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. K

    Do planck frequency photons form near black holes?

    Locally, photons "blue-shift" when falling down a gravitational field. What happens when they arrive at the Planck frequency? Do they turn back into matter?
  2. I

    Photons transmitted through transparent substances.

    I have noticed in the past that light travels through water relatively undisturbed (I have noticed that there seems to be some small amount of light reflected, but most seems to pass directly through). How does this happen, in terms of photons encountering particles? I once thought that...
  3. S

    FTL is not possible via entanglement because you need to compare both the photons

    Faster than light (FTL) is not possible via entanglement because you need to compare both the photons... or in other words entanglement is a single system that is "spread out" (across time-space) , the two photon pair behaves as a single system in which the two photons are within that system...
  4. Anymodal

    Are Photons Timeless? | Federico

    I'm not a physicist, I'm only taking my first subject on physics in Eng. here in Buenos aires, so I don't know much about it. But i find it so interesting that I spend some deep hours of thought ofently in it, and i just want to clarify this concern: According to relativity your local time...
  5. B

    How Many Photons Does a 100 W Bulb Emit Per Second?

    Estimate how many photons of visible light (500 nm) are emitted by a 100 W Bulb. I get 2.5\times10^{20} Photons/Second Am I correct? Thank you. EDIT: Added units.
  6. H

    General questions about photons and electrons

    Hello, I am a student in Physics and I have some problems to understand how photons and electrons are studied. Is there the assumption that they occupy a so little space in the "Phase space" that they can be considered has point ? Can we expect that in the real case when they "moove" they are...
  7. U

    Tracking Photons in a Double Slit Experiment

    The path of photons can be (almost) reconstructed in a double slit experiment! Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer http://www.sciencemag.org/content/332/6034/1170.abstract Discussion: http://physicsworld.com/cws/article/news/46193...
  8. M

    Why do photons come in quanta?

    Hi, Why do photons come in quanta? Why are the quanta not bigger or smaller or continuous? Thanks, Anna.
  9. G

    Delayed choice with photons - impossible?

    I have long been puzzled by the whole idea of delayed choice experiments with photons. In a photon's frame of reference there is no distance traveled between emission and absorption; the distance has contracted (Lorentz-Fitzgerald) to zero. More relevantly there is no time duration either...
  10. B

    Can DIY Photon Entanglement Be Achieved at Home?

    How do I entangle photons? INSTRUCTIONS PLS! I know this question was already asked on the site. But I am wording this differently. I do not want the process of entanglement explained to me. I want instructions on how to do it. Is it even possible without a lab? I have mirrors for oscillation...
  11. T

    Photons collected be telescope apertures

    the energy output of a star is such at the surface of the Earth it provides 5x10^-19 WM^-2 in the visible part of the spectrum... how many photons are collected per second by a 500mm diameter telescope? i don't know if there's any definite formula to drive the answer...but i tried this way...
  12. C

    Do Free Electrons Emit or Absorb Photons?

    Greetings, I know that when an electron in an atom emits or absorbs a photon, what changes is the electrons orbit. What about free electrons? If an electrons moving freely through space emits or absorbs an electron, what changes about the electron? And what determines what frequency...
  13. C

    Photon Emission from Antennas: Explained

    Greetings, As much as I know about science, sometimes something very fundamental dawns on me that I do not understand. I thought all photons were emitted by electrons. Having recently been pursuing interest in radio communication, I realized I do not understand how an antenna produces a...
  14. M

    Exploring Gamma Photon Distribution in the Galaxy: What Can We Learn?

    How many gamma photons are in the galaxy, and what can we know about the distribution? Which kind of process can absorb them?
  15. S

    Can Photons Orbit Around Large Masses? Exploring Einstein's Theory

    According to Einstein's theory of general relativity, light bends--or rather space bends--around mass. So, in theory, is it possible for a photon to orbit around a body of sufficiently large mass?
  16. C

    Demystifying Photons and Polarizers: Understanding Interaction and Measurement

    Cant we shoot a photon thorough a polarizer without having the polarizer interact with it ? Like all the ones polarized perpendicular to it will get blocked but the ones lined up will go through. But if the particle is not in a polarized state until we measure then how does this work?
  17. V

    Lumious intensity and flux of photons

    Is there any quantitative relationship between luminous intensity of monochromatic light and flux of photons(number of photons/(area*second)) ?
  18. C

    A question about power, and photons

    Homework Statement a handheld gaming device has 120 pixels per inch on its 2X3 in screen and uses a 5V, 7 watt*hour battery to power it. if each pixel is lit by a photon that is emitted from one electron flowing across the diode, will it last the whole week if you play it just in physics...
  19. N

    Protons, Photons, and ionic states of matter with one type

    Is it possible to impart heat to a cloud of protons via EM radiation? Photons usually interact with electrons, and I can't find much info on pure photon-proton interaction. Also, if you took some hydrogen ions (protons, to be specific), could you form a state of matter, such as a liquid...
  20. K

    Quantum Mech.: Photons and Tanning Bed Safety

    Homework Statement Summer has arrived and you decide to go to the tanning bed. After arriving, you recall from physics class that the threshold for tissue damage from ultraviolet light corresponds to a wavelength of 300nm. You decide to inquire about the limits of exposure. You're informed...
  21. M

    Wave lengths affect the speed of photons?

    I know that there are different wave lengths, light and photons travels through these waves. All of them travel at the speed of light. But note that Gamma rays, and radio waves both travel at the speed of light, but the waves of gamma rays are shorter. So both gamma rays and radio waves travel...
  22. S

    Do photons experience instant lifetimes?

    Do photons experience "instant" lifetimes? So we know that, for instance, the Andromeda Galaxy is about 2 million lightyears away. That means that from an observer on Earth, we would watch a photon take 2 million years to actually reach that galaxy. But if we're the photon itself, would it...
  23. C

    Gravitational Waves, Photons & the Uncertainty Principle

    If photons emit gravitons, then where does this energy come from? It can't pull it from its kinetic energy or maybe it just redshifts it but this seems weird. In order to emit gravitational waves do i haft to accelerate energy . I am trying to draw an analogy between gravitational waves and an...
  24. L

    Exploring Possible Speeds of Photons in a Gravitational Field

    Consider a photon moving in same direction with a gravitational field . So the speed of photon must become greater than speed of light. how its possible ? please answer this then I'll ask my next questions. :smile:
  25. T

    Deflection angles of photons passing by black holes

    In trying to compute deflection angles for photons given their impact parameter (closest distance of trajectory to centre of black hole if unaffected) I am trying to numerically integrate the following equation (d^2/d(phi)^2)(u)+u=3Mu^2. However I am stuck as to how to work out the initial...
  26. G

    Spherical EM wave or one or more photons?

    My understanding is that the EM field at r.t generated by a radiating source can be described as the amplitude of the EM fields at r, at time t. Is there a corresponding photon associated with that wave? A unit surface area at large r from the source will have less energy passing through it...
  27. P

    How Many Photons in a Light Wave

    I was just wondering how the concept of a single photon is compared to the concept of a light wave. Is a single photon equivalent to a single light wave, or is a single photon just a tiny instantaneous part of a light wave? That is, if a single light wave of a given wavelength strikes a...
  28. stevmg

    Are photons packets of electronic waves such as this below

    Are photons packets of electronic waves such as this below ----------\/\/\/\/\/\/\/\/\/-------------------------------------\/\/\/\/\/\/\/\/\/\/--------> Imagine the "------------------------->" line is an electronic beam.
  29. S

    What is the Minimum Detectable Power of a Light Flash on a Silicon Solar Cell?

    Homework Statement A circuit employs a silicon solar cell to detect flashes of light lasting .25 seconds. The smallest current the circuit can detect reliably is .42 \muA. Assuming that all photons reaching the solar cell give their energy to a charge carrier, what is the minimum power...
  30. QuarkCharmer

    Understanding Atom-Photon Interaction: How Long Will an Electron Hold Energy?

    I am told that when an atom absorbs a photon, it jumps up an energy level, these are discrete levels of energy etc etc. What determines how long the electron will hold this energy, and what exactly is the photon (wave) doing to up it's energy level. For ease of explanation, let's assume Neon...
  31. C

    Planck's Law/ number of photons

    Homework Statement Produce plots of I(λ, T) vs. λ for a blackbody at temperature T = 500 K. Compute the number of photons emitted with 400nm < λ < 450 nm for each temperature, assuming the total surface area is 1.0000 m2, to 5 significant figures. Homework Equations Planck's Law...
  32. T

    3 Thought Experiments on Light: No Photons or Waves

    I would like to invite your comments on three thought experiments. The first i suggest demonstrates that light cannot be either a stream of photons nor a wave. The second questions if light can also have momentum and the third asks why a theorectical truth is not observed. Here we go, 1...
  33. M

    Young Double Slit experiment, single photons

    Hello, I was wondering if I could get any help with the following question/thing: Why does a stream of photons (let's say that one is fired every 3 seconds) at a double slit as shown in Young's double slit experiment create an interference pattern although there is no interference because the...
  34. S

    Black holes and only photons going in

    Build a Dyson sphere around a black hole (this is a thought experiment), so no more mass "falls in". Line the sphere with insanely bright lights. What happens to the black hole? I ask this because Prof. Sean Carroll in "From Eternity To Here" discusses whether there is a limit to the...
  35. G

    Probability density of photons

    hi...does anybody know here...how you can calculate the probability density of photons?
  36. N

    Are Photons Bigger Than Electrons?

    I was told by a professor that electron microscopes are used instead of visible microscopes because photos are too big and make the picture blurry where are electrons are so small that they give detail. Considering photos don't really have a size, and have no mass, I don't understand this. Why...
  37. R

    Where do all the interfered photons go?

    I understand the basics of the double slit experiment. I'm trying to imagine what would happen if one slit could operate at a 180 degree phase shift, or nearly that. The obvious answer is not much except between the slits, and even less if the distance between the slits is near the wave length...
  38. D

    How to know if two photons are entangled?

    If I give you a black box and two photons go out of it (not necessarily in opposite directions), how can we know if the photons are entangled?
  39. Shackleford

    What is the value of N when evaluating the integral for thermal photons?

    I'm not sure how to evaluate this integral. http://i111.photobucket.com/albums/n149/camarolt4z28/IMG_20110317_145314.jpg?t=1300391815 http://i111.photobucket.com/albums/n149/camarolt4z28/IMG_20110317_145259.jpg?t=1300391802
  40. M

    Exploring Photons: Understanding Their Origin and Fate

    I need to explain a bit before asking my question. I have been reading about where photons come from and I feel I understand that part fairly well. I am wondering where they end up. It only makes sense that without photons being eliminated there would eventually be more and more and more. I read...
  41. J

    Do E and B Fields Distort When Focusing Photons with a Lens?

    I have read different light frequncies have different lensing characteristics. Example, Gamma frequencies not capable of being focused with a lens. This made me think, what happens to E and B fields, for visible spectrum, when you focus photons with a lens? Anything? Do the fields distort...
  42. S

    Mirrors & Photons: More Light in 6' x 6' Room w/Mirror?

    Hypothetical question: Someone is in a 6' x 6' x 6' lightly ventilated space with a mirrored ceiling and a lit candle on a small table. Is there more photons in the room due to the mirror on the ceiling? Is there twice as much light as there would have been if there were no mirror in the...
  43. A. Neumaier

    Are clicks proof of single photons?

    In the semiclassical picture known to Einstein 1905, currents are produced by discrete electrons. But now, 100 years later, this picture is known to be approximate only, and that currents in metals are in fact produced by the continuous electron fields of QED. Discrete semiclassical particles...
  44. Femme_physics

    Does the wave–particle duality apply only to electrons and photons?

    Does the wave–particle duality apply only to electrons and photons?
  45. N

    How Does Fock Space Relate to Particle Interactions and States?

    If electron acts both as a wave and a particle; how does an electron emit a photon when it is a wave?
  46. J

    Do gravitons interact with photons?

    Does string theory predict that gravitons interact with photons? Can a particle traveling linearly at the speed of light curve space-time around it? I guess a photon could instantaneously have rest mass it if were energetic enough to convert to an electron-positron pair and back again...
  47. L

    Can Photons Interact with Each Other Through Electron Mediation?

    ...Do they? Are they too small or fast? If they are LIGHT particles, still, could they? ...That is all...
  48. T

    Mirror Reflections: Exploring the Physics of Photons

    Hi. There's something I don't understand.. and it might be a silly question, but I want an answer. Light is formed of photons, and photons are reflected off mirrors. If you reflect photons off one mirror onto another mirror, then have that second mirror reflect back onto the first... why...
  49. E

    Photons & Frames of Reference.

    I accept that the FAQ section is there to cut down on repetitive questions, but this arises out of an answer in the FAQs. Would this line of reasoning not lead to the conclusion that, even without an inertial frame of its own, a photon would not be able to be stationary relative to itself?
  50. J

    Finding wave wquations for photons

    Homework Statement Use the relativistic expression for energy E^2=p^2c^2+(m_0)^2(c)^4 to find a wave equations for photons. Find a solution for ψ and compare to the electric field (hint: photons are massless, E_op=ih(d/dt) and p_op=h/i(d/dx) Homework Equations the only equations i know...
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