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. J

    Relativistic Momentum of a photon

    [b]1. An elementary particle of mass M completely absorbs a photon, after which its mass is 1.01M. (a) What was the energy of the incoming photon? (b) Why is that energy greater than 0.01Mc2? Homework Equations p (photon) = E/c p (particle) = γmv p(i) = p(f) Ek = γmc^2 -mc^2...
  2. E

    Photon + proton collision & threshold nrg

    Could anyone guide me through this problem? https://www.physicsforums.com/showthread.php?t=348863 Just need to get started. Note that this would need to be solved in the proton rest frame.
  3. P

    Reaction between photon and proton

    A high energy proton can collide with a photon to produce a charged pion and a neutron. If the photon comes from the cosmic microwave background and has an energy equivalent to a temperature of 3 K, what is the minimum energy of the proton in eV necessary to allow this reaction? I'm thinking...
  4. B

    Can Excess Photon Energy Cause an Electron to Jump Energy Levels?

    What if an electron at a certain energy level receive a little bit more energy from a photon than the difference between its current energy level and the next one, will it jump to it ? I see written in textbooks that the photon energy must exactly match the difference between two energy level...
  5. S

    Coherent state between a fermion and a photon

    Can we have a coherent state between a fermion and a photon! I mean can there ever be a fermionic polariton?
  6. S

    What happens to the freq of a photon after encountering double slit?

    1. (How) Is the frequency of a photon effected after encountering a double-slit or a half silvered mirror (say in a Mach-zehnder)? 2. Are particles such as photon, electron, assumed to be entangled, at all points in time-space, with something or the other?
  7. C

    Relation between wavefunction of the photon and the Four-potential

    Hey! Maybe this is a "piece of cake" question, but here is the thing, i have the Maxwell equations in the Lorenz gauge are \begin{array}{c} \partial_{\mu}\partial^{\mu}A^{\nu}=\mu_{0}j^{\nu} \end{array} In vacuum this gets reduced into \begin{array}{c} \partial_{\mu}\partial^{\mu}A^{\nu}=0...
  8. K

    Can we experimentally prove the momentum of a photon without mass?

    1.photon has no mass . so m=0.hence, p=mv=0.by doing some calculations , we can get that p=h/lambda.we can prove p=mv experimentally.but how can we prove the second one experimentally?
  9. R

    Energy Of A Single Photon In Em Radiation?

    Energy Of A "Single" Photon In Em Radiation? Is the energy of all photons in em radiation same? That is, say light differs from radio waves only in the number of photons per second
  10. M

    Virtuality of a nearly on-shell photon

    Hi all, It was recently pointed out to me that the finite electron mass puts a lower bound on the Mandelstam variable t describing the square of the transferred momentum in the centre-of-mass frame, t_{min}=m_{e}^{2}. This solved a problem I was worrying about (the finiteness of the...
  11. A

    How does a photon reflect off a smooth surface?

    Hello, I was just reading optics and I thought that if photon doesn't has mass it can't be accelerated.. If it is so then how it gets reflected off a smooth surface, why don't it just passes from the plane. As any reaction force by the plane couldn't accelerate it , it mustn't rebond as...
  12. B

    Source of the first Photon in the LASER

    what is the source of the first photon in the laser ? i understand there is a burst of light or electricity that excites up the medium , but what is the source of the first photon ? is it the light source or is it one atom of the medium itself releasing photon as it gets de-excited * if...
  13. Q

    Can someone travelling at c race a photon?

    In trying to understand relativity, I once read a statement that even traveling at the speed of light, an emitted photon traveling in your direction would still move away from you at the speed of light. This confuses me as I have a hard time correlating it to a real scenario. Say "The Flash"...
  14. A

    Interacting Photons at Planck Energy: What are the Properties and Possibilities?

    What are the properties of a photon with Planck energy? Is it even possible to interact with it, or does it just travel trough all matter?
  15. I

    Direction of a photon created by bremsstrahlung radiation?

    basically any time i have an arc, i have electrons flowing in the opposite direction to the field gradient. so when the electrons reach the other side, they will be stopped, which will then produce an EM wave. In, class, my professor drew the direction of radiation in a particular direction, but...
  16. 7

    Atom Emitting Photon: Energy Conservation

    I have come across a problem which is a homework indeed, but i tried to pack this question up so that it is more theoretical. What i want to know is if i am alowed to write energy conservation for an atom which emitts a photon (when his electron changes energy for a value ##\Delta E##) like...
  17. I

    Mechanism behind photon absorption and photon emission

    hi all, i learned that only certain orbits were allowed in the atom and that if the electrons occupied any of the orbits in between, that they would no longer be in a resonance orbit (i was taught that the allowed orbits were the electron probability wave in resonance and therefore no EM...
  18. Z

    Conservation of momentum of single photon passing a slit

    When a single plane-wave photon/electron passes a slit/orifice, its direction of travel becomes random. Although there is the well-known Uncertainty Principle, it is not a replacement of the law of conservation of momentum for the phonon/electron before and after passing the slit. Question...
  19. J

    Extremely energetic photon threshold

    Does anyone have any information regarding if a photon will at some point obtain mass at huge energy amounts? Let's say perhaps the photon gains so much energy that it's wavelength reaches Planck's length, is it possible that it would gain mass? Is there any information regarding this inquiry...
  20. B

    Photon mass - clarification request

    Am I correct in thinking it is currently believed that a single photon at rest has no mass a single photon in motion does have mass ?
  21. P

    What is the frequency spectrum for a single photon?

    According to the Einstein light-quantum hypothsis, the photon energy is given by E = hbar*ω. If taking the photon as a very short light pulse, then the frequency spectrum is extremely wide in terms of Fourier-transform theory. How to understand the frequency ω in E = hbar*ω if taking the...
  22. A

    How could a photon travel at lightspeed if it has mass?

    I've heard that photons have a mass. If a photon were to be stationary, would it have mass? If not, then where does the mass of a photon come from? I know that if an object has stationary mass (forgive me if this isn't the correct term), then it takes an infinite amount of energy in order to...
  23. S

    Time & Speed of Photon: Questions Answered

    I know this thread is similar to another one that was posted recently (I really enjoyed reading it), but I think I have some other questions which aren't entirely redundant. BTW, I'm not looking for really mathematically rigorous explanations, because I probably wouldn't understand them...
  24. D

    Photon energy changing due to Dopplershift?

    How does the energy of a photon change by Doppler shift? Let's say we have an IR photon, but due to Doppler shift it shifts towards UV wavelengts. Shorter wavelength/higher frequency means more energy, but where does this energy come from? How do you connect this with Photoelectric effect...
  25. T

    Photon polarization and direction

    When a photon is produced when an atom raised to excited state return to ground state it's frequency can be calculated. but can we predict the direction and polarization of that photon.
  26. 4

    Can something with zero rest mass (photon) curve spacetime?

    My understanding is that the presence of energy and matter curve spacetime. Is a photon considered energy? If so, how can it curve spacetime while having zero rest mass?
  27. T

    How does a photon know to pass through without interacting?

    How does a photon "know" to pass through without interacting? The usual explanation given for transparency is that when the energy of a photon is smaller than the band gap energy of an atom, the photons don't interact with the electrons and pass through, so the material is transparent. But...
  28. G

    Photon as a particle and e-ray and o-ray

    Wiki in birefringence says, "light with linear polarizations parallel and perpendicular ... the component with polarization perpendicular to this axis will be refracted as per the standard law of refraction, while the complementary polarization component will refract at a nonstandard angle...
  29. O

    Interaction of Photon with water

    Dear all respected expert/collegues, I'm setting out a photoelectrochemical cell. I'm facing problem to regulate temperature in my photoreactor durng reaction. Can i just dip the photoreactor inside a big water jacket? But i wondering incoming photon or light supplied by my Xenon arc lamp...
  30. C

    Does the Ward identity rescue a zero photon mass?

    In Peskin at page 248 he finds that if he calculates the vacuum polarization that $$\Pi(q)^{\mu \nu} \propto g^{\mu \nu}\Lambda^2$$ a result which violates the Ward identity and would cause a non-zero photon mass $$M \propto \Lambda$$. But as Peskin states, the proof of the Ward identity...
  31. S

    Photon flux from a light source

    Homework Statement What is the photon flux (photons/ m2 s) at a distance of 1 km from a light source emitting 50W of radiation in the visible domain, with wavelength 6000 Angstroms. Homework Equations The Attempt at a Solution Energy of one photon = hc/λ = 3.32 x 10-19J. So, number...
  32. E

    Horizon size of a photon in a matter dominated universe

    Homework Statement The maximum proper distance a photon can travel in the interval (0,t) is given by the horizon size h(t) = R(t) ∫0t dt' / R(t') Show that, for a matter dominated universe h(z) = H0-1(1+z)-1(Ω-1)-1/2cos-1(1-2(Ω-1)/(Ω(1+z))) for Ω>1 = 2H0-1(1+z)-3/2...
  33. 4

    Equation of state of photon gas

    Dear all, I am using stress-energy tensor to derive equation of state of photon gas (assuming it as a perfect fluid). I completed all the steps except one: average value of [cos(θ)]^2 over unit sphere = 1/3. I have no idea how this is so. (θ is polar angle). I tried integrating over...
  34. S

    Photon behavior after the polarizer

    Let's say we pass a photon through a 0 degree polarizer. the photon is now oriented at 0 degrees, meaning it will pass through subsequent polarizeres oriented at 0 degrees. (note - Not all photons will pass through the 0 degree polarizer, roughly 50% will pass through. we are talking about...
  35. M

    Photon frequency in different inertial frames

    Do we measure different frequencies of light in frames moving relatively at constant velocities? Because when we look at an annihilation reaction from the view of different frames, we see different energies of the reactants which seem to affect the frequency of the produced photons. your kind...
  36. D

    What is the velocity of the hydrogen atom after absorbing the photon?

    A stationary hydrogen atom with a mass of 1.67 x10^-27 kg absorbs a photon of light with an energy of 10.2eV.What is the velocity of the hydrogen atom after absorbing the photon? This is my question: Is it true that the atom has no speed since all the energy is completely absorbed at the...
  37. zrek

    Is it possible to create a higher energy photon from two lower ones?

    Is it possible for two photons to join their energy to structure a one photon of higher frequency? Let's assume that the two incoming (IR) photons are prepared specially: coming in the same time, same wavelength, phase, appropriate (complementer) spins and angle, etc. They arrive the...
  38. R

    Photon emission by an excited hydrogen atom

    Typically (in popular literature) the process of photon emission by an excited atom is considered as an instant event. But actually it is quite likely that it is a continuous process. Such processes are usually described by evolutionary differential equations (ODEs or PDEs). Assume that we...
  39. E

    Is coherence a single photon phenomenon?

    Hello folks. 2-dimensional spectroscopic techniques have been used to investigate biological systems and found that they transfer energy via coherent pathways. However, some have argued that this has no bearing on how these systems behave in nature, as the sun is not a coherent source and the...
  40. A

    Photon Shot Effect & Hanbury Brown Twiss Effect: Why No Correlation?

    1. I have never understood Purcell's explanation of the Hanbury Brown Twiss effect saying that the correlation they measured is only due to the clumping of the bosons; that the pure shot effect would not yield any correlation. Why not? 2. I was reminded of this by Purdy's recently announced...
  41. C

    Does photon exist without velocity c ?

    hi, recently i have been struggling with the concept if photon exists without traveling at velocity c ? i knew c is constant only when it travels trough space and when it travels through other medium with refractive indexes it does not exist at all .. is it true ? how do scientists define why...
  42. B

    Total Energy of a Particle After Discharging a Photon in a Moving Frame

    Homework Statement A particle of mass m is at rest in the lab frame. The particle discharge a photon with energy 1/2mc^2 to the direction of x+. A spaceship is moving at v = 0.8c in the direction x+ (in the same direction the photon is moving). What is the total energy of the particle after it...
  43. G

    I have a tricky problem involving an electron and photon.

    Homework Statement The question I have is simple enough: at what energy do an electron and a photon have the same wavelength? Homework Equations I know that for a photon, λ=\frac{hc}{E} and for an electron, λ=\frac{hc}{\sqrt{E^{2}-mc^{2}}} The Attempt at a Solution I can't just equate...
  44. W

    Photon Polarization: Linear vs Circular & Time-Dependent States

    So I know that a linearly polarized photon is in a state ψ = cos(θ)\left|x\right\rangle + sin(θ)\left|y\right\rangle What if θ depends on time maybe something like θ\equiv\frac{E_{0}t}{\hbar}? The polarization is linear at any time t, it rotates as time passes? Isn't that circular...
  45. E

    Energy shift in a photon passing by a gravitational well

    Gravity effects the energy of a photon per the equation GM/c^2r. I understand that a photon traveling on a crash course with a black hole will be blue shifted according to the hole's mass, however I'm a little confused as to what happens when a photon simply passes nearby a massive object. For...
  46. H

    Can a Photon be thought of as a bit?

    A photon is emitted and then absorbed. And since a photon travels at the speed of light, time does not exist. So that should mean that a photon is in either a state of emission or a state of absorption. Doesn't that make a photon a type of bit? Furthermore - if there is no time involved...
  47. E

    What is the maximum energy density of EM radiation and its effects?

    Hi yall, I have a few questions in regards to EM radiation and photon density, or energy density; not sure what exactly the correct term would be here. (1) Anyway, first off, excluding the possibility that a black hole would form, is there anything that would limit EM radiation energy density...
  48. S

    Can photon capture give electron and positron opposite charge

    Is it possible, in theory, that the physical entity electron e(-) and its charged opposite positron e(+) both obtained their respective electric charges by simultaneous capture, by a more fundamental quantum entity (call it FQE), of a quantum of photon with zero rest mass having equal amounts of...
  49. P

    How can a photon have hadronic components?

    the wiki article on vector meson dominance says "hadronic components of the physical photons". How can a photon have hadronic photon components? Could someone please enlighten me?
  50. Y

    Relation between RAMAN Intensity and Energy of the photon

    Hi all, What is the relation between RAMAN Intensity and Energy of the photon and/or bond stregth and/or bond energy? Is there any equation or sth explain it easily? If you can explain it in a simple way, I will appreciate. Thanks
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