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

    Uncertainty relation between number of photons in an EM field and its phase

    Derive the relation Δn.Δσ ≥ 1/2 where n is number of photons in an EM field and σ is phase Using heisenburgs uncetertainty principle? Tried subbing in frequency into heisenburgs uncertainty principle to get to the number of photons and to get rid of mometum is this the right line? Dont...
  2. J

    3 Photons going through 3 Polarizers

    Homework Statement Suppose we set up a sequence of three polarizers with orientations 45 degrees, 75 degrees and 45 degrees, where the angles refer to how much each polarizer is rotated relative to the vertical direction. If we send three vertically (i.e. 0 degrees) polarized photons in either...
  3. J

    Can you observe a photons polarization or change in polarizati without disturbing it?

    anybody?
  4. J

    Wavelength, energy and photons.

    This follows some questions/points made in my light and sight thread. I have been trying to work out the relationship/difference between photons and waves. I now think that a photon is a discrete particle like amount of energy needed to jump an electron from one orbit to another, perhaps call...
  5. S

    Why do neutrinos escape the sun's core faster than photons?

    Hi guys, I have a question about the difference in the time it takes a neutrino to escape the core of the sun compared to the time it takes a photon to escape from the core of the sun. Basically, my question is: what is the difference between photons and neutrinos that makes neutrinos very...
  6. StevieTNZ

    Two Groups of Entangled Photons

    If we have two groups of photons; each group consisting of two entangled photons. We allow one of the photons from each group to interact with another object. If we perform a polarisation measurement on each photon of one group, will the photons in the other group, independent of whether...
  7. A

    Unraveling the Mystery of Photons: What Are They Made Of?

    what are photons made off?? this might be a stupid question but lets take a proton or a neutron these "elementary" particles are made up of quarks, does photons in its particle state follow the same pattern?? if yes then what constitutes into a photon?? thanks
  8. D

    Number of photons in a mode (in a blackbody)

    # of photons in a mode (in a blackbody) I've been going over a proof again concerning the thermal average number of photons in a mode of a cubical blackbody of length 'L' at equilibrium with a reservoir of temperature 'tau' (has a small hole in it, etc. etc. etc.) The logic went as usual...
  9. 2

    Photon & Electromagnetism: Exploring the Link

    At first, I want to say that electromagnetism is my weakest physics branch so this question might sound silly. Electromagnetism unified electricity with magnetism and the quanta of electromagnetic force is a photon. Is there any link between magnets and photons? Is there any link between...
  10. X

    Do gamma and X-ray wavelength photons also exist as collapse able wave fx's?

    This question has gone unanswered by our friends in nuclear/atomic threads. My question relates to a solitary Tc^99m decay in particular, and to gamma rays in general. If light is a collapse able wave function, are different wavelength energies the same, ie gamma, x, radio, etc. My...
  11. A

    Photon Mass: Particles or Waves?

    What I have been taught in high school physics says that all particles can be interpreted as waves, and all waves as particles. By definition, particles have volume, and therefore must have mass (in a physical sense). Photons are always regarded as waves, but therefore could always be...
  12. J

    Investigating Attenuation of X-Ray Photons with Al Thickness

    Hi Would anyone help/advise me? I've done 2 experiments as part of my radiography degree. One was to investigate the effect of the ISL on a beam of x-ray photons. I could understand this, produced 2 graphs (Mean reading against distance (uGy) and Mean reading against 1/d squared) and wrote a...
  13. K

    Do photons gain mass in the presense of a black hole?

    I understand how gravity can affect photons or light waves (Einstein's equivalence principle). But I was wondering if there are any theories that predict that a photon might somehow acquire mass in the presence of a singularity.
  14. B

    What are the differences between photons and gluons?

    It is said in particle data sheets that photons and gluons both have 0 mass, 0 electric charge, and a spin of 1. If the two particles both have the same properties, then how is it possible to tell the difference between them? Can a complete table of properties comparing the photon with the gluon...
  15. vhbelvadi

    How does the number of photons determine wavelength?

    While reading on Cerenkov radiations I've been coming across statement to this effect: It has been noted that the number of photons at a particular frequency or wavelength, as it turns out, is proportional to 1/l2 How is that (mathematically) so?
  16. V

    Where did the Photons in Polariton Laser came from?

    Hello, iam new to polariton lasing and have to hold a speech about it. So I've got some questions and it would be great if someone could answer them. In a polariton laser, lower polaritons form a quasi bose einstein condensat at k=0. [My most important question:] I don't get it when the...
  17. vhbelvadi

    Number of Photons and wavelength?

    I read recently that the "number of photons emitted by a charged particle is inversely proportional to wavelength" with regard to Cerenkov radiations. (The wavelength refers to the radiation which is of comparatively shorter wavelength, towards the blue-end.) I was looking for a more detailed...
  18. StevieTNZ

    Entangled Photons: V/H & 45/135 Polarisations

    When you create a pair of entangled photons, are they simultaneously in a superposition of V and H, and 45 and 135 polarisations?
  19. N

    What is the meaning of a collinear photon in Quantum Electrodynamics (QED)?

    Please teach me this: What is collinear photon (emission)?I do not understand why call ''collinear'' photon. Thank you very much for your kind helping.
  20. J

    Photons of different energy follow different geodesics?

    a massive body like a star creates a warped spacetime in its vicinty. this warped geometry of space is reflected by the geodesics appropriate to its mass. a photon passing by this massive object is not, as per GR, "attracted" to the star via some "force", but rather simply follows what it sees...
  21. P

    Probability amplitudes and photons

    Suppose a Fock state contains 2 photons, both in the same spacetime mode and having the save (vertical) polarization. So we can write this state as |2>, or, if we want to emphacize its vertical polarization, we may write |2v> or |2v,0h>. Suppose now we want to measure polarization in the...
  22. P

    Bose-Einstein condensate of photons

    Does the photon number state of n photons in the same mode (i.e. Fock state |n>) constitute a Bose-Einstein condensate of photons?
  23. T

    How many photons are emitted by accelerating electron?

    And what determines the wavelengths of the emitted photons? In jumping from one orbital to a lower one, how many photons does an electron generally emit? Is it just one, with a wavelength determined by the distance jumped between orbitals, or many? In the case of an electron being...
  24. P

    Using Boltzmann distribution law to find Temperature (1% of photons> 1eV)

    Homework Statement Use the Boltzmann distribution function to calculate the temperature at which 1.00% of a population of photons will have energy greater than 1.00 eV. The energy required to excite an atom is on the order of 1 eV. The Attempt at a Solution I attached my attempt but...
  25. J

    Problem with quantum entanglement of photons

    I'm trying to follow the logic to the quantum theory of the entanglement of light. I'd like to ask a question about one of several problems I'm having with the entanglement. If: Photons (Light) have orthogonal electric and magnetic waves. And: In discussions of polarization of light for...
  26. A

    How to get number of photons?

    Homework Statement I have this task: A collimated monochromatic beam of photons with intensity 10^20/s and energy 10 MeV is directed to a iron absorber with thickness 3 cm. What kind of interactions are expected? Suppose, each interaction removes one particle from the beam. I need to know...
  27. D

    Calculating the number of photons in a wave

    Homework Statement I am trying to answer a relatively simple question. I have been asked to compare the number of Photons in 100W light bulb and a 100W FM Radio per second. Having crunched the numbers it appears that an FM Radio wave gives out more Photons per second than a light bulb. Is...
  28. S

    Quantum Optics help - Single photons

    Hi. Was wondering if anyone could recommend some good introductory quantum optics textbooks? I have a good grounding in quantum mechanics, but have never looked at quantum optics. Basically my problem is that I am doing my project work in this field(to start in a couple of months) and have...
  29. A

    Photons in amorphous glass slow because?

    In solids it is the interaciton b/w the phonons and photons that give a lower effective speed. not absorption & re-emmision. (see ZapperZ's post: https://www.physicsforums.com/showthread.php?t=511177 ) but in amorphous glass there are [B]no[B] phonons. So why does light 'slow down'...
  30. C

    Can Particles Like Light Be Accelerated in a Particle Accelerator?

    Is it possible to accelerate particles like light in some type of particle accelerator? if it is possible why? if it's not why not?
  31. S

    How are scientists able to distinguish between photons?

    how are scientists able to distinguish between photons? Photons are smallest quanta of EM (electromagnetic) radiation. How are scientists able to distinguish between photons except by correlating with position in time-space, frequency, spin etc? For example: In the article below: How are...
  32. B

    Are photons from a light bulb and the sun the same for our bodies?

    Photons activate hormones in us which cause us to wake up. I'm wondering if there is a different between the photons coming from the sun and the photons coming from a light bulb? Do these both have the same effect on the body? I've put this in the classical category because of optics.
  33. O

    Excitation by photons smaller than energy interval

    Hi people, Time-dependent perturbation theory allows for transitions to excited states, through a sinusoidal perturbation whose frequency is smaller than the energy difference between the states. (That is, P_{a \rightarrow b}=\frac{sin^{2}[(\omega_{0}-\omega)t/2]}{(\omega_{0}-\omega)^2}...
  34. D

    Time, Light, and Relativity: Photons from the Past

    Relativity states that as anything approaches the speed of light, time slows down. At the speed of light, time stops. If this is true then photons from 1 billions light years away would not exist in the present unless they stopped and remained stationary for 1 billion years after reaching earth...
  35. DrChinese

    Does CMBR include free photons?

    Broken off from another thread: So here's a simple question for which the answer is not so simple using current theory: Do the photons from the CMBR exist free and independent of observation? Because some people say that photons are mediators of EM force but are otherwise abstractions. For...
  36. Spinnor

    Universe with only e+ , e- , photons, and gravitation?

    Universe with only "e+", "e-", photons, and gravitation? In theory could we have a universe with only electrically charged spin 1/2 particles and antiparticles of mass m and electrical charge e and that interacted only gravitationally and electromagnetically? If so could we fine tune things...
  37. S

    Calculating energy of photons

    Homework Statement When a certain light beam strikes a certain metal, 1 eV electrons are liberated. If the beam is replaced by one of half the original wavelength, 5 eV electrons are liberated. What is the energy of the photons in the original beam? Homework Equations Part of my...
  38. N

    Phonon vs Photon: What's the Difference?

    What is the difference between the photon and the phonon ?
  39. A

    Electronic Excitation: Absorbing and Re-emitting Photons

    What's happen when a molecule or an atom is already in an excited electronic state and is hit from another photon? Example 1: my molecule absorbs a VIS photon (there's a time in which the molecule is an upper electronic state before the re-emission or etc. etc. process) what happen if another...
  40. X

    Applying directed energy to photons

    What would happen if energy would be applied to a photon in some direction? Since photons always travel at c, it shouldn't change their velocity no matter from what direction the energy would come, but what does applying energy do? I could imagine that if you applied energy from the front that...
  41. K

    What Makes Photons, Bosons, and Gluons Massless?

    According to Einstein's equation E=moc2, any object which has mass has energy, and, conversely, anything which has energy must also have mass. Then, why are photons, bosons and gluons said to be massless?
  42. M

    Light polarization vs photons spin

    If I'm not mistaken, right (or left) circularly polarized light consists of photons with right-handed helicity (or left-handed). But what are the states of the photons in the (horizontally or vertically) linearly polarized light? Are all the photons in a superposition of two helicity...
  43. Vorde

    Virtual Photons over Large Distances

    It's my understanding that virtual photons are 'allowed' because their existence does not violate the uncertainty principle (the form using ΔE and ΔT). If this is the case though, how is the electromagnetic force transmitted over large distances?
  44. R

    Photons & their relation to charge

    Hello All, I am having a doubt. Generally, the electromagnetic radiation/photons are created when electrons accelerate in a field or fall from their excited state to normal state etc. Photons are waves of electrical & magnetic fields. Since, photons originate from electrons, shouldn't they...
  45. U

    Questions about photons. Their interaction with time and their life-cycle.

    Do photons experience time? If they move at the speed of light then special relativity states that they do not, but then does that mean they do not move forward in time which means the distance they travel is instantaneous (thus it is infinitely fast), which we know is not true (as they move at...
  46. R

    Exploring the Electromagnetic Properties of Photons and Their Attraction

    Is there any electromagnetic property of photons that make them attract towards each other?
  47. K

    Center of mass of a system of two photons

    How can one calculate the center of mass of a system made of two photons? I know that they have zero rest mass so the first intuition is to say that they do not have any center of mass. However, I read about something called center of momentum which is defined as a frame in which a moving...
  48. thenewmans

    Opposite spin of entangled photons

    Quick question: Do entangled photons have opposite spin? If so, how do we know? I mean I know that they should because of conservation of angular momentum. But Cos(0)^2 = Cos(180)^2. So it looks like they’re the same. Or am I missing something.
  49. J

    Light waves, photons; sound waves, phonons; water waves, hydrons ?

    Light waves, photons; sound waves, phonons; water waves, "hydrons"? Light waves are made of photons; sound waves are made of phonons; so are water waves made of "hydrons", and if so, how would they behave, and would it be possible to make a water laser or something similar based on these particles.
  50. C

    What is the issue with photon rest mass?

    I read somewhere else on this forum that photons have zero invariant mass, only relativistic mass. Here is my problem, Einstein's equation for relativistic mass says that is invariant mass is zero then so will relativistic mass at all velocities. If a photon does have relativistic mass, then...
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