What is Photons: 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. S

    Wavelengths in fused silica and in silicon for photons

    Homework Statement Compute the wavelengths in fused silica and in silicon for photons whose free-space wavelengths are 800 nm, 1300 nm, and 1550 nm. Homework Equations This is the problem, I'm not sure what equation to use and the textbook does not seem to say, well what little I have of...
  2. B

    The Motion of Photons: Where Does the Energy Go?

    It is my understanding that a photon is essentially energy in motion. However, if by example, I were to shoot a photon in the form of light, at someones eye. Where does the photon go, once it reaches a rest position after colliding with the eye? 1. Is the energy that makes this photon...
  3. J

    Binding force applicable only to photons?

    Two Questions about fundamental forces: 1. Gravity is attractive, Electromagnetics is attractive and repulsive, Strong nuclear is attractive only (?) ... Is the weak nuclear force repulsive only ? Wikipedia is _vague_ on this. Also, I'm remembering that gravity increase with the square of...
  4. R

    Magnetism & Photons: Research, Rare Phenomena & Invisibility

    Im looking to do a little research on the effects of magnetism on photons. Can someone point me in a good direction to start? Any rare phenomena that is worth mentioning concerning this subject? Also, I read a while ago that scientists can bend light around small objects and make them invisible...
  5. J

    How do photons interact in magnetism and how is the force felt

    I understand that as one person here puts it "The electromagnetic interaction is mediated by the constant exchange of photons from one charged object to another. Some electromagnetic interactions involve "real" photons or "virtual" photons instead." But suppose I have 2 magnets and push...
  6. R

    Why doe glass get hot if the photons aren't absorbed?

    My understanding of glass is that it is transparent because photons do not have enough energy to raise the electrons in the atoms to the next energy level, thus the photon is not absorbed thus it gets through and we see it as light. But if the photon isn't absorbed, thus not leaving it's...
  7. P

    Can double slit interference be explained by the influence of previous photons?

    The usual interpretation of the double slit experiment, when done with a single photon at a time, is that the photon must interfere with itself. However interference cannot be measured in a single-photon experiment - it requires a large number of photons to manifest a discernible interference...
  8. C

    How does the amplitude on a EM wave graph correspond to the number of photons?

    Hello all! I am trying to understand ER on a more intuitive level. I can see the relationship between energy and frequency. The relationship between amplitude and photon number is less clear. So far I have E = hf. I understand that the intensity of light is a function of the number of...
  9. M

    Can photons have mass and momentum?

    do the photons have mas?or just the momentum?or both??
  10. A

    How many photons are reflecting?

    Hello, Lets we have some small diode laser pointing to a mirror. Lets say 10mW red laser something like this maybe http://www.aliexpress.com/wholesale?SearchText=10mw+laser+red&catId=0&manual=y The question: how many photons are reflecting now?
  11. M

    Photons emitted via a nuclear transition?

    Hi All, I know that when an atom is heated up electrons are excited to higher energy levels and upon returning to their ground/original state they release photons, I was wondering if the same kind of thing happens during a nuclear transition and what sort of processes are going on in...
  12. R

    Initial Acceleration of Photons

    When an electron and positron collides, they annihilate and produce photons, right? No massive particle can travel faster than light, So here is my confusion... Electrons and Positrons have mass and hence can never reach the Speed of light But When they collide they create photons. Which...
  13. B

    Photons coming from atoms, when electron changes levels

    Today I did the usual thinking, watched some courses online etc. and found out(or rather reminded myself) that the EM waves can be produced only when a charge oscillates.(or that's what the professor at Yale said) But then I remembered LED diodes, and their ability to emit light. Electron...
  14. B

    Photons coming from atoms, when electron changes levels

    Today I did the usual thinking, watched some courses online etc. and found out(or rather reminded myself) that the EM waves can be produced only when a charge oscillates.(or that's what the professor at Yale said) But then I remembered LED diodes, and their ability to emit light. Electron goes...
  15. S

    Unveiling the Versatility of Photons: From the Universe to Our Everyday Life"

    Photons seem to play an enormous role in not only our universe, but also our daily lives. from my understanding our eyes are just photon detectors... It also acts as the mediator for the EM force, is one of the few particles that can reach the cosmic speed limit, distributes heat or energy...
  16. E

    Real vs. Virtual Photons: A Comparison

    What is the difference between real photons and virtual photons?
  17. G

    How can photons arise out of pure energy?

    In the First Three Minutes, S Weinberg wrote: I thought mass gives rise to pure energy in the form of photons. I didn't know pure energy could give rise to photons, I thought photons were pure energy.
  18. B

    THE RETURN of two opposite travelling photons

    THE RETURN of two opposite traveling photons! This is a question which has been intriguing me, if two photons A and B travel in opposite direction, what would be the relative speed between them? If it is 'c', then after one year the distance between the two photons would be 2 light years...
  19. S

    How does Higgs couple to mass resulting from photons or kinetic E?

    The Higgs is supposed to couple to fermions but not photons. Nevertheless, photons add mass to systems in SR. Also kinetic energy (KE) of fermion motion adds inertia and mass to systems in SR theory. So, how does the Higgs field "tell" that this type of mass from photons and KE IS mass, and give...
  20. S

    Direct photons in one direction Help?

    Hi all, So I'm working on a project, and I'm having some design issues. I need a method of taking a white light source, and directing all of the photons that it emits in a single direction. I have a model in mind, involving an arrangement of layered components. Bottom layer: Light source...
  21. S

    Are photons necessary for an object to show colour?

    Hi, I know without light, an object can't give out radiations thus, show no colour. So if we keep any object in a completely dark room, it would not exhibit colour. Suppose we send in a large amount of sound energy into the same room. The object in the room would absorb some amount of...
  22. D

    Photons emitted by an accelerated charge

    Almost everyone is familiar with the sentence "accelerated charges radiate em waves". Nevertheless, if you are asked to derive this starting from Maxwell's equations, you might find it difficult. Surely the radiation pattern depends on the history of the motion of the charge. Then, there...
  23. S

    Calculating number of Photons absorbed by a surface

    If light with a frequency of 4.41*10^15 Hz falls on a surface that gains 1.13*10^-16 J of energy, how many photons has the surface absorbed? Probably simple, I've been using E = hf and such, but I got no idea from there Help meeee please
  24. K

    Do Photons Oscillate? Exploring EM Waves

    OK I understand it’s a silly question but I have been thinking about this all day. Say you have an EM wave moving out from its source in a transverse wave formation. Where does the oscillation come from? And (another silly question) is the oscillation affected by mass? In my simplistic...
  25. harrylin

    New loophole free EPR test with photons by Wittmann et al

    New "loophole free" EPR test with photons by Wittmann et al Recently posted under Recent Noteworthy Physics Papers, https://www.physicsforums.com/showthread.php?t=127314&page=10: Bernhard Wittmann, Sven Ramelow, Fabian Steinlechner, Nathan K Langford, Nicolas Brunner, Howard M Wiseman...
  26. R

    Photons chances of combining with electrons

    I am curious about the transparency of some materials to visible light. My physics is patchy and self taught, so I am hoping you can fill in a gap for me, or correct a misconception. So, transparency comes about when the light in question passes through a material with electrons that cannot...
  27. L

    Doppler Shift and photons with a wavelength of one Plank length

    If this is the wrong place to be posting this, feel free to lock this thread. I wasn't sure if this belonged in "classical physics" or "quantum physics"; this seemed like the appropriate place, but I'm not 100% certain, as it does involve subatomic particles. Anyways, to get to the matter at...
  28. A

    Frequency of photons emitted from electrons

    Homework Statement The energy of the electron in the first and forth levels in a hydrogen atom are: (21.76*10^-19), (-0.85) e.V.. the electron moved from the forth level to the first level with one leap. Calculate: The frequency of the emitted photon from the atomHomework Equations ΔE = hfThe...
  29. A

    Finding the energies of 2 photons from a decay of a travelling mass

    Homework Statement Mass M traveling at v=c/3 decays into 2 photons in x and -x directions. Mass M orginally formed by mass m traveling at v=3c/5 hitting a stationary mass m. This gives M v=c/3 and M=3m/√2. Homework Equations E=K + mc^2 p = gamma m v E = gamma m c^2 K =...
  30. P

    Can these two things disperse photons?What can disperse photons?

    For question 1. Can a wind blade that cuts at a cellular level disperse or effect photons? Can a person who can turn objects into dust affect the photons at all? What types of things can disperse photons?
  31. V

    Why Do Photons and Electrons Have Different Density of States?

    Homework Statement density of states of photon gas is proportional to ... (a)E^1/2 (b)E (c)E^3/2 (d)E^2 Homework Equations i know the relation for density of states of electrons which is proportional to E^1/2. So far i was thinking that electrons and photons shares the same...
  32. M

    Is Light Intelligent? Speed of Light & Quantum Phenomena

    The constancy of the speed of light = c. Is it possible that light "adjusts' it speed based on "knowing" the relative velocities of every object around itself? Is this an alternative to time and space changing with speed? Anything as bizarre as invariant speed also seems to be yet another...
  33. G

    Why photons are free from decoherence?

    Hi everybody, which are the physical laws that make photons free from decoherence? I only read about the fact that photons have no charge, but why this implies no decoherence? Thanks you all, gioia
  34. P

    Self-annihilating photons and Z bosons?

    (If my question seems irrational, it's ok - I'm an undergrad!) So I was reading up on the standard model and read that Z bosons are their own antiparticles. Does this mean that when two Z bosons are in physical contact, they annihilate? I ask this for photons too since I read this on the...
  35. M

    Moving charges: Describing its field as photons

    Hi, I was studying Cherenkov radiation and here's what I read. A charged particle moving through a medium has an electric field. This is equivalent to the particle emitting photons which are absorbed by the atoms of the medium and causes them to reorient themselves. I am a little...
  36. D

    Energy of photons and the red shift

    Was just learning red shift for my A level final exam and thought about this: Okay so I have a galaxy with a fixed amount of stars (for the duration of this thought experiment) producing a fixed power of light, which can be assumed to be from its centre. The galaxy is receding from the Earth at...
  37. S

    Do un-entangled photons exist?

    in the various QM experiments: after select event(s) have occured, we assume collapse of wave function and the photon becomes un-entangled from its twin. however does this un-entangled photon almost simultaneously get entangled with some other part of existence/universe/experimental...
  38. S

    Electromagnetic waves and photons

    Hello, why the electromagnetic waves move up and down instead of just travel straight? When an electron go in a lower energy level it relases a photon, but where does this photon came from? Thank you!
  39. D

    Why Are Photons Reflected at the Same Angle They Are Absorbed?

    I'm starting out with QM (I have a maths degree) and have come up with the following problem to help me understand QM more. Say we have a mirror lying flat on the ground. We shine a laser from the East on the mirror at say X degrees, to be reflected to the West at X degrees. At the quantum...
  40. K

    How can anyone know photons exist?

    these should be some very easily answered questions. how does anyone know photons exist? I am guessing they can be "stopped" and studdied? and also, after reading about this relativity stuff for the last few days i feel like I am getting an understanding, but please, entertain this scenario and...
  41. B

    Is the number of photons in the universe fixed

    It says clearly at the wiki article regarding the CMB that: "The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since exactly the same photons fill a larger and larger universe." I don't see how that's true...
  42. L

    Why do photons not pass straight through objects?

    so i often hear the analogy of an atom being on the same scale as a foot ball in the center of a pitch, with the electrons all in their orbitals at some large distance away. in chemistry we have been discussing the difference in energy levels of electrons and using these to describe the...
  43. R

    Questionable Question about Photons and Waves

    If a photon is a particle moving like a wave, and the color we see when viewing a stream of photons is determined by their frequency, then how many oscillations of a wave constitute a single photon? When an experiment speaks of shooting “a single photon” at a target, exactly “how much...
  44. DennisN

    Inverting the quantum cloning of photons

    Anyone keen on taking a bite at this? I understand what it's about (at least I think so), but I am certainly not qualified to evaluate it :shy:. "Inverting the quantum cloning of photons" (Sadegh Raeisi, Wolfgang Tittel, Christoph Simon) Arxiv: http://arxiv.org/abs/1111.7283 Article...
  45. D

    Center of momentum frame with photons?

    Is it possible to have a center of momentum frame in a system with a photon and a massive particle? If you were to do so in a system with two massive particles, this would involve redefining the velocity of the particles in the CM frame. E.g. if A moves towards B with speed 2v, in the CM...
  46. D

    Why can't photons reverse annihilate ?

    Why can't photons "reverse annihilate"? If electron-positron pairs annihilate to product gamma radiation, why can't gamma photons spontaneously become a electron-positron pair?
  47. P

    General Relativity: Gravitational time dilation of photons and Fabric of Space

    Consider the following, from the prospective of General Relativity: A photon's path between two points, point O (emitted point, from galaxy GO) to point R (received point, in Galaxy GR): O: point of photon's origin, in space GFO: gravitational field force at point O, due to its position in...
  48. photonkid

    What is the density of photons in a beam of light?

    I'm a "layman" when it comes to physics and I read in the FAQ that a photon is not a real particle but is described as one for the benefit of lay-people. I read that 65 billion neutrinos pass through a square centimeter every second so I wondered how many photons pass through a square...
  49. C

    Momentum of photons in CM frame.

    Homework Statement Two photons in the laboratory system have frequencies \nu_1 and \nu_2. The angle between the propagation directions is \theta. a) Write down the expressions for the total energy and momentum of the photons in the laboratory system. b) Find the photons’ frequency...
  50. S

    Can Photons Really Have Zero Momentum and Energy?

    p=mv then for a photon p=0*c means p=0. And also ke=1/2pv which is ke=0 and E=mc2 means E=0. please use easy words and we descriptive.
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