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. 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...
  2. 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...
  3. phyzguy

    Photon size and classical fields

    <<Moderator note: Split from thread Photon the smallest particle>> @Orodruin - Let me ask a question. This is an aspect of QFT that has always puzzled me. Suppose we have an RF cavity or a laser cavity with a standing E-M wave. I understand that we can view the field in the cavity as a...
  4. Keiran OConnor

    Is a Photon the Smallest Particle Known?

    Hi can I ask a question please, is a photon the smallest particle known ? If it is then how do we know there isn't something smaller as the light would blind us from a smaller particle ?.
  5. G

    B From dE=dm*c^2 to E=m*c^2: Integration constant?

    Hi, I've read some high school "derivations" of ##E=m\cdot c^2## that all considered single photons with momentum ##p=E/c## that are absorbed or emitted from some massive object, changing its mass. So they actually only showed the incremental $$\Delta E=\Delta m\cdot c^2 .$$ Most of those...
  6. A

    Robert Klauber’s second order photon propagator

    Homework Statement I have a question regarding Klauber's Student Friendly Quantum Mechanics, about renormalization in chapters 13 and 15. I have included all relevant equations in the attached document. In equation 15-105 he obtains an expression for the PI_uv(k^2) term used when calculating...
  7. Tollendal

    A Dark Photon Found by Hungarian Scientists in 2016

    In January 2016, Dr. Attila Krasznahorkay (at the Hungarian Academy of Sciences’s Institute for Nuclear Research in Debrecen, Hungary) and his colleagues published a paper announcing he had found a dark photon by firing protons at lithium-7, which created unstable beryllium-8 nuclei that then...
  8. J

    Photon Momentum/Energy: Is It Equal?

    Hi, Is the energy a photon carries with respect to its frequency the same as that of its momentum energy? My understanding is that it is by the energy relations, $$hf=E$$ $$E^2=p^2c^2+m^2c^4=p^2c^2$$ for a photon with ##m=0##, frequency ##f## and plank constant ##h## . So we have from both...
  9. klotza

    The Interaction of Sound and Light - Comments

    klotza submitted a new PF Insights post The Interaction of Sound and Light Continue reading the Original PF Insights Post.
  10. S

    I Could the speed of light just be "terminal velocity"?

    Alright, so I'm by no means a trained physicist and most of what I know comes from sporadic readings on the internet, but I had a strange theory the other day. I'm more the type to think in visualizations or analogies rather than cold hard math, so I'll explain it the way the idea came to me...
  11. olgerm

    I Any EM-field in terms of photon

    I know formula ## p=\frac{h}{λ} ## p is photon momentum h is plankc constant λ is EM-wave wavelenght but it is only valid for one wave. How to describe most general EM- field in terms of photons? Is there always discrete number of photons? If EM field is given in terms of a)EM-vectorfield...
  12. L

    B Which component of light is a photon?

    Light is an electromagnetic radiation with two wave components that are perpendicular. Between these two perpendicular waves, where is the photon in this system.
  13. Seanra

    I The speed of light in a medium and path integrals

    So I've heard from multiple sources that one explanation for why light slows down whilst traveling through mediums other than a vacuum is that the light "takes every possible path at the same time" through the medium. Below I've drawn my two possible interpretations of what that means. Can...
  14. entropy1

    Interpretation of polarisation experiment

    Consider a fully entangled pair of polarized photons, A and B, fired at two detectors with polarisation filters in front of them. I have to get a little philosophical to understand the way the interpretations of this experiment play out. My knowledge is still very basic but I'm working on it...
  15. NaiveTay

    I Can a Photon be Absorbed Without the Exact Energy Level?

    In my education of QM, I've heard countless times how energy is quantized by Planck's Constant, and how radiation is only emitted and absorbed in these discrete steps. Recently I've heard that that's not the full picture of energy, and I was hoping you could draw some clarity for me. In...
  16. S

    B Measuring photon momentum without annihilating it

    Is it possible to build an apparatus that could do the following (at least ideally in principle)? If so, what would it be like? A fairly localized light wave packet (with a corresponding spread in momentum) reaches the origin of our coordinate system from any direction in the XY plane (taking 2...
  17. marcophys

    I Down convertor 2 photon entanglement - explanation?

    Just watched many times (specifically @ 20:13 - 25:04) Ron Garret's 'Google Tech Talk' All was going well - the 2 photon experiment & concept visualised - only that the finality of comprehension wasn't delivered... perhaps because Ron was merely using this for his own purpose. This seems a...
  18. entropy1

    B Superposition of measurement apparatus

    Does measuring a polarized photon after it passed a polarization filter put the measurement apparatus in a superposition of detected/not-deteced (the photon)? Does this depend on whether the photon is part of an entangled pair?
  19. entropy1

    B Measurement and the creation/loss of information

    If we measure, say, the polarisation of a photon, the polarisation state of the photon collapses along the eigenvector of the observable corresponding to the measurement. This may seem as a loss of information of the original polarisation (for it is now collapsed into another value). However...
  20. Giorgi1997

    Photon colliding with moving electron

    Homework Statement Consider Comton scattering of a photon by a moving electron. Before the collision the photon has wave length λ and is moving in the +x direction, and the electron is moving in the -x direction with total energy E. The photon and electron collide head-on. After the collision...
  21. Stephanus

    B Is the mass of a photon always negligible compared to that of a proton?

    Dear PF Forum, Yesterday (on Sunday) my friend texted me, "the mass of a photon is 10-18 Ev/c2## If my number is correct. And I also I check it in Wiki, and it's true. Now what I want to ask is this first. I Understand, my wording is confusing. Now, I'll rearrange my question for a new question...
  22. M

    I Hydrogen Atom Photon Emission Wavelength Formula

    I am trying to calculate the Lyman-alpha wavelengths of photons emitted from different hydrogen-like atoms such as deuterium and positive helium ion 4He+, using the relation 1/λ = R*|1/ni^2 - 1/nf^2|, where R is the Rydberg constant and ni and nf are integer numbers corresponding to the initial...
  23. DavidReishi

    I Photon energies from Planck-Einstein: confirmed in practice?

    I understand that the photon energies given by the Planck-Einstein relation, though highly precise, are approximations. But have they been confirmed at all experimentally or in practice? If so, across the board or just some of them?
  24. M

    Max Photon Energy: Planck Length Lower Limit?

    Planck length is ~ 1.6\times 10^{-35} meters. Does this mean that this may be a lower limit on meaningful wavelength and therefore upper limit on energy?
  25. F

    B What is quantum equation of single photon?

    Equation of matter particles are Schrodinger,Klein-Gordon and Dirac equation.But the state of photons can not be represented by positions,then what is quantum equation of a single photon?Also what is the equation of single gluon? (quantum equation means the evolving of the state in time)
  26. S

    Quantum Book on quantum electrodynamics

    Can anyone suggest me a book on quantum electrodynamics for beginners? Thanks in advance
  27. F

    B Trajectory of a photon in a moving box

    I have a basic question about the trajectory of a photon. As can be seen in the figure above, let's suppose there is a stationary frame F and a cylinder sealed with two sided mirror is located at the origin of F. Then let's consider two cases; First is the velocity of cylinder is 0 with...
  28. entropy1

    B Photon and half-silvered mirror

    Suppose we have a photon fired at a half-silvered mirror. There is a 50% chance that the photon reflects, and a 50% chance it passes through, right? Suppose there are two detectors, one that only detects reflected photons and one that detects only passed photons. So my question is: does the...
  29. G

    B Photon: momentum without mass?

    The mass of a photon is zero but why does it have a momentum and an energy (E=mc^2=hv) ?
  30. DavidReishi

    I Square of absolute value of amplitude for a single photon

    I understand that this determines a probability, but of what exactly for a single photon? The probability that the photon will be detected on a surface where the photon is pumped, e.g. where on the surface the laser is aimed?
  31. DavidReishi

    I Technical question about modern Double Slit w/ one photon

    My question is about the experiment in which detectors are used behind the slits to determine which slit the photon goes through. Specifically, it's about the detectors themselves. What I found is that these detectors are some kind of photoelectric detectors. My question is this. During the...
  32. H

    Does an electron beam bending lose energy by photon emission

    A beam of electron in vacuum with velocity v enter a region of spa e with a electric field E. The field is such the electrons circle with radius r. The electrons are now accelerating at constant tangential speed. Because this is not an atomic orbital then by classical physics the electrons...
  33. LarryS

    I EM Wave Components for a Photon?

    Although mass-less, a photon, like any other quantum particle, has a frequency/wavelength associated with its energy/momentum. If we have a group of photons all with the same specific energy, then each photon can be represented by a little, complex-valued plane wave. Photons are also EM waves...
  34. Z

    Question about Photon energy in Nuclear/Atomic Physics

    I picked up a book titled "Physics in Nuclear Medicine" that, while a slightly outdated edition, will presumably still have a bunch of valid fundamental information inside. One thing I noticed in it talking about EM radiation behaving as packets of energy was that it states: "The energy of the...
  35. CollinsArg

    About light -- why when I look at a star I see rays around it?

    Hi! I'm not studying physics and what I know about light is mostly what I've read on the internet and what I've learn thinking and observing the nature. I come to realize that the space is full of light. When I look at the black space, I don't see light but I know that is light everywhere up...
  36. A

    I How does a photon view the universe?

    This question has been bugging me quite some time now. I'll start presenting my background for the problem: Fact: Photons are time-dependent oscillations of electric and magnetic fields as described by Maxwell's equations. Now, I've heard a lot of people, including professors saying that a...
  37. Ryaners

    Electron in 1-D box: photon absorbed?

    I don't know where I'm going wrong with this problem - I was so sure I had it right but the online grader tells me otherwise :oldfrown: Homework Statement An electron in a one-dimensional box has ground-state energy 2.60 eV. What is the wavelength of the photon absorbed when the electron...
  38. S

    I Derivation of the photon energy

    Hi everyone, Anybody knows if the photon energy (frequency * Planck's constant) can be derived from more basic physical principles or this formula was "invented" to explain the photo-electric effect or black body radiation? If you know a way to derive it, please share. Thanks a lot !
  39. T

    I Time in gravitational reference frame?

    Hi, so Newton said that gravity was like or equivalent to a force? When I learned about SR we were talking about the passing of time as defined by a photon bouncing between two parallel mirrors. So when we're sitting our two parallel mirrors in a gravitational field, even though I suppose...
  40. StanEvans

    Photon Reflection: Questions & Answers

    I was wondering while doing some work on the photoelectric effect about how it is that photons interact with their environment. The question that I have is why or how does a massless particle such as a photon reflect of a surface such as a mirror. My first thoughts went to collisions with the...
  41. throneoo

    Average photon energy from a galaxy cluster

    Homework Statement I want to calculate/estimate the average photon energy from a galaxy cluster incident on the mirror of a X ray telescope (Chandra to be specific) . The cluster has redshift z and at constant uniform temperature T Homework Equations Specific Intensity of thermal...
  42. W

    Compton Scattering; Relation between scattering angles

    Homework Statement Show that the scatter angles of the photon (θ) and electron (Φ) in the Compton effect are related by the relation: ##cot (θ/2 )=(1+\frac{hf}{mοC^2}) tan(Φ) ## Where f is the frequency of incident photon 2. The attempt at a solution I wrote down the equations of conservation...
  43. M

    I Can an Electron Absorb a Photon if the Next Energy Level is Full?

    Can an electron absorb a photon and jump to a higher orbit if the next level is already full of electrons? please explain what happens and also link info that might help me with electron/ photon interaction. I have many questions. Thanks
  44. yango_17

    Electron in a box - wavelength of photon

    Homework Statement a) An electron is trapped in a one-dimensional box that is 526 nm wide. Initially, it is in the n=2 energy level, but after a photon is absorbed the electron is in the n=7 energy level. What is the wavelength of absorbed photon? b) Eventually, the electron ends up in the...
  45. M

    Calculate photon flux at a distance from the source

    Homework Statement A 100W sodium lamp(lambda=589nm) radiates energy uniformly in all directions. (A) At what distance from the lamp will a totally absorbing screen absorb photons at the rate of 1.00 photon/cm2.s? (B) What is the photon flux on a small screen 2m from the lamp? Homework...
  46. Anchovy

    Getting from electroweak W_1,2,3 and B to W+-, Z and photon?

    I'm trying to understand how the Z and the \gamma are interpreted as combinations of the and B gauge fields. I'm comfortable with where the W_{1}, W_{2}, W_{3} and B come from, and I get that the W_{3} and B get related to the Z and the \gamma by the 2x2 matrix containing the weak mixing angle...
  47. sciencejournalist00

    Photon anti-bunching as necessary for BS entanglement

    The green dots represent photons in an anti-bunched state, squeezed light. The red dots represent photons in a semi-bunched state, laser light The blue dots represent photons in a bunched state, thermal light The complex setups used in entanglement experiments only squeeze light in amplitude...
  48. L

    Momentum of a photon heading towards a spherical mass

    Homework Statement A distant observer is at rest relative to a spherical mass and at a distance where the effects of gravity are negligible. The distant observer sends a photon radially towards the mass. At the distant observer, the photon's frequency is f. What is the momentum relative to...
  49. carllacan

    Polarization of Photon: Exploring Lorentz Condition

    My Quantum Field Theory notes, after explaining the Lorentz condition, say this: I have some questions about this. 1) What exactly does the polarization of a photon mean? 2) Why do the degrees of freedom of the potentials determine the polarizations of the photon? 3) If instead of the Lorentz...
  50. J

    Curious question about light/photons

    Hi there, If a photon wavelength (yellow) is isolated from the other wavelengths that sunlight emits (once it gets to earth), can that particular particle/photon be distorted slightly into a wavelength that is shorter (perceived as green)? If yes; please explain. If no; please explain. Thank...
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