# Photons Definition and 122 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. ### I Question about electromagnetics (waves and particles)

I saw that we can talk about the light as particles (photons ) or as an electromagnetic wave , the question is that do we represent other electromagnetic waves (like microwaves or radio waves ) as particles (like we do with light ) ?
2. ### A Massive QED

I was reading Diagrammatica by Veltman and he treats the photon field as a massive vector boson in which gauge invariance is disappeared and the propagator has a different expression than in massless photon. After some googling, I found that this is one way to formulate QED which has the...
3. ### I How do photons transfer energy?

Hi! So I know about the electron-photon interaction but what about photon-photon interaction? I mean, I do know there is a very small chance for them to interact, but how else do they transfer energy in order to get from Sun to Earth, for example? When it comes to sound waves I get it, for...
4. ### Photon and Neutrino detector and photon trawl device

Summary:: Hello I am a writer and presently working on treatment for a science-fiction story. So I am not a scientist, just a neophyte interested in science and wanting to write a fiction that would not be too far-fetched and that would make some sense for everyone, including the scientific...
5. ### I A quasi-analogy of wave-particle duality?

One Major question I have about wave-particle duality of say a photon... Could we describe it like a rock falling vertically into a still pond. Around this point of contact we establish a circular wall which detects the contact of the wave. Two things are evident here: the rock keeps on moving...
6. ### Calculate number of photons absorbed

I've tried to solve this by calculating the number of photons. I've done this by calculating the energy of one photon, taking h*c divided by lambda. h*c is 1240 eV*nm and lambda is 10 nm. This gives me 124 eV. I then divide the total energy by the energy of one photon, 50 keV/ 124 = 403 photons...
7. ### I Can two photons cancel each other?

in classic electromagnetism, two plane waves with the same frequency and -1 phase difference can cancel each other. due to conservation of energy, I assume that photons (that so far we have treated as sort of EM energy chunk traveling through space) can't truly cancel each other. then, what...
8. ### B Photon, gluon, Higgs

The photon and the gluon in the Standard Model do not interact with the Higgs field and are hence massless and travel at the speed of light. Is there a simple explanation why these two elementary particles are the exceptions?
9. ### I How does this experimental result show photon emission?

First I'll explain my understanding, because I'm not very confident in it. The main point is that the electrons are ejected and then accelerated to a very high kinetic energy. Then they start smashing into the anode. Most will go through a series of collisions before completely stopping, so that...
10. ### I EM field strength in curved spacetime

It seems a gravitational field does not alter the electromagnetic field strength. Is this correct? My reasoning: With no gravity, field strength is: F_{\mu\nu} = \partial_\mu A_\nu - \partial_\nu A_\mu Introduce gravity: \partial_\mu A_\nu \rightarrow \nabla_\mu A_\nu = \partial_\mu A_\nu +...
11. ### I Momentum of photons

We know that photons (light) are massless but they have momentum. Now suppose I am in the space far away from planets/stars that there is no external force exerts on me, if: 1- I turn on a flashlight (torch), would I be pushed in the opposite direction which the flashlight is facing (Newton's...
12. ### I To which particles does ##p=mc## apply?

In A.P. French's Special Relativity, the author said the following, As I understand, photons are massless, so I don't think the last equation above applies to photons, but then, when deriving it, he used an equation proper to photons (##E=pc##). So in which context is ##m=p/c## valid?
13. ### I Speed of a photon in one frame as viewed from another frame

According to the 2nd postulate of Special Relativity, speed of light in vacuum is the same in all inertial reference frames. If I take a beam of photons and see the other photons in the beam from a frame of reference of a single photon, do they look stationary or moving at the speed of light...
14. ### Photons bending

Are photons vibration or is vibration the transport medium for light? In what kind of medium are photons more transferrable?
15. ### I How to Determine a Photon's Wavefunction After it Collapses?

Suppose one measures the position of a photon without destroying it. From my understanding, the wavefunction of the photon should collapse, and will return to a more spread out state over time. How would one calculate this, specifically the rate at which the wavefunction spreads out from the center?
16. ### I What will eventually happen to all EM waves / photons?

I'm not an expert in this matter, and at best only aware of some superficial facts and a layman's understanding of them. So please forgive me for any ignorant mistakes in my thoughts, and kindly point them out to me. Going by the Lambda-CDM model, the expansion of the Universe will eventually...
17. ### The attractive force of photons on other photons & matter

Summary: massless particles (or at least photons) are attracted to other photons and to matter, but which is most attractive, and why ... Summary: massless particles (or at least photons) are attracted to other photons and to matter, but which is most attractive, and why ... I am really...
18. ### Finding the unitary matrix for a beam splitter

Hello, I have some trouble understanding how to construct the matrix for the beam splitter (in a Mach-Zehnder interferometer). I started with deciding my input and output states for the photon. I then use Borns rule, which I have attached below: To get the following for the state space...
19. ### How can I calculate the number of photons with this data?

The problem says: A radio station emits electromagnetic wave with a frequency of 100MHz (102*106 Hz). a)What's the energy of this radiation's/glow's photon? (Solved, i found 6,63*10-26 J) b)Compare your calculation with the energy of another visible radiation/glow, with a wavelength of 600nm...
20. ### Photons in a Laser Cavity

Homework Statement The laser cavity is formed by two mirrors separated by 15 cm. One of the mirrors has an ultra-high reflectivity and the output mirror has the much lower reflectivity of 99.5 %. How many photons are there in the cavity?[/B] The power of the laser is 1mW and the wavelength is...
21. ### Quantum state of entangled photons

Homework Statement Suppose two polarization-entangled photons A and B in the following Bell state: $$\Phi=\frac{1}{\sqrt{2}}\bigl(\left|H_{A},H_{B}\right\rangle + \left| V_{A},V_{B}\right\rangle\bigr)$$ 1. What is the state if the photon A passes through a...
22. ### I About Stimulated Emission

I understand the concept of stimulated emission and how it works as light amplification, but a certain technicality in its process eludes me. How is the inciting photon actually interacting with the electron that falls to a lower energy level? In every physical interaction that I know of there...
23. ### I What is meant by coupling?

Hi, I have come across the word coupling a few times looking at surface plasmons and surface plasmon polaritons. I was wondering if anyone could give me a better understanding of what it means for something like a photon to couple to a plasmons to creating a plasmon polariton. From what I'm...
24. ### How is light reflected?

Simple question. Are photons reflected as is from a surface like a mirror, or is the reflecting surface atoms capturing the photons and re-emitting them?
25. ### I What happens to the energy lost by photons in gravity?

After I read Martin_K's post of 4:14 Oct. 30 on the frozen image of an object just before it fell through a black hole's event horizon, the next few minutes I was jumped by a handful of related ideas. First, the frozen image scenario is illustrative because when photons are frozen in place...
26. ### I Difference between an electromagnetic field and a photon?

So I'm kind of confused. The way I understand it, an electromagnetic field is just a regular electric field viewed from a relativistic point of view, meaning that since we see the charges moving relative to us, we feel like the particles and the fields created by them come closer together (I...
27. ### Why is a state with large number of photons not classical?

In the last paragraph of these notes, https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2016/lecture-notes/MIT8_04S16_LecNotes3.pdf, it says how a state with large number of photons is not classical. Why is that? I thought quantum mechanics' laws were most applicable when we...
28. ### On board a laser powered solar sail

Can an on board laser be used to propel a solar sail space craft if the laser is pointed at the sails ? Would Newtons third law affect the laser and maybe prevent the ship from moving ? Thank you for answering my very ignorant questions. NineNinjas911
29. ### B Do photons interact with spacetime?

The way I understand this is that Relativity says space-time is like a field that's affected by the way mass moves through it. Photons are massless so is this why the speed of light is the same in all reference frames?
30. ### Do photons carry the electric field?

In basic electrostatics any charged particle will produce an electric field at every point in space, and will have electric filed lines spreading out radially. E = kQ/r^2 The Standard model of particle physics says that the Photon is the force carrier for the Electromagnetic force, just like...
31. ### Difference between emission due to absorption and reflection

What is the difference between the photon interaction mechanisms in emission due to absorption and reflection?
32. ### B Why do scientists claim that there's true randomness?

When we look at the cars on the road, it appears like where they are driving is random, their directions appear as random. But the drivers don't drive into random directions. It appears random to us because we don't know the thoughts and intentions of the drivers. If we knew almost everything...
33. ### Image formed by photons

General question regarding how images are formed. As you move your eye "detector" around a illuminated room. The is image of lets say "a book" is in every position in the room at a given time correct--even before you "look" at it? The photons reflected off the book have formed an image of the...
34. ### I Two coherent sources

If you have two similar coherent sources which are separated from each other by a barrier. Now one source sends particles one by one into one slit and the other sends particles into the other in a double slit interference experiment. Now, the photons are always undistinguishable, so they should...
35. ### Reasoning for photon having no mass

Other than relativity is there any theory, proof, experiment, etc. that tells us a photon has no mass? i.e. Is the concept of zero mass solely derived from relativity
36. ### B Light, Photons, Waves, Particles. Wave, Particle Duality.

Me and my friend have recently (half a year ago) had a huge debate, between ourselves, about the wave-particle duality. We took sides in light being a particle or a wave. I was for particle he was for waves. At the end of a hot-filled week of arguing, the debate ended up with the acceptance of...
37. ### A Why light beams attract or repel each other even when they don't have charge

Hi, Can someone please explain as to why light beams attract or repel each other even when they don't have charge. Seems like it behaves like two current carrying parallel wires. There is very little material about this which goes completely above the head. Thanks
38. ### Problem about momentum uncertainty

Homework Statement According to the Big Bang model of cosmology, the universe has been expanding since some initial time (call it t = 0) when the temperature was infinite. At early times, the temperature T scales as t^1/2 . The current temperature is about 3K. Consider the part of space which...
39. ### B Why do photons allow Doppler shift

If we (a detector) are moving toward a star that emits a single photon (due to its distance) and that photon hits our detector, it will be blue shifted. My question is why. If the color of a photon is a reflection of its energy level and since the speed of the photon is always coming at us at...
40. R

### B How does force transfer through an object microscopically?

I heard that you can never really touch anything. I also heard from an article that the reason why your butt doesn't fall through your chair is due to forces. Here is a short excerpt: "Cracking like lightning through the void, all the specks of electrons and the specks of nuclei are constantly...
41. K

### I Does the wave function shorten when approaching light speed?

Relative to the observer, objects shorten when approaching the speed of light exponentially. Does this rule also apply to the wave function? Does this rule also apply to massless particles like Photons? Or am I just simply forgetting something?
42. C

### B Aim the solar furnace up! :)

If one was to create an array of X amount of mirrors tracking the sun and shining on a targeted spot in the sky to create ions at specific elevations similar to a ladder, could this cause an ion column to create a continuous charge of energy from the upper atmosphere to hit the ground? Lets...
43. ### I Are any electrons ejected below the threshold frequency?

I've been reading about the photoelectric effect, and something got me thinking. If the frequency of light shone onto the metal is below the threshold frequency, no electrons are liberated from the surface of the metal, since electrons absorb quanta of energy, so if that light is shone for a...
44. ### Infrared radiation from friction

Hello all, I was driving down the road yesterday, and I realized that I don't really have a solid grasp on how frictional forces cause infrared radiation. Can anyone explain, or direct me to a resource that explains, how this happens at the atomic level? I am thinking that the work done...
45. ### A Simple way to create entangled photon pairs?

Two photons arrive at a hypothetical 50:50 Beam-Splitter with no phase shift between reflected and transmitted modes. One enters the Left side and the other the Bottom side of the BS as shown in Fig.1 of the link below: https://drive.google.com/open?id=0B5JsDLKoUSA5emk5Qk9nUHVIelE Each photon...
46. ### I Wave-Particle Duality of Electrons

The wave-particle duality of light was demonstrated first with Thomas Young's 1801 Interference Experiment...and then more clearly with the Double Slit Experiment. Both of these were done with light (so photons). My question is -- How did we come to understand the same of electrons? Did we...
47. ### B Are vertical lines from squinting a Quantum phenomenon?

When looking at a night light with almost closed eyes, I notice that the light becomes a vertical line. When tilting my head 90 degrees, the line becomes horizontal. Can this be explained by Heisenberg's Uncertainty principle? This question has been asked in another thread : "Squinting at...
48. ### How many Photons per second are entering the pupil?

Homework Statement (a)Estimate the number of photons per second emitted by a 100-W lightbulb, assuming a photon wavelength of 550nm.(b) A person can just see this bulb for a distance of 800m, with the pupil dilated to 7.5mm. How many photons per second are entering the pupil? Homework...
49. ### I Interpretation of the photoelectric effect

The photoelectric effect is usually presented as an example disproving classical electromagnetism as viable model for interaction of light with matter and as evidence of quantization of energy in the electromagnetic field, i.e. the existence of photons. I would like to discuss a thought based on...
50. ### Rotate only H-Pol Component of Light beam by 90 degrees

Is there a single optical component that will do the following? Do nothing to the horizontally polarized input, but rotate the vertically polarized input by 90 degrees, so that both beams finally emerge with the same polarization? I am looking for a single element that will replace the...