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. Jovian_Dsouza

    B Does an electron turn into a positron when hit by a photon?

    I read somewhere that an electron travels forward in time and a positron travels backwards.And when a photon hits the electron the direction of time for it reverses and it becomes a positron.Does an electron really turn into a positron when hit by a photon? why?
  2. Uriel

    I What does it mean -- polarization of a single photon?

    Hi. I have a rather silly question. When speaking about a single photon? What do people mean when they speak of the polarization of a single photon. For instance, in classic electromagnetic theory, this would be the direction in which the electric field of the wave is oscillating . But does...
  3. S

    I Can black holes split photons into elementary particles?

    Hello all, Is this a thing? I understand that a theory of quantum gravity is necessary to explain the physics at the core of a black hole, but it seems a black hole is the only environment energetic and dense enough to ignite a mass-energy feedback loop where colliding photons release the...
  4. C

    Measurement standard for joules = Momentum in kilograms

    Basically, I found the energy of a photon using its frequency ((6.8*10^15) and E=(3.6496394*10^-16)) and so I then used "p=E/c" to find the momentum and got 1.2173887*10^-24 however I don't know the measurement of the momentum. Is there a better equation to use to find momentum, and what would...
  5. B

    B Uncertainty Principle and a photon

    Let a photon of a definite wavelength (hence a definite momentum ) start it's journey at time 0. After 8.3 minutes it hits a detector on earth. So it's position is exactly known (in fact it can be predicted for any time less than 8.3 minutes). So we have particle with definitely known momentum...
  6. T

    I Is the velocity of two photons of different energies same?

    Is the velocity of two photos of same radiation(light) with different energies same ?? (also in Same medium) Will energy alter the velocity of photon?
  7. D

    How photons exceeds their velocity?

    Is there a theory that explains the mechanism under which photons exceeds the speed of light? It should refer to cases including photons generated in an oscillating charges, dipoles, inhalations, different kind of excitation, accelerated particles, scattering phenomenons etc. Please note that...
  8. T

    I Can electrons transition collectively, emitting 1 photon?

    Basically, I'm asking about loosely-bound electrons, e.g. everyday static electricity. Can their combined wavefunction evolve to a low-energy state "at once", so that they emit a single high-frequency photon, rather than multiple photons of energies suggested by the potential difference? If...
  9. C

    Electron vs Photon question? Significance of momentum?

    Homework Statement , 2. Homework Equations , 3. The Attempt at a Solution : [/B] If an electron and photon have the same energy, the electron will have a shorter wavelength, and a larger momentum. The shorter wavelength makes it useful for electron microscopes, outperforming optical microscopes...
  10. Kostik

    A Help with Weinberg p. 72 -- time dt for a photon to travel a distance d⃗x

    Hi all, and thanks in advance. I am an old guy learning GR for fun. Reading Weinberg's "Gravitation and Cosmology". PhD in math 1998, so I read all books like I read math books: every character, every word, every line, every page extremely carefully. I am stuck on the stupidest thing. On p.72...
  11. Vitani11

    What is the energy of the decay photon towards the state....

    Homework Statement A certain molecule has a characteristic rotational energy of 8.81x10-4 eV. What is the energy of the decay photon towards the state with angular momentum quantum number l? Homework Equations E = Erot (L(L+1)) The Attempt at a Solution Why would this not be 12 multiplied by...
  12. F

    I Laser Beam through Calcite crystal = Photon Entanglement?

    If I shine a laser beam through a calcite crystal, the beam is split in two, as shown below. Are the photons in the split beams in an entangled state?
  13. A

    I Photon absorption and speed of wave function propagation....

    Hi all, Whew, last question for a while: I think I already know the answer, but want to confirm (e..g, I think this thread basically answers the question, https://www.physicsforums.com/threads/propagation-of-wavefunction.152053/) As an example, let's say I have an electron (in free space or...
  14. P

    I Experimental check of photon mass and Coulomb's law

    Despite some effort I still don't understand how Coulombs law can be used for experimental search for mass of photon. From wikipedia: If a photon did have non-zero mass, there would be other effects as well. Coulomb's law would be modified and the electromagnetic field would have an extra...
  15. Metals

    B Photoelectric effect and atomic excitation

    A few quick questions I'd like cleared up:1) Alkali metals are said to have a really low threshold energy, enough for visible light to cause the photoelectric effect. Does this mean if I aim a flashlight (turned on) at a piece of sodium, I could ionise it? Simply flashing a light over a piece of...
  16. M

    I Beam in specific photon number state?

    Can a beam of light be objectively in a specific photon number state (Fock state), let's say ##|2\rangle##? Or is it (the specification of the state) detector/observer dependent? I.e. we can only say that a beam of light is in a Fock state, but which exactly is detector dependent (detection time...
  17. Green dwarf

    Is electromagnetic radiation a form of kinetic energy?

    Hi, I'm a high school science teacher. Most textbooks classify EM radiation as kinetic energy. But this doesn't seem right to me. As a photon is massless it's hard to see how it can have kinetic energy which is 1/2 mv^2. It could be said that it has energy hf and therefore mass hf/c^2. Then its...
  18. UHECRon

    A Photon densities within galtactic disc compared to CMB

    I am interested in what the average photon densities are within galactic environments due to non CMB sources. In extra galactic environments, the CMB certainly dominates, however as you enter more dense and luminous areas (e.g. , the galactic disc) broad spectrum photon densities from local...
  19. Elvis 123456789

    Photon beam is incident on a proton target produces a particle

    Homework Statement A photon beam is incident on a proton target (at rest). Particle X (and nothing else) with rest mass M=1.232GeV/c2 is then produced. Use m_p =0.938GeV/c2 as the proton mass. a) What is the energy of the photon beam, in terms of GeV? b) What is the momentum of the moving...
  20. C

    B Photon Dimensions: Wave or Particle?

    I know photon is considered a point particle, so it means it has no physical dimensions, if I'm not wrong. Plus, it has a probability wave function associated to it. It also interacts as a wave which consists of an electrical and magnetic field perpendicular to each other and delayed by a...
  21. S

    A Probability of photon emission from quantum dot

    Hi guys, I am looking for a formula which I am sure exits but I cannot locate it. The problem is that a quantum dot absorbs a photon of wavelength λ0(dot is semiconductor or could be any other material). Assuming that it reemits a photon, what is the probability that this emitted photon will...
  22. K

    B Can a Magnetic Field Affect the Path of a Photon?

    Electrons have spin 1/2 and interact with a magnetic field. Since photons have spin 1, I suppose they also feels a force on a magnetic field. So that means that light can be deviated from its original path by a magnetic field?
  23. Tareq Naushad

    B How to make the wave collapse in double-slit experiment

    I successfully created the fringe pattern at home with a simple laser light and a black plastic sheet with two thin cut as double-slits. I then used two mobile phone cameras at two sides in hope that the wave function of light will be collapsed. But nothing happened i.e. the fringe remained...
  24. Alfreds9

    Where could I look for down-to-earth X-ray facts?

    Hello, I'd like to know which book or papers I'd read to understand, possibly without too rigorous mathematics knowledge, how do photons like X-rays or gamma ones reflect off surfaces and attenuate through solids or generally speaking, facts about how do they interact with the real world. So...
  25. G

    I Photon Interval vs Areal Density for Changing Intensity

    When one reduces the intensity of let's say, an incandescent bulb (by varying the resistance, as seen in many homes), which decreases more, the photon frequency (not related to wavelength, but the time interval between photon emissions) or the areal density of the photons? To what extent does...
  26. R

    What is the effect of light on the weight of a glass prism?

    Homework Statement Figure 1.1 shows an equilateral glass prism illuminated by a 100 W laser beam of wavelength λ = 600 nm. The refractive index of the glass of the prism is 1.50 at λ = 600 nm. The path of the light in the prism is parallel to the base of the prism. The momentum of a photon is...
  27. EBPoncho

    PET/CT Gamma Ray Protection with Lead Apron

    I was informed I might get a better response by posting to this forum, so sorry for the double post. Hello all, I am currently attending a service course for a PET/CT system for my employer. We have been covering the physics behind the function of detectors and my instructor said something...
  28. T

    Understanding Photon Interference: A Look at Wavelength and Molecule Distance

    [Moved from https://www.physicsforums.com/threads/if-a-photon-were-trapped-between-two-perfect-mirrors.880490/ because it's a different question] Hi, ed. Could you explain what you mean by interference between photons? Is there a wavelength smaller than 0.1nm for EM radiation?
  29. P

    I 'If a photon were trapped between two perfect mirrors....'

    If a photon were trapped between two perfect mirrors perpendicular to its axis of motion, and they were gradually brought together until they were touching, so that the distance between their faces was 0m, would the photon be "trapped" between the mirrors? Without space in which to move, how...
  30. V

    Analytical solution of photon diffusion in finite media

    Homework Statement I'm trying to derive an analytical expression for the photon backscatter flux in finite turbid media using the diffusion equation and the method of images. What I want to write is: for a given volume (x,y,z), where a coherent light source is incident on the x-y plane and z is...
  31. Cobalt101

    I Delayed choice two slit experiment - photon

    Is it possible that the reason the delayed choice two slit experiment outcomes occur is because for the photon, traveling at the speed of light, the past, present and future are all as one, and therefore it "knows" what is going to happen and therefore always "makes the right choice" at the...
  32. A

    I The speed of a "photon" in a multipath experiment?

    Let's say here are two ways for a photon to go between point A and B and a detector at B detects photons with some probability based on interference. Mathematically, it's calculated by carrying phases along each path and then adding them up regardless of how much time it takes to travel along...
  33. P

    I Can one determine the velocity of a photon in the fourth dimension using limits?

    Can one shed light on the velocity of the photon through the fourth dimension x4 using limits? To begin with, please study the mathematics from Brian Greene’s book An Elegant Universe. The upshot is that the faster an object moves through space, the slower it moves through the fourth...
  34. A

    I What is the velocity of the photon through the fourth dimension x4?

    What is the velocity of the photon through the fourth dimension x4? Photons are real, physical entities. The fourth dimension is a real, physical entity. Therefore, photons must have a relationship with the fourth dimension. They must have some velocity relative to it. What is the velocity...
  35. A

    I Can the Planck equation be derived by considering phonons?

    As far as I understand, phonons are just thermal vibrations of atoms in a lattice and blackbody radiation is just the radiation emittied due to thermal oscillations accelerating the atoms back and forth. Is there any example of a derivation of the Planck equation from considering black body...
  36. Chris Frisella

    I High Energy Photon Recieved as Lesser Energy Photons?

    Is it possible (and has it been observed) where a single high energy photon is emitted, but then is received as multiple lower energy photons?
  37. bhobba

    I What is a Photon? - Physics Basics Explained

    Hi Guys and Gal's In answering a question in general physics I came across the following which explains at a reasonably basic level what a photon is, spontaneous emission etc at the level of basic QM with a bit of math: http://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf...
  38. C

    B Accuracy in speed of photon - uncertainty?

    Hi there! I wanted to ask you, we know that light, photons have a certain fixed speed generally. Wouldn't this imply, considering Heisenberg's uncertainty principle, that the uncertainty for the speed (momentum) of the photons, should be infinite? But we know for sure, for example, that...
  39. kiwaho

    A What is the photon direction in Z-pinch

    In high energy Z-pinch, lots of X-ray emit. Which direction is the highest density of photon radiation?
  40. LarryS

    I How are protons, photons and quarks related electrically?

    When protons, due to their electric charge, interact with photons are the quarks somehow also involved in this same electric interaction? After all, the quarks do have fractional electric charges. Thanks in advance.
  41. F

    I Two photon absorption or multi-photon absorption?

    Hello Forum, What does the process of two-photon absorption photon entail? That, in general, a molecule or atom (composed of many electrons) that is illuminated by an electromagnetic field can only absorb one photon at a time except in those special cases where two photons can be absorbed...
  42. Deepak K Kapur

    Mass of a Photon: Rest & Moving Mass

    Photon's rest mass is zero. What's its 'moving' mass? Thanks.
  43. kiwaho

    B What is the photon direction?

    If a accelerated electron 10keV fly through a decelerating electric field that is set by 10KV high voltage, then after exit, the electron all kinetic energy is lost and become 0 velocity. Of course, braking radiation will happen. My question are: 1. what is the direction of radiation photon...
  44. A

    Understanding the Energy of a Photon: A Closer Look at the Underlying Formula

    please look at he underligned formula. is it a axiom we have to ake for granted ?
  45. J

    B Photon in a Box: Is Light Affected by Volume?

    Ok I am no physicist, by any means but a thoughy struck me the otherday. If i where to build a box with a volume of 1 cubic plank length , and the inside of which was a perfect mirror, and i contained within it a photon or multiple photons, how would this affect the speed of light? I believe...
  46. T

    A Qn on photon statistics (second order correlation function)

    I am trying to better understand the concept of second order coherence G2(τ) (in particular G2(0)) and a few questions have arisen. Note that I am trying to get a physical idea of what is happening so I would appreciate it if your responses can keep the math to the minimum possible. :) How do...
  47. S

    B Photon red shifting and Doppler effect

    When thinking of sound waves (or more commonly seen waves) the doppler effect is fairly obvious. A car is moving away from you, it's horn's membrane is fluctuating, increasing & reducing air pressure over time. So if it's moving away from you the sound's peak would occur further away...
  48. G

    I Velocity Addition: Physical Meaning of Non-Singularity at c?

    Hi. In the derivation of the relativistic formula for adding velocities, the Lorentz factor drops out. Mathematically, the formula works for inertial frames with relative velocity c and even gives an answer to Einstein's famous question about what happens if you drive at the speed of light and...
  49. Coldslooks

    Number of emitted photons per second

    Homework Statement The average wavelength that LED emits is 500nm. The electrical power of the LED is 300mW and the efficiency of turning energy into light is 66%. Number of emitted photons in unit time is? Homework Equations ##E_f=hcf## ##\frac{Pt}{E_f}=number of photons## The Attempt at a...
  50. entropy1

    I Macro state of a measurement device and correlation

    I have a question that seems to reflect my main concern with QM. Here it is: Consider a series of polarisation-entangled photon pairs that are sent in opposite direction to two measuring devices (e.g. at opposite ends of the universe). The measurement consists of detection of a photon after...
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