Information AboutPhotons |
| CATEGORIES ABOUT PHOTON | |
| fundamental physics concepts | |
| bosons | |
| electromagnetism | |
| optics | |
| quantum electrodynamics | |
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In Quantum Physics , the photon (from Greek φως, "phōs", meaning ''light'') is the Quantum of the Electromagnetic Field ( Light ). That is to say, electromagnetic fields are made up of large numbers of photons, and the Electromagnetic Interaction is mediated by the exchange of Virtual photons. The term photon was coined by Gilbert Lewis in 1926. The photon is one of the Elementary Particles . Its interactions with Electron s and Atomic Nuclei account for a great many of the features of matter, such as the existence and stability of Atom s, Molecule s, and Solid s. These interactions are studied in Quantum Electrodynamics (QED), which is the oldest part of the Standard Model of Particle Physics . In many circumstances, a photon acts as a classical particle, for instance when registered by the light-sensitive device in a Camera . In other circumstances, a photon acts like a classical Wave , as when passing through the Optics in a camera. According to the so-called Wave-particle Duality of Quantum Physics , it is natural for the photon to display either aspect of its nature, according to the circumstances. Normally, light is formed from a large number of photons, with the Intensity related to the number of them. At low intensity, it requires very sensitive instruments, used in Astronomy or Spectroscopy , for instance, to detect the individual photons. In many cases, people may say a photon "sometimes acts like a wave and sometimes acts like a particle". This is slightly misleading, because a photon ''always'' acts like ''both''. For example, when shooting single photons through a Slit , a detector can detect each photon when it hits — but over time, the detector will detect the same Diffraction Pattern as it would if the photons were given off all in one burst. This is because any given Trajectory the photon could take has a certain probability that is dictated by the properties of an electromagnetic wave. SYMBOL A photon is usually given the symbol γ, the Greek Letter Gamma , although in Nuclear Physics this symbol refers to a very high-energy photon (a Gamma Ray ). PROPERTIES Photons are commonly associated with , Frequency , Energy , or Momentum , and that light or fields interact with matter in discrete units of one or several photons. Photons are Fundamental Particle s. They can be created and destroyed when interacting with other particles, but do not decay. A photon of a definite frequency is not a localized particle. Photons thus exhibit a position-frequency Uncertainty Relation similar to that of matter particles and exactly analogous to the Bandwidth Theorem of Classical Optics . According to the Quantum Electrodynamics of the Standard Model , photons have zero Rest Mass and zero Electric Charge , but they do carry Energy , Momentum and Angular Momentum . Although the photon is generally accepted to be massless, experiments may only show that its mass is consistent with zero. A conservative upper limit for the mass of the photon, given by the [http://pdg.lbl.gov/ Particle Data Group], is 6×10-17 EV , based on changes in Magnetohydrodynamics which would contradict Solar Wind observations. As massless particles, photons must always move at ''c'' (often called the Speed Of Light In Vacuum ). Because of Special Relativity , photons always move at a constant speed with respect to all observers, regardless of the observers' own velocities. The energy and momentum carried by a photon is proportional to its frequency (or inversely proportional to its wavelength) with a constant of proportionality equal to the Planck Constant . The momentum carried by a photon can be transferred when it interacts with matter. The force due to photons interacting with a surface is called Radiation Pressure , which may be used for propulsion as with a Solar Sail . Photons are deflected by a Gravitational Field twice as much as Newtonian mechanics predicts for a Test Mass traveling at the speed of light. This observation was a key piece of early evidence supporting Einstein's theory of Gravitation , General Relativity . In general relativity, photons (as well as any other object in a free fall) always travel in a "straight" line, taking into account the Curvature of Spacetime . (In curved space, such lines are called Geodesic s). Creation Photons are produced by Atom s when a bound Electron moves from one Orbital to another orbital that has less energy or when a free electron becomes bound by an atom. Photons can also be emitted by an unstable Nucleus when it undergoes some types of Nuclear Decay , producing Gamma Radiation . Furthermore, photons are produced whenever Charged Particle s are Accelerated . Atoms continuously emit photons due to their Collision s and interactions with each other. The Wavelength Distribution of photons in thermal equilibrium with emitting atoms is related to their Absolute Temperature by the Planck Distribution . The Spectrum of such photons at a temperature ''T'' peaks around 2.9 mm/''T'', determined by Wien's Law . For the Sun (5780 K) this is around 500 nm ( Visible Light ); for the Earth (~300 K), the peak is around 10 μm (infrared). As temperature is further increased, the photons will reach higher frequencies, such as Ultraviolet , X-ray , Gamma Ray s. Radio , Television , Radar and other types of Transmitter s used for Telecommunication and Remote Sensing routinely create a wide variety of low-energy photons by the Oscillation of Electric Field s in Conductors . Magnetron s emit Coherent photons used in household Microwave Oven s. Klystron Tube s are used when microwave emissions must be more finely controlled. Maser s and Laser s create monochromatic photons by the same Stimulated Emission process. More energetic photons can be created by Nuclear Transitions , Particle-antiparticle Annihilation , and in High-energy Particle Collisions . Spin Photons have Spin 1, and obey Bose-Einstein Statistics , making them Bosons . Photons mediate the electromagnetic interaction; they are the Gauge Bosons of Quantum Electrodynamics (QED), which is a U(1) Gauge Theory . A non- Relativistic spin-1 particle has three possible Spin states (−1, 0 and +1). However, in the framework of Special Relativity , this is not the case for Massless spin-1 particles, such as the photons, which have only two spin projections, Helicities , corresponding to the right- and left-handed circular Polarization s of classical electromagnetic waves. The more familiar linear polarization is formed by a Superposition of the two spin projections of a photon. Quantum state Visible light from ordinary sources (such as the Sun or a lamp) is a mixture of many photons of different wavelengths, phases and polarizations. One sees this in the Frequency Spectrum , for instance by passing the light through a Prism . In so-called "mixed states", which these sources tend to produce, light can consist of photons in Thermal Equilibrium (so-called Black-body Radiation ). Here they in many ways resemble a Gas of particles. For example, they exert pressure, known as Radiation Pressure . On the other hand, an assembly of photons of long (many Wavelength s) Wave Function can also exist in much more Coherent State s, called Bose-Einstein Condensate — such as in the light emitted by Laser . The high degree of precision obtained with laser instruments is partially due to coherence laser photons. |
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