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. R

    Travelling the Same speed as a photon

    Hi, i have a question about SR. would happen if two bodies were moving away from each other at exactly .50c, and one body emitted a beam of light the opposite way it was traveling just as the other was passing? I know that simultanety is relative, but what if someone moving on the body that...
  2. T

    Landau-Lifschitz : Photon helicity states

    Good morning everyone ! I've been reading discussions on PF for a long time, but here I'm stuck on a little problem that really annoys me and I couldn't find answer anywhere, so I guess it was time to register. :> I've been focusing on quantum electrodynamics for a couple of weeks now as part...
  3. P

    Is there a limit to how much energy can be stored in a photon?

    hi all, I have a couple of questions. is there a limit to how much energy can be stored in a photon? And why does the spectrum only go to gamma rays? And what is the higest observed energy and what made it ? Thanx in advance
  4. jaumzaum

    Photon of intermediary energy - Spontaneous Emission

    I'm studying spontaneous emission and I'm in trouble to understand the following: In a hydrogen atom we have distinct energy levels with different energy values associated to them. E1 = -13,6 eV E2 = -3,4 eV E3 = -1,51 eV To go from the first to the second energy level we need a photon...
  5. zrek

    Electron jump and the photon freq

    I found the explanation of the radiation here, in the document of the Purcell Simplified: http://physics.weber.edu/schroeder/mrr/MRRtalk.html I like it, however I'm ucertain how to fit this to the light, as photon. Am I thinking correctly, that this figure fits also the case when a...
  6. A

    Neutrino is faster then photon (light) so how can be this possible?

    If I reach the speed of light which is 300 00 km/s the time will stop! and if I travel faster then light i'll travel to the past (Please Correct me if I'm wrong) I read that the Neutrino is faster then photon (light) so how can be this possible ?! Because if it is really faster than light that...
  7. Z

    Photon absorption - Newton vs. Einstein

    A note from Newton's Principles definition #4 in 1687: Impressed Force - This force conflicts in the action only; and remains no longer in the body when the action is over. In Einstein's second paper on relativity in 1905, he explicitly concludes: "Radiation carries inertia between...
  8. D

    Estimating the energy of an emitted photon in gamma decay

    Homework Statement Consider a nucleus which is initially at rest and in an excited state with energy Ei. It then decays to a lower energy state with energy Ef by emitting a gamma-ray photon. Show that the energy of the photon is approximately given by Eγ≈ΔE−(ΔE)^2/(2mc^2) where ΔE = Ei - Ef...
  9. S

    Can Free Fundamental Particles Absorb Photons?

    I recently learned that a free electron can't absorb a photon and derived it by showing it would be impossible to conserve both momentum and energy if that were the case. It seems like the same argument would extend to other fundamental particles. Is it true that no free fundamental particle can...
  10. A

    World's record for highest freq photon ever seen, nature or lab?

    Hi all, What is the highest frequency photon ever seen, whether in nature or the lab? This is based on an earlier post of mine, but I think has merit as its own post. Since E=hf, there is no maximum freq or energy of a photon, it seems, on a max freq of a photon. It seems that such high...
  11. L

    Photon Wavelength: Particle or Wave?

    I understand that electromagnetic radiation is a photon. But one thing that is continuing to bug me is the question how does a photon, which is defined as a particle not a wave, have a wavelength? Do photons travel as packets in some sort of a compression type wave similar to sound?
  12. E

    Describing the Life of a Photon

    I heard Neil deGrasse Tyson talk about the life of a photon that was emitted from the Sun and hit oneself. He said that its entire life was an instant. I hope I'm not misquoting or misunderstanding what he said. With that in mind, does that instant take into account the thousand of years...
  13. S

    Photon interacting with 1D-boundary

    Good evening fellas, I'm in a bit of a conundrum: lately I've been considering a 2D array of atoms and the interactions that may arise when studied with photon beams. Up until now, I was using 3D systems and Dark Field Imaging, but when I switched to 2D, I got some unexpected intensity at the...
  14. S

    Photon interacting with 1D-boundary

    Good evening fellas, I'm in a bit of a conundrum: lately I've been considering a 2D array of atoms and the interactions that may arise when studied with photon beams. Up until now, I was using in 3D systems, but when I switched to 2D, I got some unexpected intensity at the boundary of the...
  15. L

    Can a Photon Have a Perfectly Smooth Orbit?

    can a photon have a perfectly smooth orbit? say for e.g. you have a photon orbiting a point, if its wavelength were to become twice the diameter of its orbit then would the wave not become a replica of the orbit offset by the amplitude? similarly say the amplitude is the radius of the...
  16. J

    Photon Amplitude: Is It Right or Wrong?

    Does it make sense to talk about the amplitude of a photon? In my mind, the amplitude of the photon is the maximum strength that the electric and magnetic field gets as they oscillate. If you were to change the amplitude of a photon (say increase), then the maximum strength of the e&m fields...
  17. S

    Photon conservation in Raman amplification

    Part 1: Homework Statement Use two coupled-wave equations for the Raman amplification process depicted to the right to show that for every photon at Stokes frequency omega_s created (destroyed) one photon in the laser pump field omega_L is destroyed (created). I have the coupled-wave...
  18. A

    Is the Transmission Probability of a Vertically Polarized Photon Really 1.0?

    I think I understand this only up to a point. 1. Photon spin is quantized to be +1 or -1 and these represent left- and right-hand circular polarization. 2. A photon can have a superposition of 2 spin states in any proportion. 3. Having probability amplitude of 0.5 of spin 1 and 0.5 of spin -1...
  19. O

    Photon wavelength probability distribution for blackbody

    Hey everyone, This is my first time posting on PF! I want to model the photons ejected from a blackbody source at temperature T. The question I want answered is: given a photon is detected, what is the probability of the photon having a wavelength λ? This amounts to just attaining the...
  20. 2

    Does photon lose its momentum?

    Hello. I know that light exerts pressure. My question is how can light exert some kind of a force? If it exerts a force, it means that there has to be a change in momentum but light clearly can't go slower and it doesn't change its wave length after the bounce. If it is able to move a solar...
  21. S

    How can a photon move something( light moves), if it doesn't have a ma

    ...a mass? not matter how fast it hits an object, it doesn't have mass. F=ma. (explain without any equations or without too many equations.)
  22. U

    What is the excitation energy of an atom after a photon collision?

    Homework Statement There's an elastic collision between a photon of energy E and an atom in an excited state. After the collision, the energy of the photon is still E but its direction changed of angle of 180° and the atom is now going back with velocity Bc. If the atom is in his ground state...
  23. U

    Elastic collision between a photon and an electron

    Homework Statement Hello everybody , a/There's an elastic collision between a photon of energy E and an electron at rest. After the collision, the energy of the photon is E/2 and propagates in a direction making an angle theta=60° with the initial direction. Find E. What kind of photon is this...
  24. F

    Scattering of an unpolarized photon off an electron

    Hi all! I have been told that if an unpolarized photon hits an electron in a Thomson scattering the outcoming photon will be polarized because of the electron's spin. I didn't understand what it means, nor do I get how an electron reacts to an unpolarized photon: in Classical Electrodynamics...
  25. S

    Work function and energy of X ray photon emitted by anode

    Hi guys, I'm constantly bothered by one assumption in my textbook..it says that the photon emitted by the accelerating electron boiled off from the cathode colliding into the anode, has E=hf neglecting the work function, since its negligible. I'm curious whether it's E=hf plus or minus the work...
  26. P

    QFT Beginner Seeks Help: Photon Self Energy Feynman Diagram

    I am a beginner to QFT and I try to plot the Feynman diagram for the photon self-energy. Following Mandl-Shaw book (page 109 Eq. 7.22) \int d^4x_1 d^4x_2 (-1)\mathrm{Tr}(iS_F(x_2-x_1)\gamma A^-(x_1) iS_F(x_1-x_2) \gamma A^+(x_2)) but when I try to convert it to momentum space I get \int...
  27. A

    Photon Arrival Rate: Doppler Effect Explained

    Let a monochromatic plane wave impinge on a collector of area A, delivering power P. Let the average photon rate of arrival on the collector be B. Now make the collector move toward the source at velocity v. By the Doppler effect, the frequency, and hence the energy, of the photons will...
  28. S

    Exploring the Possibility of Monochromatic Photons in Electromagnetic Waves

    When electromagnetic waves of different frequencies interact, they give rise to secondary wave structures called envelopes in which individual waveforms form at the rear and die out at the front. These envelopes are called groups and they travel with a velocity called group velocity and the...
  29. T

    Would Photon Clocks be possible?

    My question is as follows: A photon clock (two 100% reflective mirrors exactly parallel and photons whizzing in between) actually possible. My question is made assuming we could get the mirrors exactly parallel. So really, I suppose my question is: Are 100% reflective mirrors possible to make...
  30. C

    Photon Wavelength of He+ Emission Spectrum

    Homework Statement An electron of wavelength 1.74 \times 10^{-10} m strikes an atom of ionized helium (He+). What is the wavelength (m) of the light corresponding to the line in the emission spectrum with the smallest energy transition? Homework Equations Kinetic Energy...
  31. Z

    Photon with negative energy, Compton Scattering

    Hello PF people, Homework Statement In a Compton scattering event, after the collision, the Photon has an energy of 0.12 MeV and the Electron has an energy of 0.04 MeV. Find the following: i) The Wavelength of the photon before the collision. ii) The scattering angle for the photon...
  32. D

    Heisenberg Uncertainty Maximum Lifetime of Photon

    Homework Statement In special conditions, it is possible to measure the energy of a gamma ray photon to 1 part in 10^15. For a photon energy of 50 keV, estimate the maximum lifetime that could be determined be a direct measurement of the spread of photon energy. Homework Equations...
  33. J

    What determines if a photon is absorbed?

    Say it has xnm wavelenght. It hits an atom that reflects it, then another different atom absorbs it, why? Also what determines if particles like free neutrons or some hadrons will absorb a particular wavelenght? Why does gold absorb all wavelength except yellow and why does it also seem...
  34. M

    Can a Single Photon Be Directly Observed?

    Is it possible a single photon to be seen by using any existing scientific equipment? I mean seen , after it is caught and enlarged on a photo or computer monitor, the way we can see a very small object. I mean a photon directly , not a picture of effect it may produce when interact with the...
  35. C

    The photon, time and entropy

    The photon, "time" and entropy (ignore original title, in fact id appreciate if a mod would change it. It was not my intention to have a conversation about time, lol!) Can we view a closed system of just photons, as ever, or usually, being subject to entropy (defined by the whole 2nd law...
  36. N

    Intro chem help Visible light spectrum? and energy of photon? help

    on a visible spectrum scale what are the numbers for the colors of ROYGBIV? for example is violet 400-500nm and red is 700-800nm? how about the other colors? If the energy of a photon is 2.63 x 10^-19 J, how can we tell what color this is? i was looking at this electromagnetic spectrum and it...
  37. C

    Time in the reference frame of the photon and aswell curved spac

    Time in the "reference frame" of the photon and as well curved spac Hi there! I understand the tendency of physicists to stick to the math, and the logic itself, and to often avoid attempting to conceptualize a process or law, but that's kinda the opposite of what I'd like to do here. I...
  38. J

    Phase of photon from excited atom

    Imagine that an excited hydrogen atom in the symmetric 2s state emits a photon. We use a birefringent crystal to split the photon into vertical and horizontal components. Is the phase relation between these components random or is it the same for every photon emitted by a similarly excited...
  39. S

    How to reconcile photon not experiencing time with it's travel time?

    If a photon does not experience space-time does that mean it cannot "degrade/fray"? Is there a way to reconcile the, say, eight minutes, time the photon (approx) took to travel from sun to Earth with the zero time that passed in the (frame of reference of) photon? Where was the photon, in a...
  40. S

    What is a Photon? - Mass, Momentum, Speed & Wavelength

    What is a photon? Does it have mass?(if no then why it has momentum) it travels with speed of light? Has it some wavelength associated with it,that can be calculated by De Broglie wave equation?
  41. F

    Using recoil of atom to find correct in wavelength of emitted photon

    So firstly, the book supplies the answer of \Delta\lambda = \frac{h}{2mc} I use energy and momentum conservation. I say the total energy is that of the incoming photon, which is then changed to energy of the photon released, and kinetic energy of the atom, from the recoil. I say the same...
  42. D

    Special relativity from the frame of refrence of photon

    we know that particle having rest mass cannot reach speed of light,c. but if we apply results of special relativity to a photon,then it seems that for a photon 1)there is no time and 2)length is contracted to zero. 1) so does that...
  43. F

    Photon energy and specific heat capacity

    If I use the answer given in the book for part a, I can get the correct answers for b and c. However, I do not know what I have done wrong in part a? My best guess would be that assuming the blood evaporates at 100 degrees is incorrect? The answer given in the book is 5.1*10^{-3}J
  44. J

    Why is a photon considered a point particle despite having a wavelength?

    So if an electromagnetic wave can have a wavelength measuring 100 km (an arbitrary measurement of course), why is a photon a point particle. Is it a point, or a varying size maxing out at 100 km? Are the perpendicular magnetic fields spanning 100 km at the peak of the cycle?
  45. W

    Photon emitted when proton changes state

    Homework Statement a proton is confined in an infinitely high square well of length 10 fm. If the proton transitions from n=2 to ground state determine the energy and wavelength of the photon emitted Homework Equations E = \frac{h^2 n^2}{8mL^2} E = \frac{hc}{\lambda} \ \ or \ \...
  46. K

    How Do Photons Initiate the Process of Vision?

    Photon help please -- how does vision work? how does a photon of activate the molecular pathways that lead to “vision”? Need help asap can't find anything on this.
  47. E

    Photon into matter - antimatter pair production question.

    Hi there. Just a few quick questions: What causes a photon to become an antimatter / matter pair? Is it just random? Can a photon be influenced to encourage it to change? Also what is the process called? Do we say the photon has decayed? That doesn't seem right to me. In addition, if...
  48. S

    Experiment on mass of a photon

    Ive been trying to find if anyone has done a simple experiment to test relativistic mass of a photon. I was hoping if anyone has found documentation or results of the following experiment: In a vacuum fire a stream of photons at different frequencies at a pressure plate and measure the...
  49. A

    The Compton Effect: Find the angles of scattered photon and electron

    Homework Statement In the Compton scattering event seen in the figure, the scattered photon has an energy of 120 keV and the recoiling electron has an energy of 40 keV. Find: a) The wavelength of the incident photon b) The angle θ at which the photon is scattered c) The recoil angle...
  50. S

    Effect of photon detection mid-way between (double) slit and screen

    In a single particle double slit (self-interference) experiment: the which-way information is got by having the detection instrument (or at-least part of the instrumentation) right next to the slits.To make it easier, Let's assume the following x-coordinates: at source of photon x = 0 slits...
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