What is Photon: Definition and 1000 Discussions

The photon (Greek: φῶς, phōs, light) is a type of elementary particle. It is the quantum of the electromagnetic field including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always move at the speed of light in vacuum, 299792458 m/s (or about 186,282 mi/s). The photon belongs to the class of bosons.
Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, their behavior featuring properties of both waves and particles. The modern photon concept originated during the first two decades of the 20th century with the work of Albert Einstein, who built upon the research of Max Planck. While trying to explain how matter and electromagnetic radiation could be in thermal equilibrium with one another, Planck proposed that the energy stored within a material object should be regarded as composed of an integer number of discrete, equal-sized parts. To explain the photoelectric effect, Einstein introduced the idea that light itself is made of discrete units of energy. In 1926, Gilbert N. Lewis popularized the term photon for these energy units. Subsequently, many other experiments validated Einstein's approach.In the Standard Model of particle physics, photons and other elementary particles are described as a necessary consequence of physical laws having a certain symmetry at every point in spacetime. The intrinsic properties of particles, such as charge, mass, and spin, are determined by this gauge symmetry. The photon concept has led to momentous advances in experimental and theoretical physics, including lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. It has been applied to photochemistry, high-resolution microscopy, and measurements of molecular distances. Recently, photons have been studied as elements of quantum computers, and for applications in optical imaging and optical communication such as quantum cryptography.

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  1. D

    Intro Physics Photon and charged particle interactions with matter

    I am about to teach some of an introductory course for bachelor students in the field of medical physics. More specifically the topic "Photon and charged particle interactions with matter" in respect to radiation therapy (again, medical physics). I know there are a lot of topics within radiation...
  2. RUTA

    Insights The Schwarzschild Metric: Part 2, The Photon Sphere - Comments

    Greg Bernhardt submitted a new PF Insights post The Schwarzschild Metric: Part 2, The Photon Sphere Continue reading the Original PF Insights Post.
  3. Pushoam

    Change in the energy of photon due to recoil of the nucleus

    Homework Statement Homework EquationsThe Attempt at a Solution Since energy of photon is very low compared to the rest mass energy of the nucleus, I consider non – relativistic calculation. Conservation of linear momentum gives : momentum of nucleus = momentum of photon = p...(1)...
  4. entropy1

    Photon absorbtion and conservation

    If a (polarized) photon is absorbed by a polarization filter, does its energy go into the filter? I am wondering if that is the case to obey conservation laws. And if it passes, is its original polarisation direction somehow conserved?
  5. A

    B Planck-Einstein relation and the Photoelectric Effect

    Hi everyone, I just have some confusion regarding Planck's and Einstein's equation. The following is an explanation of the photoelectric effect using Einsteins theory: Light is composed of photons. Each photon has energy hf and mass hf/c^2. When ultraviolet photons are brought to rest by zinc...
  6. U

    Direct Modulation and Photon Lifetime - Laser

    Hello, There is a thing I struggle to understand on laser physics. There is a modulation method called direct modulation for semiconductor lasers where by changing the current we modulate the light which is emitted form the laser cavity. There is a picture below It is stated in...
  7. A

    How can I get only one single photon with a coherent light source?

    Hi! We will make a lab analogous to the stern gerlach experiments but with polarized light. How can we get only one single photon in experiment with a coherent light source? I'm going to make a lab where we need to get in only one single photo at a time. I have read that you can use some...
  8. Serifos

    B A photon left galaxy G, 12 by ago and is reaching Earth now

    ... where was our current place in spacetime, call it A, when that photon started its journey 12 by ago? Did A even exist 12 by ago?
  9. I

    B Beam splitter, one photon, one detector?

    I would like to know the results of a few scenarios with a beam splitter. (1) You send a single photon through a half silvered mirror with a reflector at either side, as above, but instead of having two detectors, and a 50% chance of either going off, you just have one detector, and where...
  10. C

    Photon Detection: Learn About Interaction, Detection & Angles

    Hello, I have always been curious about physics and decided to try this forum. However I am a bit concerned due to my lack of knowledge on the subject, so I’m hoping my questions won’t be unacceptable for the forum. I have always wondered about photons and how they interact with the world. If I...
  11. S

    B Do Gluons Emit Photons?

    I've been googling this and can't come up with a direct answer. Can a gluon emit a photon? I know. 1. All Standard Model particles with mass interact via the weak force. 2. All quarks and gluons interact via the strong force. 3. All quarks, charged leptons and massive weak force bosons emit...
  12. reese houseknecht

    Calculating how many Joules are required to make a photon

    A double covalent bond of nitrogen (N2) is 15.58eV Now let's go into a example. if i have a laser that is 477nm and i do 1240/477nm then i get 2.6eV per photon. Now if i do (15.58eV / 2.6eV) it equals ~6 photons. Now to figure out how many Joules are required for 1 photon I do (6.626*10^-34S *...
  13. B

    Knowing the Energy of a photon, what is the charge of a species?

    Homework Statement An atom or ion with only one electron is excited from the ground state to the first excited state (n = 2) with a photon of 1.47E-17 J of energy. What is the charge on the one-electron species? A. 0 B. +1 C. +2 D. +3 E. +4 Homework Equations E=-(2.18E-18)(Z2)/(N2) The...
  14. M

    Photon Energy that can Produce an Electronic Transition

    1. The problem statement, all variables, and given/known data After thinking over your beloved but erratic instructor’s lectures on the Bohr model of the atom, you come to the sad but inevitable conclusion that he has, yet again, lied to you. You have been taught that to make an electronic...
  15. E

    Dark current and dark count in photon collection devices

    Does anyone know of a simple relation between dark current and dark count in photon collection devices?
  16. morrobay

    B Entangled Photon Measurements: A(t) = d/c and B(t) = 5d/c

    Entangled photons in superposition state : | Ψ} = (|HV} - | VH})/1/√2 are sent from source to polarizers and then to detectors A and B ( in this setup distance from source to detectors is equal). If A measures vertical photon she knows immediately that B photon must be horizontally...
  17. G

    I Does QM allow for a photon to travel at 800,000 km/s?

    I realize this type of question has been asked elsewhere on competitor websites but I want to make it more precise. Usually, the person asks if anything in QM is possible to which the answer is no, for example, a photon cannot have spin 0, hence some things in QM are impossible. But what about...
  18. T

    I Photon states should not evolve?

    Since proper time for photons doesn't change, i.e. in their reference frame time doesn't change, then it should be that photons don't change their quantum mechanical state, or the equivalent in Maxwell's theory. One could say, well they don't experience time, but we do. Okay, but since their...
  19. S

    B Collision of photon and proton

    In what condition does a photon hit a proton? What happens to the energy of the photon in that case? Does proton emit another photon and goes down to the initial energy state?
  20. weezy

    I Why do people keep saying photons are timeless?

    If you search for "does a photon experience time", almost every other link says that they travel at the speed of light and so STR tells us that its clock doesn't tick at all. However why do they use the arguments for special relativity which was developed for massive particles moving close to...
  21. J

    How could the "size" of a photon be measured?

    I'm looking at the possibility of trying to measure the length of photons as a Science Fair project. I know that photons are often considered to be "point particles", and I know there are fundamental limits on measurements due to uncertainty in position and momentum. I also know that according...
  22. K

    B Photon detection in the EPR experiment

    In the photon version of the EPR experiment, how is the final polarization state of the photon detected? I have read a number of high level descriptions of the EPR experiment, but I am having trouble with understanding the detection part. Here is my understanding, please correct me where I am...
  23. S

    B How did Caltech's photon teleportation work?

    This page doesn't explain it well imo. How does quantum entanglement convey info? http://science.howstuffworks.com/science-vs-myth/everyday-myths/teleportation1.htm
  24. mpolo

    Does the Photon Defy the Rules of Special Relativity?

    Is it fair to say that the photon does not follow the rules of Special Relativity? I ask this because the photon relative to all other frames of reference has zero mass even though it is traveling at the speed of light. If we increase the speed of any other particle to the speed of light its...
  25. K

    How can a Photon apparently transfer Momentum without any Mass?

    Due to Compton - Scattering it seems that a Photon changes the Momentum of the electron, although it doesn't have any mass. How should I imagine this?
  26. D

    I Why isn't the photon energy h nu over two?

    Can anyone counter my argument that the photon energy is properly half of the generally accepted value? It is a short argument, as follows: Accepting the Planck hypothesis that the energy of the standing-wave electromagnetic modes of a cavity oscillator at thermodynamic equilibrium is E = n h...
  27. abilolado

    B Photon producing Electron-Positron pair's effect on c.

    Hello all, disclaimer here, I haven't taken quantum mechanics yet, all I know of it comes form books and PBS Space Time Series. Since a photon can, at any given instant, "split" into an Electron-Positron pair, does it mean that if this occurs between point a and b, the measured speed o that...
  28. Charles Link

    I Establishing consistency between a wave model of the photon and a particle model

    Note: added to the title should be "and a particle description". ## \\ ## The intensity (energy density) of an electromagnetic wave is proportional to the second power of the electric field amplitude, i.e. intensity ## I=n \, E^2 ##, apart from proportionality constants. Meanwhile the energy...
  29. lalo_u

    A Photon propagator in Coulomb gauge

    My aim is to derive the photon propagator in an Coulomb gauge following Pokorski's book method. In this book the photon propagator in Lorenz gauge was obtained as follows: Lorenz gauge: ##\partial_{\mu}A^{\mu}=0## It's proved that ##\delta_{\mu}A^{\mu}_T=0##, where...
  30. P

    B How much space does a single photon occupy?

    If a photon leaves a source 4,2 light years away how far apart will it be from a similar photon it was adjacent to (say less than 10^3 wavelengths) when it departed. Does the inverse square law mean that individual photons get further apart and stay the same size or do they occupy a larger space...
  31. G

    I Gamma radiation, photon energies and wavelength question

    I haven't though about this from such a perspective but today while reading wikipedia (yes yes not the best source) I got confused, now the "eV" is said to measure the energy gained by an electron between a potential difference of 1V. I assume particle physicists use this measurement because its...
  32. weezy

    I Interference of a single photon in an interferometer

    I think we can all agree that when we are shooting many photons one by one, through an interferometer, we can eventually land up with the interference pattern. This can be explained by saying that two photons combining in some areas to give four photons and in some places annihilating each...
  33. Timothy Schablin

    Why Does an Electron Release a Photon After Absorption?

    After an electron absorbs a photon, it will move to a higher energy state. It then releases a photon and returns to its ground state. But why does the electron release the photon? Why does it not remain in that energy state? What forces it to return to ground state?
  34. G

    I Scintillation photon energy conversion?

    Hello, I read that in a scintillator before the photomultiplier tube is a crystal or sometimes gas or even sometimes a plastic, in other words a material which exerts luminosity under ionizing radiation. Say in the doped crystal for example, a high energy photon of say several Mev hits the...
  35. M

    From whence cometh the photon?

    Hi, I am a new member here. I am a non-physicist. I have been told that the electron is an elementary particle. I have been told that when an electron moves to a lower shell there is a photon emitted. If the electron is elementary where does the photon come from? Thanks in advance for your...
  36. infinitebubble

    I Photon Sphere and time travel around a Black Hole?

    Reading the post below on event horizon of a black hole (BH) got me thinking about the photon sphere of the BH. We all know light will travel around this photon sphere and how light from a source would completely travel back to it's source if one could see it real time, we all know this from...
  37. zonde

    B Photon Position: Is It Less Classical Than Massive Particles?

    I am puzzled by that statement. I do not intend to question the statement that "photon doesn't admit to define a position observable". My confusion is with the conclusion that photon is less "classical" because it does not have position observable as I can reach only weaker conclusion that...
  38. C

    A Simple way to create entangled photon pairs?

    Two photons arrive at a hypothetical 50:50 Beam-Splitter with no phase shift between reflected and transmitted modes. One enters the Left side and the other the Bottom side of the BS as shown in Fig.1 of the link below: https://drive.google.com/open?id=0B5JsDLKoUSA5emk5Qk9nUHVIelE Each photon...
  39. Y

    I Photon Timelessness: Does It Exist?

    I have read that photons do not experience time... If that's the case then if a particular photon is emitted by a body then that should exist in every time relative to us i.e that same photon should be there at exactly the same point forever.
  40. Arman777

    I Emitting Photon and Energy Change

    I know that my title is not very suitable for this question, but I couldn't find right words to describe the situation.I read a book called '101 Quantum Questions ' and there writer states that when electron absorbs energy and goes higher energy level, we don't know when and where it will come...
  41. Ephemerality

    I Is this an accurate article on the shape of a photon?

    I came across this article on the shape of a photon https://cosmosmagazine.com/physics/what-shape-is-a-photon However, the first image confused me, is it showing the wave function? Most other questions regarding what a photon "looks" like seemed to dismiss the question.
  42. H

    I One by one photon delay in double slit experiment

    In a 'one photon at a time' double slit experiment, does the interference pattern still emerges if the time delay between individual photon emissions is increased to minutes or even hours?
  43. Cecilie Glittum

    I Spin and magnetic field of a photon

    Is the spin of a photon pointing in the same direction as the magnetic field of the photon?
  44. Andy_K

    B Double Slit Experiment: Timing & Reflection Interference

    Dear All, I have a couple questions on the double slit experiment I hope you can help shed some light (or photons) on. =) Arrival Timing of Photons In a normal double-slit experiment like the above setup, do photons always arrive at the detector at a constant speed (basically, speed of light)...
  45. GAURAV DADWAL

    Why Can't a Photon Be Completely Absorbed by a Free Electron?

    I have read in textbook that if a photon were to collide with a free electron it's an impossible situation for photon to get completely absorbed by an electron . The situation seems possible by conservation of energy but I am not able to understand the true reason behind the statement .is there...
  46. Andy_K

    B Double Slit Experiment: Detection around slit?

    Dear All, I have a question on the double slit experiment. From the references I've read so far, they are focusing on the results of the detector AFTER the 2 slits. Let's say when photons are fired, how many % actually pass through the slits and get detected at the end? Would some (and how...
  47. E

    How Does LIGO Detect Gravitational Waves Using Laser Interferometry?

    Homework Statement In-phase light from a laser with an effective power of 2x105J and a wavelength of 1064nm is sent down perpendicular 4km arms of the LIGO detector. (i) Determine the number of photons traveling in the interferometer arms. (ii) Assuming the detector is sensitive enough to...
  48. M

    I Neutrinos interact with photon in "level tree"?

    i found reading the possible interaction of neutrinos with photons to not "level tree", what's the meaning of level tree? and how is that posibble ?
  49. A

    I Change in electron energy level and photon emission....

    Hi all, This is likely a naive question, following up on something @vanhees71 posted some time ago in another thread: My question is the following - if we take an electron that has, for example, absorbed a photon, is the portion of the wavefunction representing the electron in a lower energy...
  50. P

    I L due to Azimuthal (1/r)x(1/r^2) Poynting vector components?

    The basic idea: I am interested in the possibility of an azimuthally-directed Poynting vector component which drops with the inverse cube of the distance (or as 1/r^3), primarily because it suggests the possibility of emitting field angular momentum, allowing for a uni-directional torque to be...
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