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. Uriel

    B Are photons really necessary in physics?

    I've read two articles about the historic development of the concept of photon and how some effects which originally were explained using photons have been later explained by other means. And after this two quick readings I have a lot of questions. First of all, do we really need a concept of...
  2. 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?
  3. B

    Are Man-Made Lights Sources Photons Man-Made?

    This is going to sound like a dumb question, but if you have a man made light source, ie; a lamp, are the photons coming out of the lamp also classified as man made?
  4. Elvis 123456789

    Photons emitted by Hydrogen and helium atoms

    Homework Statement An excited hydrogen atom can emit photons of various wavelengths. a) What is the maximum wavelength of the Balmer series (in nm) (5pt) b) What is the minimum wavelength of the Balmer series (in nm) (5pt) c) Corresponding to part b), what is the kinetic energy of the recoil...
  5. 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...
  6. K

    (SP. RELATIVITY) Annihiliation of Electrons into Photons

    Homework Statement [/B] An electron e- and positron e+ annihilate to produce two photons. a_ Why are two photons produced rather than one? b_ Assume that the e- and e+ are at rest just before they annihilate. In their rest frame, what are the energies and momenta of the photons? Define the +x...
  7. M

    I Two single-photon Fock states vs one two-photon Fock state

    What is the difference between two single-photon Fock states ##|1\rangle |1\rangle## and one two-photon Fock state ##|2\rangle## (all in the same mode)? In both cases the mean photon number is 2. How do we distinguish them experimentally?
  8. Elvis 123456789

    Question concerning photoelectric effect lab

    This isn't really a homework question but I do have to know it for my lab report so I figure this is a good place to post it. So for my lab we had the setup that is displayed in the picture attachments. My question deals specifically with step #9 of the lab instructions. I'm assuming that the...
  9. N

    Stargazing H.E.S.S. gamma ray telescope got an image of the moon

    https://en.m.wikipedia.org/wiki/File:Moon_egret.jpg Can we use better telescopes of this type (ground based gamma rays telescopes) to image the surface and subsurface of a planet with a lot of resolution?
  10. A

    I Clarification virtual photons, real photons

    I need clarification on the difference between a virtual photon of a transmitted force and “radiated photons” of accelerated charges. Imagine two electric charges separated by a distance “d” but not moving. They exchange virtual photons which tell the electric charges to attract/repel. How are...
  11. T

    Can Low Energy Photons Cause Box Explosions?

    Can you pump enough low energy photons into a box that the rate of dissipation is overwhelmed and cause the box to explode?
  12. superkraken

    B Do photons have mass and how does it affect their existence?

    this is the first time I am learning about photons and i have asmall doubt. if photons are massless then how come light dosent escape from black holes. or if they do have mass the by the eqn m=m0/(1-V2/c2)then theyll have infinite mass and turning on a torch may be like getting hit by a bullet...
  13. 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...
  14. N

    B Can we know what exists in planets' subsurfaces?

    imagine this situation: there's a very similar planet to Earth with almost the same atmosphere and surface. we want to know what lies beyond the surface to know if life exists there. so could we send or observe radio waves or microwaves with a similar telescope as arecibo that penetrates several...
  15. C

    B Heisenberg's uncertainty principle and photons

    Hi everyone, Apologies if this is common knowledge or a silly question, I'm just coming back to physics and I've been looking through the double slit experiments ( both double slit and delayed choice quantum eraser ) and it got me thinking about the uncertainty principle. With a photon of...
  16. Sophrosyne

    I Wavelength of photons exchanged between charged particles

    When two electrons approach each other, there is a repulsion between them by the exchange of a photon as the electromagnetic force carrier. Is there a general range of wavelength of such photons? Does it depend on how rapidly these electrons are approaching each other?
  17. Ontophobe

    Where do photons *Actually* come from?

    1. When an electron drops from one orbit to a lower orbit, it secretes a photon. 2. Any body with a temperature above absolute zero secretes black body radiation. Black body radiation can't possibly be caused by electrons dropping orbits, because there are only so many electron orbits, and...
  18. Tareq Naushad

    Which properties of photons carry info on size,width,height?

    I know when we see an object then the frequencies of photons reflected from the object carry the information about the colors. Frequencies of visible spectrum helps us to visualize colors. However which properties of photons help us to estimate distance,speed,size,width,height,depth, plainness...
  19. K

    I Virtual photons [in electrostatic interactions]

    I've been looking through the internet and I haven't found anything too clear on this: is it correct to assume a electrostatic interaction, say, between two stationary electrons as an exchange of virtual photons?
  20. Tareq Naushad

    When I lit a laser ray for 10 seconds from a distant planet

    I am on Alpha 4 light years away from earth. On 1st Jan, 2016 I lit a powerful laser light towards Earth for 10 seconds and then switched it off. My friend on Earth knows that he has to detect that light after 4 years on the night of 1st Jan, 2020 from a space station using a very powerful...
  21. Jlister

    Absolute Relativity (who needs virtual photons anyhow?)

    I've seen, in the description of a black hole's photon sphere, a mention of a "light-like ds." I was happy to see it because I've been thinking about a photon's physical environment; at the speed of light, it would experience no radial dimension & no time dimension. Might it make more sense to...
  22. 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?
  23. F

    I Where do high energy background photons go?

    The temperature of expanding Universe is cooler and cooler.The most contribution of energy of background photons(CMB) are of photons having energy ~3kT.Then where have the high energy background photons gone?
  24. E

    I Planck formula and density of photons

    Hello! Let's consider again a system of atoms with only two permitted energy levels E_1 and E_2 > E_1. When electrons decay from E_2 level to E_1, they generate a photon of energy E_{21} = E_2 - E_1 = h \nu. The number of photons (per unit frequency, per unit volume) emitted by such a system in...
  25. 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.
  26. A

    Electrons producing gamma photons in accelerators

    When electrons produce photons in accelerators what is the mechanism called? and who was the first to do it?
  27. Madi Araly

    I Uncertainty Principle.... Intent Behind It?

    I've been pre-occupied with Heisenberg's uncertainty principle for around four years now, and I've come to fabricate a lot of questions. The most pressing one, however, is as follows: To me, the uncertainty principle seems to reference our (relatively) poorly controlled methods to measure a...
  28. F

    B Are photons particles or waves?

    I've heard of photons being described as a wave/particle duality. But what evidence is there that individual photons behave like anything other than a particle? I can see how photons en masse can display wave/particle characteristics, but what evidence is there that any individual photon...
  29. B

    B Is dark matter a sea of massive photons?

    Is the "missing mass" the mass of the photons connected to and neighboring the matter which are displaced by the matter?
  30. 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...
  31. A

    Is there any absolute proof that photons exist?

    I mean, look this stupidity: [Mentor's note - link to crackpot site deleted] This guy denies that light photons exist, and that we are 'magically creating it' like cyclops X-Men This is worst than flat-earthers, I wonder If there is some evidence or is it unfalsifiable, like solipsism? Because I...
  32. Surya97

    I Electric and Magnetic Fields, Photons, Charges

    I know that magnetic fields create moving charges (an electric current) normal to the plane of the magnetic force lines. I also have heard that magnetic and electric fields create each other in a perpendicular direction to the other (badly worded). Electric currents are moving charges (usually...
  33. secur

    I SR Pseudo-Paradox: Photons Affected by Emitter?

    This question is a new (to me) wrinkle on the old Special Relativity spaceship pseudo-paradox gedankens. Suppose you observe two spaceships motionless relative to you, side-by-side a mile apart. First, a rifle is fired from one at a target on the other, the bullet hits the target. Now they...
  34. victorhugo

    I Photons being released from fire/heat

    (correct me if I'm wrong; When new bonds form in the process of combustion, the energy being released is given as kinetic energy to the molecules (heat) and the kinetic energy is (somehow) absorbed by electrons to reach and excited state, eventually dropping down again and releasing photons of...
  35. S

    I "Asking photons where they have been" - Danan et al

    This paper https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.111.240402 was discussed briefly in another thread on Klyshko photons. I have a couple of questions to anyone who has studied the work. The authors themselves note that "...even Maxwell’s equations for the classical...
  36. N

    The law of conservation of energy is wrong?

    e few days ago i talked with my teacher about the energy in the universe being constant. but we were completely confused when we came to the concept of: "because of the universe expansion everything moves away from each other. and the same goes for the wavelenghts in light. because of the way...
  37. S

    I Gravitational Mass of Photons: Passive & Active

    As I understand a photon has zero rest mass (as far as we can tell) but it does have a passive gravitational mass in order for it to be able to respond to gravity. But I've been shown that passive gravitational mass should be equal to active gravitational mass, and if this is true and photons...
  38. C

    Do Photons from one star affect other stars?

    I am not a physicist but have heard that photons from the sun effect the flight path of asteroids? I was just wondering if these same photons would ever be able to effect nearby star systems? Therefore could there be a slight push between star-systems pushing them further apart? (Does this come...
  39. J

    B Entangled photons thought experiment

    Following scenario: One million entangled photon pairs are created and are sent to Alice's and Bob's laboratory such that Alice receives 1 million photons(or any other large number) of which each is entangled to one of the million photons Bob receives, their laboratories being at a distance to...
  40. L

    I Photons, Higgs -- Mass question

    I am really not sure. I watch Susskind lecture But I am still confused. He sad electron in motion emits photons. Quarks in motion emited gluons. Is that correct? Could quark be in motion? Also why all matter is made on fermions? And does Higgs gave mass to all particles? Why photon after all...
  41. Keiran OConnor

    Detecting Photons: Uncovering the Invisible Particles

    How can we detect photons without mass ? In the experiment that was showing two photons hitting the detector when only 1 was fired through the slit, how did technology detect photons without mass to hit the detector ?
  42. Green dwarf

    Photons always take the quickest route

    My understanding is that the path of a photon between any two points A and B can be worked out by finding the route which will get it there in the least time. This seems to be true both when a photon passes near a heavy mass and deviates because of gravity (as predicted by general relativity)...
  43. Toreno

    I Is nuclear fusion induced by gamma photons possible?

    Hi folks, IMO, it should be possible to initiate a nuclear fusion of H1-H2 and H2-H2 in crystals of Lithium Hydride induced by gamma photons emitted form decay of Na24. Could anyone please verify if the following is correct? By using nuclear photodisintegration effect, we need a gamma photon...
  44. J

    I Properties of Photons: Orange vs. Green

    How are the photons in a monochromatic beam of orange light (for example) different from the photons in a monochromatic beam of green light? I'm trying to understand the properties of individual photons. -- Jeff, in Minneapolis
  45. J

    I Proper mapping of the second postulate into math

    The second postulate of SR is telling us that light always travels at C in a vacuum(absent of gravity) measured by any observer independent of the source or inertial frame the observer is measuring the light from. However, light is made up of photons which do not travel like ping pong balls in...
  46. A

    Magnetism, electricity, and photons

    If photons are part of a magnetic field like electrons, which electrons can be read with an amp meter, what can measure or count photons? Electrons are not counted but the magnetic field changes when more current flows and the ammeter reads a change in the magnetic field. What type of light are...
  47. F

    I How can photons interact if they travel at the speed of ligh

    I was listening to a podcast about the solar neutrino problem, and they discussed how we have deduced that neutrinos are not massless due to the fact that they interact with other particles (even if this interaction occurs rarely). I paraphrase: "a particle traveling at the speed of light is...
  48. Zahidur

    Does light always travel at light speed?

    I've been told contradicting ideas about this. I've been told that light doesn't travel at a constant speed everywhere (i.e. light slowing down in speed after entering a more dense medium). However, I've also read that light speed is constant everywhere (i.e. if you could travel close to the...
  49. S

    I Spatially Resolved Hong Ou Mandel - "Coalescing" Photons

    I'm reading this article https://www.osapublishing.org/ol/abstract.cfm?uri=ol-40-7-1540. It is a Hong-Ou-Mandel interferometer in which they can record exactly where each photon of a pair ends up, with a resolution of maybe a hundredth of the output port areas. So you see either two dots in the...
  50. E

    I Calculation of mean energy value of photons in recombination

    I am interested in the calculation of the mean energy value of CMB (Cosmic Microwave Background) photons from which the recombination is performed. The subject on French Wikipedia says : "Intuitively, one might say that recombination occurs when energy average of photons is of the order of the...
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