Questions tagged [photons]

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 interactions.

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Why is photon quantized in vacuum? [duplicate]

Apparition of quantized energy levels in quantum physics is usually explained in analogy with sound waves in a box (like in music instruments): the wave has to satisfy the boundary conditions and ...
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Why does decreasing the wavelength of light while maintaining intensity decrease current in photo electric effect [duplicate]

I understand a photon with a smaller wavelength is more energetic so for a given intensity, less photons are incident on the electrons and so less photo electrons reach the detector per second. ...
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Question about pair production

Pair production can't occur in vaccum because momentum cannot be conserved because the photon does not have a rest frame of reference, that means that the produced pair has a center of mass frame of ...
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Is acceleration of particle by photon a continuous process?

There are a lot of questions, so I assigned a number to each one. Let's assume we have electron which is not moving in some inertial frame of reference. Let this electron be hit by a photon. Since ...
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Pair production problem, how is energy not conserved here?

The problem is asking me to find the minimum photon energy that would produce an electron-positron pair when it collides with a free electron at rest. This is my attempt at trying to conserve both ...
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How do photons encode images?

When light bounces off an object, the light then travels into the eye of an observer who can then reconstruct the image. Do the photons encode the image and how do they do so?
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Do photons bend spacetime or not?

I have read this question: Electromagnetic gravity where Safesphere says in a comment: Actually, photons themselves don't bend spacetime. Intuitively, this is because photons can't emit ...
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What happens if I replace an IR laser diode with CD burner laser diode in green laser pointer [closed]

I was wondering what happens if CD burner photons hit the crystal NdYO4 and KTP 780 nm wave length where original 808 nm IR pumper diode. I am confused with NdYO4 crystal I know that KTP is a ...
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Why is the photon emitted from a bound electron dropping energy levels not exactly equal to the difference in energy levels [duplicate]

In a problem sheet there was a question asking us to guess why a photon released from an electron dropping energy levels does not have the exact difference. My guess is that the electron needs some ...
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Is it possible to manipulate the polarization of unpolarized light within a medium via Electric field?

You see this setup above. There is an unpolarized light source travelling through a transparent dielectric medium. Think of this triangular shaped medium as a capacitor. there are 3 conductive sides ...
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Photons and electromagnetic waves

If an electromagnetic wave is constituted by a continually accelerating charge, then how can a photon be neutral?
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Is the Compton effect observed during regular x-ray diffraction?

Here we have the standard set up to observe the compton effect. From what I understand, due to the particle like nature of light, when the x ray photons collide with the electrons in the scatterer ...
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What experiment would confirm De Broglie equation on photons?

If we want to check experimentally that, for a photon: λ=h/p (De Broglie equation) Has such experiment been carried out? What is/would be the experimental setup? Wikipedia doesnt show such protocol ...
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Four-velocity vector of light

Please note that my question is not a duplicate, it is not about the speed of light, my question is only technical about the four velocity vector for light, its definedeness, value and constantness. ...
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How do electrons ever receive the amount of energy needed to move up energy levels?

Suppose there is a (blackbody) electromagnetic radiation source. It should emit a finite amount of photons every second with an intensity against frequency graph looking similar to a Maxwell Boltzmann ...
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Does optical density depend upon concentration of a solution?

Does optical density vary upon the amount of salts dissolved in water?
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Why can't we have a massive photon in the Standard Model of Particle Physics

I've heard that in the Standard Model of Particle Physics you can´t have a massive photon whatever you do but I'm having a few problems showing that. I understand that the trick to make this work is ...
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How do I measure the (near) infra-red radiation intensity?

I need to measure how much of the radiation is absorbed by a specific fabric (as in "what percentage of photons is absorbed"). I am interested in 2 LEDs: 650nm (red) and 800nm (infra-red). The ...
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Can Compton scattering increase a photon's energy?

I've read (in Hagai Netzer's book The Physics and Evolution of Active Galactic Nuclei) that "Comptonization in the [black hole's accretion] disk atmosphere can increase the energy of some photons..." [...
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Would an observer going faster than the speed of light in a vacuum be able to observe photons that they emit? [closed]

If I could exceed the speed of light in a vacuum, would I be able to see photons that I previously emitted? Would this be theoretically possible?
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Spatial wave-function of a single photon and its measurement

In the last decade there were several papers claiming that they've measured a "transverse quantum state" / "quantum wave-function" / "spatial Wigner function" of a single photon: Measurement of the ...
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In what sense do photons exist?

In a photon's frame of reference travel is instantaneous, so presumably a photon cannot exist in its own frame of reference. I am not aware that a "beam" of photons interacts via gravity despite ...
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Why can we parameterize a null geodesic such that its velocity is four-momentum? [duplicate]

One principle in general relativity is that the wordlines of massless particles are null geodesics. It also seem to be a commonly stated fact (for instance see eq. (3.62) in Section 3.4 of Carroll's ...
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Photons’ Speed (light speed) in vacuum [duplicate]

In a vacuum, are photons always traveling at 100% light speed? Can photons go any slower in a vacuum? They have no mass which means they can go at 100% light speed, but do they have to?
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Why do we assume the photon has lost all its energy to the electron in the photo electric effect but not the Compton effect?

I was just revising the Compton effect for my exam and was wondering why in the case of the Compton effect we consider the photon as having collided with the electrons and only partially losing its ...
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Black hole gravity and light [duplicate]

When an object is squeezed to its Schwarzschild radius it becomes a black hole (made by density) and its mass does not change (its gravity doesn’t change), but if its mass doesn’t change (its gravity ...
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Quantum mechanics of refraction and reflection? [duplicate]

I learnt that light bends when passing through different mediums is due to the epsilon which is how much the strength of electric and magnetic field in the direction of travel changes in the material, ...
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When do we need fully quantum mechanically treatment of photon?

I'm wondering when do we need fully quantum mechanically treatment of photon.(i.e. quantized photon and quantum matter.) I kind of know that for sub-Poisson(photon statistics) light, fully quantum ...
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Question about field quantization

I'm reading a paper by Rubin, Klyshko, Shih and Sergienko titled "Theory of two-photon entanglement in type_II optical parametric down-conversion", 1994 (link to the paper). I'm stumped by equation ...
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When an electron around an atom drops to a lower state, is 100% of the energy converted to a photon?

Let's say we have an electron around atom. Let's say the electron drops into a lower electron shell. Is 100% of the energy difference converted to a photon? Does the atom recoil at all? Is the any of ...
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Question about Homodyne detection

In this Review of modern physics, section "II. A.", optical homodyne detection is briefly reviewed. In equation (9), it is stated that the fields after the beam splitter are $$E_{1}=\frac{E_\text{L}+...
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Atomic model - absorption [duplicate]

I have a question regarding the absorption of light in e.g. hydrogen atoms: I didn't quite understand how the electron knows that it can only absorb photons of the right energy. I thought that when ...
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Are photons “born” travelling at the speed of light? In what manner do they propagate through space? [closed]

When an electron jumps from a high to a lower energy level and a photon is created, can the photon be detected from all vantage points or only from a discrete position? Does the atom nucleus cast a ...
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Why is the magnitude of a photons 4 momentum vector 0 if it has momentum?

I've just started studying 4 vectors. I understand because a photon has 0 rest mass and travels at c, you can not define a 4 velocity for it, and as momentum 4 vector = mass x velocity 4 vector, it ...
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How does the electron know to “release” the photon at the same angle at which it got absorbed? [duplicate]

This is a quote from Wikipedia: In classical electrodynamics, light is considered as an electromagnetic wave, which is described by Maxwell's equations. Light waves incident on a material induce ...
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What happens after barium sulphate absorbs gamma rays?

BaSO4 will happily absorb gamma-rays, presumably bumping electrons up into outer orbitals. What happens next? I assume fluoresence as electrons drop back. Will the BaSO4 be safe to handle immediately ...
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What is the radiation density in a spherical oven? Does this pertain to the universe? [closed]

Imagine a spherical oven with radius R. The inside of the spherical wall is at temperature T (everywhere) and is assumed emit perfect black body radiation (everywhere). (1) What is the energy ...
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Time-of-flight vs. which-path information for single photon interference

I wonder how different path length and therefore different times required for each path lead to interference. Wouldn't it be in principle be possible to tell which path was taken by looking carefully ...
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Lasers: why when a photon kicks a excited electron, the two released photons have the same polarization?

When a photon kicks a excited electron, why do the two released photons have the same polarization? I was playing with some lasers and am trying to understand the principles at play. I just read ...
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Is luminous flux proportional to number of photons?

I am writing up an experiment to with measuring the probability that a given photon passes through a polarizing filter. For this I need to find the ratio of the number of incoming photons and the ...
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What is the set of independent parameters that can be used to describe a quantum particle like photon or electron?

How many independent parameters are there for fully describing a quantum particle like photon or electron?
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Tensor product of photon number states

I'm looking to compute the tensor product of photon number states. I suspect this is a fairly simple quantum optics problem, but am having the following problem. Consider a qubit which is in the ...
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Casimir effect: where did the pressure come from?

From my understanding of Casimir effect, there exists a difference in energy between plates and that of the outside environment. How did this translate to a pressure difference and eventually where ...
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What is the practical Length of photons, in vacuum for example [duplicate]

From practical point of view we know the width of photons is proportional to wavelengths (experiments for light going through tiny holes: it starts with a specific size of the holes, etc). And nobody ...
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Is the whole electromagnetic spectrum made of quanta?

My question is probably stupid (and maybe has already been answered on other posts), but I just started investigating quantum physics and I am struggling to understand the real meaning of "quantum". ...
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Can two people see the same photon?

In a dark room there are two people and a very faint candle. Then the candle emits one photon. Is it true that only one person can see the photon? Why? And are there any experiments? Edit 2019/4/23: ...
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If you could bottle a photon would it have mass? [duplicate]

Ok. A photon has no rest mass for the good reason that it has no rest energy. But what about a standing wave? There is fair belief that particles are standing waves, so perhaps nothing rests. So. ...
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Threshold frequency in photoelectric effect [duplicate]

If I take a metal sample and I throw light on it, and the emitted photons have frequency less than the threshold frequency required to make an electron come out of that particular metal. So the ...
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Why energy matters for light to escape event horizon?

I like to know why photon with sufficient energy can avoid being pulled into the event horizon? Most light will get trapped and forms into photon sphere while some can escape, isn't light supposed to ...
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What is the length of a single photon?

I don't mean the wavelength, I mean the full length in the direction of propagation. What is the total extent of a single photon in space? Does it even have a length? Edit to clarify: Let's say a ...