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. M

    What happens to photons that don't get absorbed by a chromophore in the eye?

    I was explaining the physics of colors to someone in another forum, but I came to a point where I wasn't sure what happened next, what happens when a photon hits a chromophore in the eye and isn't the right energy to be absorbed by it. So, suppose a "green" photon hits a chromophore that...
  2. T

    Photon Speed in a Wave: How Does It Work?

    I was in math class today as we were learning about sine waves and frequency, and i kinda related it to light. Then I started thinking about the speed of the photon in the wave, and if the photon moves in the wave at the speed of light at different frequency's then would one particle reach a...
  3. T

    Proton-proton collision and virtual photons

    a) Two protons, approaching each other with 3keV of energy each, collide head on. Calculate how close they get to each other. b) Estimate the momentum transferred by one virtual photon at the closest approach. c) Hence, estimate roughly how many virtual particles are exchanged during the...
  4. E

    Light Speed: Photons & Bending - Educate Me

    New to the forum. In discussing the speed of light, will photons vary in speed when light is bent? Black holes, magnetic fields, atmospheric refraction, other phenomena capable of bending light changes photon direction, which should in turn slow photons. Educate me...
  5. I

    Feynman says photons ARE particles

    but isn't a particle just an idea? like the point in geometry i guess what I'm having trouble understanding is the divide between theory and reality is explaining how a system behaves the same as saying what the system is?
  6. B

    How photons create an image via the brain

    im trying to understand how photons allow the brain to create an image. I understand that light is made up of photons. These photons are emitted from an energy source and reflect off an object and hit the retina which produces electrical signals that the brain creates an image from. What i don't...
  7. Y

    Speed of photons through dual slit.

    Let us say we have source that can emit a single photon. We can in principle detect when the photon leaves the source due to the momentum kick. Now let us say this single photon passes through both slits and forms a dot on the far right of the screen. Now if we draw paths through the two slits...
  8. J

    The Nature of Photons: Do They Truly Travel Between Points A and B?

    Do photons actually "travel"? since, at the speed of light, time does not exist and there is effectively no distance between any 2 objects in the universe, and since QM demonstrates that photons "take all possible paths" (ie, they are essentially everywhere between the time they are emitted and...
  9. U

    Searching Book about QED; compton scattering of polarized photons

    does anybody know a book, where the compton-scattering with respect to the initial photon-polarisation is calculated? (beside landau-lifschitz-berestetzki)
  10. J

    Relativity and the mass of photons.

    If we rearrange Einstein's mass-energy equivallency equation we get. m=\frac{E\sqrt{1-\beta^{2}}}{c^{2}} A photon moves at the speed of light making its beta=1. This would make its mass=0. This would insinuate that in the original equation E=\frac{mc^{2}}{\sqrt{1-\beta^{2}}} E=\frac{0}{0}...
  11. menniandscience

    Can Photons Bounce Back? | Meeting of Photons Hello!

    hello! q: can photons bounce back from photons? i'm thinking about a ray of light in space crossing in front of my eyes but i can't see it off course since it dosen't hit my eyes or particles like dust. so if i want to see it, can i turn on a flashlight making the ray hit by the rays of the...
  12. V

    Photons and human distance perception

    Hello, I don't even know if this two are related in the way I think, but this is what i would like to know. for example If we traveled far away from the planet Earth and looked back at it (no telescopes), we would see a blue marble but would not be able to see for example our familly...
  13. W

    Angular dependence of electron-positron to two photons

    Hello, First of all, it is not a homework question, just something I wonder about. The dominating electron-positron annihilation process at low energies is photon pair creation. What is the angular dependence of the total cross section? For some reason I expect the head-on collision to...
  14. N

    Behaviour of photons in a 'sealed room'

    Hi could some one help me understand the relationship between photons and a sealed room to which no photons can escape. Say there is a light bulb in the room producing photons that are all the visible spectrum and the wall paper is made of beta carotine from carrots so some of the photons are...
  15. D

    Electomagnetic wave frequency and photons

    If i calculate the frequency of light with λ = 600nm ... Frequency (v) = c/λ = 49.9654 x 1012 Hz (waves per second) Can i say this is the frequency of photons i.e. 49.9654 x 1012 Hz (waves per second) Thanks DD
  16. P

    Stimulated photons direction: book suggestion.

    Hi everyone! I will very thankful to whom can suggest me some texts or webpages where I can find a demonstrated explanation of why photons emitted by stimulated atoms travels in same direction of the photons which interact with the atoms. Every author says photons are coherent, same phase...
  17. N

    Wave/particle duality - electrons and photons

    I understand that: 1. Electrons can interact with each other in both particle-like and wave-like ways (eg. can exhibit wavelike interference in double slit experiment) 2. Photons can interact with each other in both particle-like and wave-like ways 3. Electrons and photons can interact...
  18. C

    Finding Force Exerted by Photons on a Sphere

    I was given this question in my book … A sphere of radius 1cm is placed in path of light of large aperture. The intensity of light is 0.5W/cm^2. The sphere completely absorbs photons falling on it. Find the force exerted by them on the sphere. I solved the question and I have given the...
  19. M

    Why do the photons produced by a laser have the same phase?

    Hi. This has been destroying my mind all day. Take, for example, a semiconductor laser with no internal photons initially. By spontaneous emission, one electron falls from the conduction band to the valance band and emits a photon with the bandgap frequency and a random phase. As this photon...
  20. L

    Heisenberg photons and particle velocity measurement

    Hi, [c=speed of light] if a particle travels at a velocity x/t and momentum p, such that (c-x/t) < (delta-x)*(delta-p) (ie the Heisenberg limit), how could one tell whether what one was looking at was a photon traveling at speed c or a particle traveling indeterminately CLOSE to c? also...
  21. K

    Unexploited Photons: What Happens to Solar Energy?

    While I’m waiting until I can afford some photovoltaic panels, I wonder what is happening to all the photons pummeling my roof now? If the (light) energy from one square cm of solar radiation is converted into electrical energy and then converted to heat by Aunt Matilda's blow-dryer, what...
  22. I

    Ratio of number density of photons to baryons

    Homework Statement photon/baryon ratio = Np/Nb Np, is the number density of the CMB photons Nb, is the number density of baryons Evaluate this ratio at the redshifts z=0 and z=100. From my understanding, if interactions are negligable, particles cannot simply disappear. Therefore...
  23. C

    Correct me if I am wrong, but atoms emit photons when they jump energy

    Correct me if I am wrong, but atoms emit photons when they jump energy levels. Right?
  24. A

    Why energy level transitions only absorb or emit photons？

    The question occurred to me when I was thinking about black radiation. It is critically depend on the fact that the photon number is variable. Therefore I wonder why energy level transitions of atoms can only emit out the massless photons, but not any other kind of particles, even some with...
  25. TrickyDicky

    Are virtual photons and electron-positron pairs related in the polarized vacuum?

    How are virtual photons and virtual electron-positrons pairs of the polarized vacuum related? Are they interchangeable, do virtual pairs give rise to virtual photons? Or none of the above?:)
  26. O

    Effect of 3D photons on a 2D universe. Help me understand?

    Hi, I just saw this video on youtube, originally from Carl Sagan's 'Cosmos', about a 4th spatial dimension. http://www.youtube.com/watch?v=Y9KT4M7kiSw&NR=1" So what happens when the photons that are traveling in 3D hit an object in a 2D plane universe. To a 2D object it seems like photon...
  27. B

    How close can 2 coherent photons be?

    By coherent photons, I mean ones that are in phase (like what comes out of a laser). I would guess that they can be no closer than the size of the atom that emitted them. If adjacent atoms simultaneously emitted photons, that would give the minimum separation. For the photons to be any closer...
  28. J

    Can Single Photon Beams Exhibit Coherence?

    For any light beam it is normal to characterize the coherence length and time. This is done by measuring the ability light to interfere with itself. Does it make sense to talk about the coherence of a single photon beam, such as the one used in the double slit experiment? Can a single photon...
  29. B

    Photons in Empty Space: Is Space Full?

    If a sun is giving off photons in every direction in space, does that not mean that all of empty space is full of photons? I know photons have no mass but that still means it is not a perfect vacuum right?
  30. C

    Light, photons, reflection and percentages

    Hey all, I am new to these forums. I am thinking of taking a distance learning degree in the near future from Open University in Physics, so decided to do some reading up first as it has been a while since I hav etuched physics. Anyway I am reading a book at the minute "Quantam Theory cannot...
  31. O

    Photon Emission and Energy Levels in a Hydrogen Atom

    Homework Statement A hydrogen atom is excited from the state with n=1 to that with n=4. Calculate the energy (in eV) that must be absorbed by the atom. Calculate and display on an energy-level diagram the different photon energied (in eV) that may be emitted in order for the atom to reach...
  32. T

    Why are photons emitted when electrons drop energy levels

    Hi everyone, So I'm doing some research on fluorescence and I'm puzzled. I understand that running a current through fluorescent gasses like neon cause the valence electrons to jump up an energy level, and I understand that photons are emitted when they fall back down into their default...
  33. P

    What Does 0.1 Expected Photon Per Pulse Mean in Quantum Cryptography?

    Maybe a stupid question, but anyway. The paper "Experimental Quantum Cryptography" by C. Bennettet al. discusses laser pulses of 0.1 expected photon per pulse. What does that mean physically? Is it that 90% of laser pulses are not pulses at all, containing no photons and thereby no energy? Or...
  34. A

    Question about Steradian and Radian for Compton.s Photons

    Homework Statement In an experiment there was found equal compton scattered photons per steradian in the direction 150* as in 90* Homework Equations What relation in number of scattered photons in 150* and 90* would you get if you define number of photons per radians instead? The...
  35. M

    How Do Photons Relate to Light's Wave-Particle Duality?

    Hi, I was hoping you could help me in my understanding of light as both a wave and a particle. In particular, let's say we have light propagating at some frequency v. Does each oscillation of the electric field correspond to one photon? That is, if the wave has v cycles per second, does that...
  36. V

    Radius of Photons? - Get Answers Here

    I was wondering if there is an estimated radious to the Photon. As far as I know a Photon is a particle with no mass and an amount of energy determined by h * f. Anyone knows if there is an estimated radious of a photon? thanks.
  37. J

    Detecting single photons - conceptual question

    Imagine I have a detector which is "single photon sensitive". What that means, I gather, is that if I shine a pulsed laser at the detector with just the right amount of attenuation, I should get to the regime where there is less than 1 photon arriving each time the laser fires a pulse, so the...
  38. K

    What is the relationship between antenna tuning and photons emission by antenna?

    If electric energy is supplied in the form of high voltage & low current & if the same energy is supplied in the form low voltage & high current will there be in difference in the photons emitted by the antenna?the frequency is same for both the cases.One electron can interact with just one atom...
  39. C

    Is it Possible for Nuclei to Emit Visible Photons?

    Can a nucleus in an atom emit visible light . I am aware that the nucleus has discrete energy levels, And it seems that they only emit gamma rays, but could we make them emit visible photons. I have just begun to study very basic nuclear physics in my classes. And i am taking EM theory...
  40. A

    Fraction of stimulated photons that escape from a laser cavity

    How to calculate the fraction of stimulated photons that escape from a laser cavity with alpha=0.1 cm^-1 length of cavity=1mm refractive index of laser cavity=3.2 the photons are assumed to be escaping into air so approx refractive index is 1. Im assuming that alpha is some sort of...
  41. P

    Exploring Free Electron Reflection of Photons

    How do free electrons of metals reflect photons? I had read that they first absorb and then emit the photon but if they are free (not bound to an atom and thus not fixed to any old energy level) why should they lose their energy on the first place? Surely they don't have to drop to their...
  42. S

    Do Photons Spin in Our Reference Frame?

    hi, i have read that photons must have an angular momentum because they have no rest mass. does anyone there understand why?
  43. E

    Photons and and equivalent number of atoms on gravity

    due to e=mc2 would massless photons and an equivalent number of mass-energy atoms have the same effects on gravity? Does mass play any special role in gravity or only energy?
  44. T

    Do Photons Have a Gravitational Effect?

    According to GR energy creates gravity. Photons have no mass but have energy, so do they create gravity?
  45. O

    Origin of Photons Irrelevant in Double Slit Experiment?

    If I "create" a stream of photons in the lab or if I spectographically select a stream of photons from a star, I get the same experimental result in a double slit experiment? In other words there is no difference between photons whose point of origin can be deduced to be eons ago vs those...
  46. S

    Photons reflecting off mirrors, wheres the flaw?

    Hello. Consider 2 spacecraft isolated in space, with a mirror on their back, facing each other. One light pulse is shot between them. The photons reflect off the first one, transferring a momentum off 2p, travels in the opposite directions, reflects off the second spacecraft , giving it 2p...
  47. W

    Photons live in a singularity?

    Photons "live" in a singularity? I did some searches, but couldn't find an answer to this on this forum. Since a photon can travel 1 km, 1 lightyear, 1 million lightyears in ZERO time from its own perspective, and if we did a though experiment from the point of view of a photon, does that...
  48. M

    How is heat caused by photons?

    When certain photons with the right energy hit an atom's electrons, the electrons of that atom can shift positions. Visually this is what I'm talking about: http://www.youtube.com/watch?v=L0UJhSvPE5A&feature=related" From my understanding Heat=the vibration of atoms When I rub my hands...
  49. C

    Very basic question(s) about photons

    I'm a beginner at quantum mechanics and I was just wondering about: 1) How photons can interact with charged particles when they are uncharged themselves. Also in the same regard according to my (school) textbooks light is an electromagnetic wave. But it also says that photons are uncharged and...
  50. L

    Why photons don't fall to Earth?

    Some introductory line about me apologizing for my ignorance and inability to find answers on interwebs. :smile: If photons get affected by gravitational field (or follow the curvature of space-time created by nearby massive objects) and have in essence some non-rest mass how come they...
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