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. victorhugo

    B Silly questions about the behaviour of photons

    Please bare with me as I haven't studied much at all about light in high school physics. When we see an object as red, does that mean the way that photons bounce off of it cause only light at the wavelength of 'red' to be reflected? If so, wouldn't that mean that the light reflecting off of a...
  2. N

    I How to determine the energy of photons released in decay

    What equation would I need to use to determine the energy of photons released when radio-isotopes decay? I would like to figure out the energies of photons released when tritium and plutonium 237 decay.
  3. entropy1

    B What is ¨unpolarized¨ light exactly?

    I learned that light can be linearly and circularly polarized. It can also be a mix of several simultaneously. Is there also a polarization state that is neither linear nor circular? (ie not polarized?) If so, what is it?
  4. entropy1

    B Entanglement, correlation and randomness

    I was wondering. In this example I use polarized photons, but maybe it is applicable to electrons and spin also. We can prepare two completely unentangled polarized photons, and send them in opposite directions to two detectors preceded by a filter at particular angles. Both of them will show a...
  5. entropy1

    B Thought experiment violates Heisenberg? (of course not)

    Suppose we have an elementary double-slit experiment: A laser fires individual photons through a double slit at a detectionscreen made of atoms. As we fire photons, an interference pattern emerges, exposing the momentum of the photons (the frequency of the laser). So, we have registration of...
  6. DavidReishi

    I Photons on a CCD Vs. the magnetic or electric vector?

    If my understanding of the theory is correct, the fact that very low light is detected on a CCD at points demonstrates light's behavior as a particle. For, if light acted as a wave in this instance, we would see not points but vectors (lines) registered on the CCD, that is, representing entire...
  7. A. Neumaier

    Insights Misconceptions about Virtual Particles - Comments

    A. Neumaier submitted a new PF Insights post Misconceptions about Virtual Particles Continue reading the Original PF Insights Post.
  8. DavidReishi

    I Technical question about modern Double Slit w/ one photon

    My question is about the experiment in which detectors are used behind the slits to determine which slit the photon goes through. Specifically, it's about the detectors themselves. What I found is that these detectors are some kind of photoelectric detectors. My question is this. During the...
  9. DavidReishi

    I Inverse square law and ensembles of photons

    I'm trying to visualize the effect of the inverse square law, not on a direct source of light, but on scattered light carrying visual data, such as that responsible for our everyday sight of things as well as our images of Earth from satellites. It seems to me that it should be true that, while...
  10. zonde

    I Are photons from atomic cascade directly entangled?

    As far as I know first entanglement experiment used polarization entangled photons from atomic cascade. As I understand atomic cascade produces entangled photons from two electrons that emit photons by falling from the same orbital to the same lower orbital (so they have the same spin). Now it...
  11. T

    I Do the Energy-Momentum Transformations apply to Photons?

    I apologize in advance for this dumb question, I think I know the answer but I just want to be sure.A photon has energy E = pc = hf Do the Energy-Momentum transformations: apply exactly to photons? Or must we introduce certain corrective terms? Let's say all this takes place in free space.
  12. entropy1

    B Interference patterns of electrons vs. photons

    I understand that fermions are subject to the Schrödinger equation, but photons are not. I understand that interference patterns of electrons are governed by the Schrödinger equation, but with photons it is different. If I understand correctly, then what is the nature of this difference?
  13. StanEvans

    Photon Reflection: Questions & Answers

    I was wondering while doing some work on the photoelectric effect about how it is that photons interact with their environment. The question that I have is why or how does a massless particle such as a photon reflect of a surface such as a mirror. My first thoughts went to collisions with the...
  14. Karan Punjabi

    EM Waves & Photon Explained - Learn Here

    Guys, I am studying about electromagnetic waves and I'm not able to differentiate between a photon and a em wave or I'm not understanding it. Please help me to solve it out. Thanks
  15. S

    Understanding Photons: How They Emit Different Types of Energy

    I know that photons give off light (in a "stream") but i recently learned that photons can give off all kinds of energy (or waves), like infrared, ultraviolet, gamma rays, and more. My question is: Do photons give off all kinds of radiation or do u have to influence them somehow; How do you make...
  16. chimera27

    Gravity & Photon Trajectory: Is Wavelength a Factor?

    This seems like an intuitive question, but I've seen some rather contradictory answers on it and am not sure what to think. What I'm curious about is if two photons, A and B, with wavelengths 900nm and 200nm respectively, both start from the same point on the same trajectory passing near a...
  17. C

    Particles, Photons and Matter: Exploring the Invisible Forces of Our Universe

    [Mentor's Note: Post split off from this thread: ']https://www.physicsforums.com/threads/are-tracks-in-collision-experiments-proof-of-particles.857255/] I'm sorry, If a photon is a particle of energy, or bosom, and neither matter nor energy can be created nor destroyed, what happens to the...
  18. nmsurobert

    How Many Photons Are in Each Bit of a 10 Gb/s Optical Signal?

    Homework Statement An optical communication system operating at λ = 1550nm is transmitting pulses at 10 Gb/s. The magnitude of the optical pulses is the same. Calculate the number of photons received within each bit. Assume that the received average optical power is 10 mW. Homework...
  19. Comeback City

    Mathematical Proof that Photons have No Rest Mass

    Using these equations I am about to prove that photons have a rest mass of zero (mathematically) ________________________________________________________________________________________ E=hc/λ Photon Energy Equation E2=(pc+mc2)2 Mass-Energy Equivalence with Momentum Equation p=h/λ Momentum...
  20. Raiden60

    Can You See a Photon Beam Between Two Reflective Surfaces?

    Let's say you have 2 100% efficient reflective surfaces exactly parallel to each other. If you theoretically managed to place photons at exactly 90° angle of incidence. So this beam of visible light (and I want to stress that) is going to stay there as long as there is no interaction, the beam...
  21. sciencejournalist00

    B Is entanglement preserved on re-emission of photons

    What happens if I create entangled photon pairs and then pass them through a phosphorescent coating so they are absorbed and re-emitted? Is entanglement preserved? In my case, I have succeeded and created UV entangled photon pairs in my latest experiment but I could not see them with my naked...
  22. entropy1

    Probability of entangled photons passing filter independent?

    Consider two polarisation-entangled photons A and B fired at two polarisationfilters that are at a certain angle α. Are the probability that A is passing its filter and the probability that B is passing its filter indepedent probabilities? I am aware that is probably an incredibly stupid...
  23. sciencejournalist00

    Can calcium ions be used as a source of entangled photons

    By heating calcium in a tantalum oven and subjecting the calcium vapor stream to blue laser, calcium atoms absorb the blue photons and emit two photons of lower energy as they decay twice. Once they decay from the highest excited state to an intermediate excited state, and then from the...
  24. M

    Entangled Photons: What Happens to Photon B? Can Two Combine?

    If a photon A is entangled with photon B and one somehow destroys photon A, what will happen to photon B? Will it also get destroyed? And can two entangled photons combine into one?
  25. SireJoe

    Is harnessing quarks and gluons for observation a possibility in the future?

    Are we able to observe other particles without the use of photons? Are we able to harness electrons or positrons to do this? I ask because of quantum entanglement. Creating a use out of quantum entanglement seems to rely on observing one of the particles and thus either getting its spin or...
  26. sciencejournalist00

    Is this mathematical proof that beamsplitters entangle photons

    Prof. S Lakshmi Bala from Department of Physics, Madras, India writes a blackboard of equations which show how beamsplitters used alone affect the wavefunctions of input photons. It seems that it depends on the number of photons you use and in which input port to get you a different entangled...
  27. 6

    2 photons, same starting point/time, same end point

    How would 2 photons with the same starting point& time and same end point view each other's positions, time and distance from start to end point. I recognize this is likely a bad question, so any input would be helpful.
  28. Raptor112

    Bose Equilibrium Distribution and Atomic Units

    Homework Statement For my project I need to compute the average the number of photons given by the expression: ##\bar{n}= \frac{e^{-\bar{h}\omega/\kappa T}}{1-e^{-\bar{h} \omega / \kappa T}}## where ##\kappa## is the Boltzmann constant and ##\omega## is the oscillator frequency. For the...
  29. L

    How does a banana change the color of light?

    I understand the basics of how vision works, but correct me if I'm wrong at any point. Electromagnetic radiation is emitted from the sun as a byproduct of fusion, in the form of photons. These photons travel a vast distance at an incomprehensible speed, and bounce around the atmosphere a bit. A...
  30. Molar

    Do Photons Challenge the Concepts of Mass and Energy in Physics?

    1. From E=mc2 , m= E/c2 "c" being huge, "m" should be small. 2. m= mo/√ [1-v2/c2] if v= c, then m should reach infinity. 3. Photons moving with speed of light are massless particles. All the above statements are confusing me. (1) doesn't match with (2), (2) doesn't match with (3). Please...
  31. edguy99

    Quantum Field Theory intro with flying field disturbances

    Great youtube introduction video about Quantum Field Theory (QFT) from a couple of days ago by Dr Don Lincoln @fermilab. The video and description of a particle being a disturbance in a field and flying through the air at 3:25 is especially compelling.
  32. ellipsis

    Average number of photons in a cubic meter

    It seems to me that light travels (in a room) from everywhere, to everywhere else: A complicated, messy, interconnected network of photons of varying wavelengths which somehow avoid ever colliding with each other. This is what allows me to see things, and other people to see other things...
  33. E

    Do Photons Have Mass and Why Does it Matter?

    hi I was wondering if some one could give me a simpler explanation as to why photons have zero rest mass and of any subject areas I should read up on to better understand this. can we please bear in mind I'm just in 15 so do not have a university grade knowledge. but this is an are of interest...
  34. M

    How hot does Gaseous H2O need to be to emit IR Photons?

    I've read that H2O has an absorption/emission band around to 10 micron range. What conditions are required for photons of this wavelength to be emitted by H20 gas? In particular, how hot would the gas have to be? What amount of pressure is required? Under everyday conditions (like steam possible...
  35. S

    Question about photons traveling on same path

    if photons are traveling on the same path, and in only one direction, are they sometimes passing each other? Or are they staying in sequence? Assume the sources for these photons are variously moving in relation to each other. .
  36. E

    Ionizing Photons: Photoelectric Effects Compared

    This is my first time on this forum and my knowledge may be a bit basic but I was wondering how does the photoelectric effect in the ionization of atoms differ from the photoelectric effect that makes mirrors reflect. They both have incoming photons that give their energy to electrons which move...
  37. AdrienJ

    [Video] Carbon nanotubes & single photons

    Hi, I'm currently doing a PhD about how to use carbon nanotubes as an efficient single photon source. On my spare time I've done a short video to explain it to my familly and friends. I thought you might be interested, so here it is :
  38. M

    Can circularly polarized photons interfere with themselves?

    Hello, I was studying the Bell-state quantum eraser and saw that they placed quarter-wave plates in front of each slit to mark the photons with either clockwise polarization or counterclockwise polarization that were previously either horizontally or vertically polarized. So, now I'm wondering...
  39. W

    A From how far away is radiated energy of photons effective?

    I'm faced with a problem here and would really appreciate anyone's help. The question is paraphrased as follows: The eye can respond to single photons but there are certain requirements from the eye: it must receive at least 100 photons/s to process it. Assume the eye to have an area of 4E-6...
  40. Z

    Are photons really anti-correlated off their basis vectors?

    The wiki page https://en.wikipedia.org/wiki/Bell's theorem states the following which I agree with: Suppose the two particles are perfectly anti-correlated—in the sense that whenever both measured in the same direction, one gets identically opposite outcomes, when both measured in opposite...
  41. P

    Photons & Waves: A Laymen's Query

    Moderator's note: this post follows from the Is light a wave or a particle FAQ Nothing to add, rather just a query. 1) Do Photons move in a wavelike manner, as given by De Broglie's Equation? 2) If so, will the wavelength obtained over there be different from that of the electromagnetic wave...
  42. entropy1

    Are photons passing a polarization filter randomly?

    This may be a dumb question, but maybe someone can help me out: Consider a pair of entangled photons A and B, fired at respectively Alice and Bob who both let it go through a polarisation filter at different angles. Now Alice establishes that half of the photons get through her filter, and half...
  43. entropy1

    B Proof Photons Exist? Evidence & Facts

    This may seem an odd question, but I'd really like to find out: is there proof that photons actually exist?
  44. Ryan Reed

    Do Electrons Excite the EMF or do the Virtual Photons?

    Do electrons excite the EM field or do they just create virtual photons that do? If you could only see EMF excitations, would you see the electrons or virtual photons coming from the electrons?
  45. entropy1

    Bell-state entanglement swapping and retrocausality

    I thought there were already some posts about this, but I can't find them.:frown: In this article of Anton Zeilinger et al. they perform an experiment which suggests FTL or retrocausal influence. Alice and Bob both produce their own polarisation-entangled photon pair, both send one photon of...
  46. S

    Collision of two photons using four-momentum

    Homework Statement Two photon of energy ##E_1 ## and ## E_2## collide with their trajectory at an angle $\theta$ with respect to each other. a) Total four-momentum before collision? b) square length of 4-momentum in lab frame (LB)and in center of momentum frame (CM)? c) 4-momentum of two photon...
  47. K

    Phase of coherent states and single photons

    Hello I am confused and I can't figure out what is meant by phase when it comes to quantum mechanics for single photons and coherent states as I am a new to this field. Best regards
  48. moenste

    Photons of light emitted within the D-lines per second?

    Homework Statement (a) Calculate the energy of one photon of light emitted within the D-lines of a sodium lamp if the wavelength of the D-lines is 589 nm. (b) In a 200 W sodium street lamp, 30% of input electrical energy is emitted within the D-lines. How many photons of light are emitted...
  49. Sterling Lutes

    Amount of single photons to make white light?

    First off I hope I'm putting this in the correct forum. My question is more than just the minimum amount of photons to make a single blip of white light, but more so the base photons in the visible spectrum of light. We have all seen a prism split light into violet, blue, cyan, green, yellow...
  50. Garlic

    Photons passing through transparent material

    Transparent materials such as glass can let visible light pass through nearly undisturbed. I don't understand how photons just pass through atoms in that material. I can understand the concept of atoms not absorbing the photons in that specific wavelengths, but how can photons avoid deflection...
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