Photon Totally Explained

Everything about Photons totally explained

In physics, the photon is the elementary particle responsible for electromagnetic phenomena. It is the carrier of electromagnetic radiation of all wavelengths, including gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. The photon differs from many other elementary particles, such as the electron and the quark, in that it has zero rest mass;
   Apart from having energy, a photon also carries momentum and has a polarization. It follows the laws of quantum mechanics, which means that often these properties don't have a well-defined value for a given photon. Rather, they're defined as a probability to measure a certain polarization, position, or momentum. For example, although a photon can excite a single molecule, it's often impossible to predict beforehand which molecule will be excited.
   The above description of a photon as a carrier of electromagnetic radiation is commonly used by physicists. However, in theoretical physics, a photon can be considered as a mediator for any type of electromagnetic interactions, including magnetic fields and electrostatic repulsion between like charges.
   The modern concept of the photon was developed gradually (1905–17) by Albert Einstein to explain experimental observations that didn't fit the classical wave model of light. In particular, the photon model accounted for the frequency dependence of light's energy, and explained the ability of matter and radiation to be in thermal equilibrium. Other physicists sought to explain these anomalous observations by semiclassical models, in which light is still described by Maxwell's equations, but the material objects that emit and absorb light are quantized. Although these semiclassical models contributed to the development of quantum mechanics, further experiments proved Einstein's hypothesis that light itself is quantized; the quanta of light are photons.
   The photon concept has led to momentous advances in experimental and theoretical physics, such as lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. According to the Standard Model of particle physics, photons are responsible for producing all electric and magnetic fields, and are themselves the product of requiring that physical laws have a certain symmetry at every point in spacetime. The intrinsic properties of photons—such as charge, mass and spin—are determined by the properties of this gauge symmetry.
   The concept of photons is applied to many areas such as photochemistry, high-resolution microscopy, and measurements of molecular distances. Recently, photons have been studied as elements of quantum computers and for sophisticated applications in optical communication such as quantum cryptography.

Nomenclature

The photon was originally called a “light quantum” (das Lichtquant) by Albert Einstein. in which photons were “uncreatable and indestructible”. Although Lewis' theory was never accepted—being contradicted by many experiments—his new name, photon, was adopted immediately by most physicists. Isaac Asimov credits Arthur Compton with defining quanta of light as photons in 1927.
In physics, a photon is usually denoted by the symbol

gamma

, the Greek letter gamma. This symbol for the photon probably derives from gamma rays, which were discovered and named in 1900 by Villard and shown to be a form of electromagnetic radiation in 1914 by Rutherford and Andrade. In chemistry and optical engineering, photons are usually symbolized by

h 
u

, the energy of a photon, where

h

is Planck's constant and the Greek letter

u

(nu) is the photon's frequency. Much less commonly, the photon can be symbolized by hf, where its frequency is denoted by f.

Physical properties

massless, has no electric charge and doesn't decay spontaneously in empty space. A photon has two possible polarization states and is described by exactly three continuous parameters: the components of its wave vector, which determine its wavelength

lambda

and its direction of propagation. The photon is the gauge boson for electromagnetism, and therefore all other quantum numbers—such as lepton number, baryon number, or strangeness—are exactly zero.
Photons are emitted in many natural processes, for example, when a charge is accelerated, during a molecular, atomic or nuclear transition to a lower energy level, or when a particle and its antiparticle are annihilated. Photons are absorbed in the time-reversed processes which correspond to those mentioned above: for example, in the production of particle–antiparticle pairs or in molecular, atomic or nuclear transitions to a higher energy level.
In empty space, the photon moves at

c

(the speed of light) and its energy

E

and momentum p are related by

E = c p

, where

p

is the magnitude of the momentum. For comparison, the corresponding equation for particles with a mass

m

E^

where, as above,

E

and

p

are the polariton's energy and momentum magnitude, and

omega

and

k

are its angular frequency and wave number, respectively. In some cases, the dispersion can result in extremely slow speeds of light in matter. The effects of photon interactions with other quasi-particles may be observed directly in Raman scattering and Brillouin scattering.
Photons can also be absorbed by nuclei, atoms or molecules, provoking transitions between their energy levels. A classic example is the molecular transition of retinal (C₂₀H₂₈O, Figure at right), which is responsible for vision, as discovered in 1958 by Nobel laureate biochemist George Wald and co-workers. As shown here, the absorption provokes a cis-trans isomerization that, in combination with other such transitions, is transduced into nerve impulses. The absorption of photons can even break chemical bonds, as in the photodissociation of chlorine; this is the subject of photochemistry.

Technological applications

Photons have many applications in technology. These examples are chosen to illustrate applications of photons per se, rather than general optical devices such as lenses, etc. that could operate under a classical theory of light. The laser is an extremely important application and is discussed above under stimulated emission.
Individual photons can be detected by several methods. The classic photomultiplier tube exploits the photoelectric effect: a photon landing on a metal plate ejects an electron, initiating an ever-amplifying avalanche of electrons. Charge-coupled device chips use a similar effect in semiconductors: an incident photon generates a charge on a microscopic capacitor that can be detected. Other detectors such as Geiger counters use the ability of photons to ionize gas molecules, causing a detectable change in conductivity.
Planck's energy formula

E=h
u

is often used by engineers and chemists in design, both to compute the change in energy resulting from a photon absorption and to predict the frequency of the light emitted for a given energy transition. For example, the emission spectrum of a fluorescent light bulb can be designed using gas molecules with different electronic energy levels and adjusting the typical energy with which an electron hits the gas molecules within the bulb.
Under some conditions, an energy transition can be excited by two photons that individually would be insufficient. This allows for higher resolution microscopy, because the sample absorbs energy only in the region where two beams of different colors overlap significantly, which can be made much smaller than the excitation volume of a single beam (see two-photon excitation microscopy). Moreover, these photons cause less damage to the sample, since they're of lower energy.
In some cases, two energy transitions can be coupled so that, as one system absorbs a photon, another nearby system "steals" its energy and re-emits a photon of a different frequency. This is the basis of fluorescence resonance energy transfer, which is used to measure molecular distances.

Recent research

The fundamental nature of the photon is believed to be understood theoretically; the prevailing Standard Model predicts that the photon is a gauge boson of spin 1, without mass and without charge, that results from a local U(1) gauge symmetry and mediates the electromagnetic interaction. However, physicists continue to check for discrepancies between experiment and the Standard Model predictions, in the hope of finding clues to physics beyond the Standard Model. In particular, experimental physicists continue to set ever better upper limits on the charge and mass of the photon; a non-zero value for either parameter would be a serious violation of the Standard Model. However, all experimental data hitherto are consistent with the photon having zero charge The best universally accepted upper limits on the photon charge and mass are 5×10⁻⁵² C (or 3×10⁻³³ times the elementary charge) and 1.1×10⁻⁵² kg (6x10^-17 eV), respectively .
Much research has been devoted to applications of photons in the field of quantum optics. Photons seem well-suited to be elements of an ultra-fast quantum computer, and the quantum entanglement of photons is a focus of research. Nonlinear optical processes are another active research area, with topics such as two-photon absorption, self-phase modulation and optical parametric oscillators. However, such processes generally don't require the assumption of photons per se; they may often be modeled by treating atoms as nonlinear oscillators. The nonlinear process of spontaneous parametric down conversion is often used to produce single-photon states. Finally, photons are essential in some aspects of optical communication, especially for quantum cryptography.

Further Information

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