The photon is the quantum of the electromagnetic four-potential, and therefore the massless bosonic particle associated with the electromagnetic force, commonly also called the "particle of light". Use this tag for questions about the quantum-mechanical understanding of light and/or electromagnetic ...

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Does the entropy of the universe change as expansion exceeds the speed of light?

The potential encoded information in a photon that is at the edge of the observable universe would seem to be lost as the universe expands. Does that loss of information contribute to the overall ...
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What could cause a diode laser to be emitting the half-harmonic of the fundamental frequency?

I have a 405nm laser which is seemingly outputting a small portion of 810nm light. I am wondering what mechanism this could be caused by. Is this a down-conversion phenomenon or perhaps just another ...
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Can a photon have little to no energy and/or speed?

Can a photon move more slowly than the speed of light and behave 'non-relativistically,' so to speak. Perhaps another way to express my thought is: could we stop a photon from moving?
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Brightness of light sources

I would like to know what determines the brightness of light.I'm confused,After hours of reading i got these definitions mixed up i need to link them together : Light intensity Brightness of light ...
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Why doesn't light vibrate in-situ?

Light always moves in a straight geodesic path (shortest distance between 2 points in flat space where gravity is homogeneous) across 3 dimensions of space and 1 dimension of time. It is consists of a ...
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Photon Energy and Einstein Equation $E=mc^2$ [duplicate]

If the mass of a photon is zero and these ones travel to the light speed, how may I explain Einstein's equation $E=mc^2$? It is well known that the energy associated to a photon may be calculated ...
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79 views

What happens to theoretical physics if a photon has non-zero mass?

I want to know the theoretical implication if photons have a non-zero mass. What happens to the Maxwell equations? What happens to QFT? If the photon have mass it can decade?
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How can we see objects that are so far away? [duplicate]

The HUDF used to be the deepest image of the universe ever taken by the Hubble telescope, the furthest star in this image is 59000 light years away. The star in question: Now Imagine a light ...
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1answer
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During transition of the electron from high to low energy state, is the photon released always of same energy?

Is it not possible that during transition of an electron from higher to lower energy state, it can release multiple photons of low energy instead of a single photon of the exact energy difference ...
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Why are non-linear optics called non-linear?

Looking at the wikipedia article on nonlinear optics you can see a huge list of frequency mixing (or multi-photon) processes. What makes these different from single-photon interactions? More ...
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How do solar cells increase the lifespan of the Sun?

While reading "Physics of Solar Cells" by Würfel, I came across an amusing statement: An interesting aspect arises if we had to pay for solar energy, but could also get a refund for energy ...
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Don't photons pass photons move away at speeds faster than light? [duplicate]

I understand there is a longstanding rule that nothing travels faster than the speed of light... but if you had two flashlight aimed at each other and turned them on inside a vacuum the photons from ...
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How is polarization vector in QFT related to polarization in classical electrodynamics?

As i know in classical electrodynamics polarization shows the orientation of the electric vector in a plane perpendicular to the direction of propagation of light. But in quantum field theory ...
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Quantization of energy of phonons

when taking into account of energy of photons, the relationship $E=nh\nu$ stands because it is said that the Energy is proportional to the frequency of the electromagnetic wave, however when the ...
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What forms does the energy of a photon get converted to after absorption

Assume an object which has a high absorption rate of light. Now my question is, if you shine light on the object, what all forms of energy does the object acquire? I know there will be an increase in ...
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71 views

Why are photons bosonic?

I am studying the quantization of the electromagnetic field. My text quantizes by changing amplitudes to ladder operators, by putting in an action and by imposing bosonic commutation relations upon ...
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4answers
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Photon Emission/Absorbsion from the Photons Perspective [duplicate]

First some assumptions. 1) Photons travel at the speed of light. 2) From the photon's reference spacetime is contracted to 0 length in the direction of photon travel. 3) From the photon's reference ...
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Spin zero photons

As I understand it, the reason why there is no Spin 0 Photon is because the polarisation of an EM field lives in two dimension. Hence we only have two basis vectors, yielding two pairs of ladder ...
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Why is a photon its own anti-photon?

Two properties of a photon that I have considered in trying to answer this myself : Photons are electrically neutral, so there is no need for "anti-photons" to preserve conservation of charge. Take ...
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Compton effect confusion

In a certain question we were told that a photon collided with an isolated electron and as a result its wavelength changed. We were told to calculate the initial and final momenta of the photon which ...
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1answer
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Reflection of light at a microscopic level [duplicate]

I always read that light is reflected by a mirror! My question is how does reflection takes place at microscopic level? Mirror is just atoms and electrons are revolving around the nucleus ! So where ...
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Are there any naturally occurring examples of photons without mass?

I read that a photon is said to have zero mass at zero velocity. Does this mean that they only exist in a state of probability until observed && interacting with some system? And then when ...
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does photon travels in a helical path in an optical vortex?

The wave front of the optical vortex beam is helical. Does it mean that the photon travels in a helical path? When the optical vortex beam is focused on a screen, an annular ring with dark center is ...
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Photons at Event Horizon

The escape velocity at event horizon of a black hole is same as speed of light, So this questions is not about all photons travelling into a black hole, but for those which are tangential to the event ...
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Photoelectric effect – Why does one electron absorb one photon?

When I read about the photoelectric effect, I came across this: "The electrons could not absorb more than one photon to escape from the surface, they could not therefore absorb one quanta and then ...
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How many times can a photon be absorbed?wC

I am just being introduced to quantum physics. I know that in order for a transition in the energy level of an electron to take place, the photon energy must be equal to the difference in energy ...
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Calculating the Magnetic Component of a Photon

I've been trying to figure this out for some time. I have found some formulae on other sites that claim to allow me to calculate the magnetic component of a photon, but I have seen so many variants of ...
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What about a surface determines its color?

Light falls on a surface. Some wavelengths get absorbed. The other are reflected. The reflected ones are the colors that we perceive to be of the surface. What is the property that determines, what ...
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Do photons have/contribute to mass?

Do electrons gain mass by absorbing photons during excitation and releasing them by returning to the ground state? Do photons have mass in the form of energy by $E=mc^2$?
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Are photon energies conserved in general relativity?

As I understand it, both Maxwell's wave equation and the null geodesics of general relativity are scale invariant. Thus an electromagnetic wave can be shifted along a null geodesic without changing ...
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Does the Photoelectric Effect cause any kind of decay?

From my understanding, the Photoelectric Effect knocks electrons off of some metal using photons. Since electrons are being thrown out of the metal, does this cause some kind of decay?
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Deflection Effects on a Spinning Solar Sail

Suppose I have a solar-sail-powered starship flying directly away from a star. The sail is flat and perpendicular to the direction of travel. Now, in order to make the trajectory more stable (we ...
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photon absorption by atoms causes heat?

I have came up with a weird doubt: photon absorption by atoms causes heat? I mean, I was always told that if the photon's frequency is the magic one, the atoms absorbs the photon and goes to an ...
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2answers
71 views

Can photons decay without interaction?

Can photons decay like other particles without interacting with other particles or fields, i.e. by just "being"? In case the answer is "no" - does this have anything to do with them travelling at c, ...
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74 views

Angular momentum conservation at quantum level

how angular momentum of system is conserved when electron jumps higher energy state to lower energy state and photon is emitted(circularly polarized)? i read somewhere that it is NOT conserved .Why?
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Relativistic momentum of an electron

Suppose we have some interaction between a photon and an (initially) stationary electron, and we wished to find the final momentum of the electron. Should we solve this using conservation of momentum, ...
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350 views

Is the virtual photon a particle?

I just saw an exam question: Write down the antiparticle for the virtual photon. The answer was "virtual photon". Is the question even a meaningful one? If so please explain why?
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Is pair production only with $\gamma$ photons?

in my revision guide the section on pair production only mentions it happening with gamma photons, so the question arose whether this is the only way it can happen? This is what the book says: "Pair ...
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Amount of energy received from photons when traveling at near-light speed? [closed]

Special Relativity says that when you are traveling in a rocket ship at near-light speed, in effect your clocks "tick slower", however from your reference frame every one else's clocks "run slower". ...
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Luminal motion and the uncertainty principle

I had a thought the other day about a connection between special relativity and the uncertainty principle. According to special relativity, you need an infinite amount of energy to accelerate a ...
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1answer
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Common CCDs and photons

Is it possible to capture single (refracted?) photons with a CCD in a typical digital camera? Are these more likely due to coatings, etc? I have minimal knowledge of physics, but am curious.
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What happens to the planck distribution if the temperature is set to zero?

BE Problem I am currently working on modelling the density of states and optical conductivity of graphene utilizing the GW algorithm. In calculating the exchange self energy of the system, the ...
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1answer
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2nd law of thermodynamics - thought experiment

I have designed this simple thought experiment that seems to contradict 2nd law of thermodynamics. Could you please find a mistake in my reasoning? ...
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3answers
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Kinetic energy and Potential of a photon

How does the potential and kinetic energy of a photon relate? Do they mean the same thing? Also how does De broglie wavelength and Potential relate?
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Is light a particle with EMF or does it travel in wave? [duplicate]

Is light a particle which has a electromagnetic field around it OR does the particle itself travels in a wave like motion? IS it just the EM field which moves in a wavy motion like ripples? (Please ...
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Double-double-slit with entangled photons

Edited question to face complaints about ambiguity Following experimental setup. Bob uses a nonlinear crystal to create two entangled photons A and B. Consider a standard pair of EPR-entangled ...
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Saturation current vs frequency

In the photocurrent experiment, if the intensity of light is kept constant, but the frequency is slightly increased, what's the effect on the saturation current? I think saturation current should ...
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1answer
56 views

Has someone measured that in the far field of radio waves, the electric and magnetic field oscillate synchronously?

That in near field both fields oscillating 90° displaced is obvious since in antenna rod a lot of electrons will be accelerated and and this oscillation of electric field is escorted by alignment of ...
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Perpendicular doppler effect photons

If I have a photon with frequency f in the lab frame and the observe it in a inertial frame moving perpendicular to the photon. The direction of the velocity of the photon must change but does this ...
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Entanglement of two particles with same values of the entangled parameters?

The question For an entangled state consisting of systems A and B, if A is measured when does the wavefunction at B collapse? contains next entangled state: If there are two systems A and B, with ...