.

At the beginning of the 20th century, Einstein’s theories on the corpuscular nature of light will give rise to photon and quantum optics. Physicists are then forced to admit that light has both the properties of a wave and a particle. From there, Louis de Broglie considers, through wave mechanics, that if the photon can behave like a corpuscle, then, conversely, corpuscles such as electrons or protons can behave like waves.

Geometrical optics

(Geometry of reflection and refraction of light rays. )

The geometric optics introduced by Alhazen developed on the basis of simple observations and is based on two principles and empirical laws:

• rectilinear propagation in a homogeneous and isotropic medium;
• the inverse return principle, which expresses the reciprocity of the light path between source and destination;
• the laws of Snell-Descartes for reflection and refraction.

Problem solving is done using geometric constructions (lines materializing the rays, calculations of angles), hence the name of geometrical optics. It gives good results as long as one does not try to model phenomena related to polarization or interferences and that no dimension of the system is comparable to or less than the wavelength of the light used.

Geometric optics allows to recover almost all the results concerning the mirrors, the diopters and the lenses or their combinations in doublet and optical systems constituting in particular the optical instruments.

Moreover, in the context of the Gaussian approximation, geometric optics gives linear mathematical relations allowing the use of mathematical tools such as matrices and computerization of computer calculations.

Wave optics or physical optics

(When oil or fuel is spilled, colourful patterns are formed by thin-film interference.)

Whereas geometrical optics is a purely phenomenological optic and does not make any hypothesis about the nature of light, except possibly that it carries energy, wave optics (sometimes called “physical optics”) model the light by a wave.

The model of the scalar wave (Huygens-Fresnel principle) makes it possible to interpret diffraction phenomena (when passing through a hole, a narrow slot, near an edge, etc.) and interference. The calculations are then based on the sum of the amplitudes of sinusoidal waves which are superimposed, which sum, according to the phase shift, can lead to a zero result. The superposition of two beams can thus give darkness. This is observed in the dark areas of interference or diffraction patterns.

We must then consider that it is a transverse wave, if we want to interpret polarization phenomena. Finally, Maxwell will understand that light waves are only electromagnetic waves characterized by a range of wavelengths that makes them visible to humans.

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

This approximation consists of using the rays of geometrical optics to estimate the fields on a surface and then integrating these fields over the entire illuminated surface to determine the transmitted and reflected fields.

In the optical and radiofrequency domains, this approximation is used to calculate the effects of interference and polarization and to estimate the diffraction effects. Like all high frequency approximations, the approximation of physical optics becomes more relevant as one works with high frequencies.

Because of the assumption made about the electrical current density on the surface of an object, this approximation is all the more correct when the objects studied are large in front of the wavelength and with smooth surfaces. For the same reason, this approximate current density is inaccurate near discontinuities such as edges or boundaries between the illuminated area and the shadow areas.

Quantum optics

(Experiments such as this one with high-power lasers are part of the modern optics research.)

Problems related to blackbody radiation and the photoelectric effect led to the conclusion that light was composed of energy bundles (licht quanta, in German, according to Einstein).

Later, the Compton effect led to consider light as made up of particles in their own right: photons.

These are characterized by a zero mass, a velocity equal to c (celerity of light), an energy E = hν, where ν is the frequency of the associated electromagnetic wave, and a momentum p = ℏk with ℏ = h/2π where h denotes the Planck constant and k the wave vector.

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

With quantum optics we abandon all certainty, we reason only in terms of probabilities:

• probability of a photon being emitted or absorbed by an atom;
• probability that a photon emitted by an atom has a given energy;
• probability that a photon will disintegrate;
• …

Optics of charged particles

The optics of charged particles, electronic optics and ion optics, correspond to the image production using electron or ion beams refracted using electric and / or magnetic fields (electromagnetic lenses). Advantages: small wavelengths associated with these beams, which allow to obtain resolution powers much higher than those obtained with instruments using visible light.

Optics and biology

Light plays a fundamental role in Living, which has developed many ways to use it, even to produce it (bioluminescence).

Bio-optics uses light and its absorption by living to study the environments, aquatic in particular.

Ecologists and biologists study the way in which living organisms have learned during evolution to use and manipulate for their benefit the absorption of light (base of photosynthesis in particular, but also of vision), transparency, diffraction , interference, reflection and antireflection, diffusion, light, optical guidance, and the lens, dynamic camouflage (in the chameleon, octopus and cuttlefish in particular) or bioluminescence …

These “solutions” developed by living organisms to use light (or produce it via bioluminescence) are also of interest to biomimetics.