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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Compton scattering from moving frame

How does an observer see a Compton Scattering from a reference frame moving with constant speed $\vec{v}$ in the photon direction? (Assuming electron is at rest in the lab frame)
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Why does Feynman say the color of a photon is a property of the source?

Note:I am new to Path Integrals/Lagrangian QFT. In the following link, Feynman says that the color of a photon is a property of the source and that "Energy is already contained (in his Feynman ...
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Could we have the same speed in water with two beams of light and matter?

Could we have the same speed in water with two beams of light and matter? Or the speed of light of the light beam and matter beam both equal 225,000,000 m / s in water. the photon goes at the speed of ...
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Which particles can be described as an excitation of the electromagnetic field?

In my teacher's class notes, came across this definition of light today: Light is an excitation of the electromagnetic field, with photons being the lowest energy excitation. And the way in which it ...
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What happens, electromagneticaly, when you push something?

Scenario : I understand that if I push a boulder the mechanical structure of my body is being used. Cells, molecules, atoms and ultimately the electromagnetic force of my electrons are being used to ...
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Do photons travel instantaneously? [duplicate]

Special Relativity tells us - the faster things travel their time is slower relative to a stationary observer. Do massless particles, like photons travelling at the speed of light, “experience” zero ...
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What is the connection between quantum optical photons and particle physics' photons?

There has been quite some debate amongst users with different backgrounds about the usage of the word photon.[1, 2] The most apparent disagreement was on whether a photon has a wavelength. I don't ...
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In empty space what happens to the lights energy that is lost to the doppler effect go to? [duplicate]

When light makes a long distance journey it's frequency shifts toward longer wavelengths. With $E = hc/\lambda$ being the amount of energy in any given photon (see photon energy); as it travels trough ...
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EM transitions - selection rules

In atomic physics we say that only transitions with $\Delta l = \pm 1$ are allowed. This is since photons are bosons. But for example in nuclear physics we also consider higher order EM transitions ($\...
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What is the current limit for a photon's rest mass?

Before you start yelling, I know that a photon according to theory has zero rest mass. But it is sort of difficult to measure zero, so what is the current best measurement upper limit of this zero? A ...
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Why is there destructive interference in one path in Wheeler's delayed choice experiment?

In the diagram below I've drawn Wheeler's delayed choice experiment, in the case where a 2nd half-silvered mirror has been inserted near the detectors. ...
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Is there interference and which-way-information?

Since interference is possible with a single photon, both interfering signals of a photon, after passing a beam-splitter, arrive at the detector with a phase-shift. No upper limit of this phase-shift ...
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Why can't light deflect a freely suspended magnet if it is an EM wave?

Light is an EM wave but why doesn't it deflect a magnet or show magnetic interaction with magnetic particles?
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Do anti-parallel photons affect each other gravitationally? [duplicate]

We know that two parallel photons have no gravitational effect on each other because they never pass through each other's light cone. The question is, what happens to anti-parallel photons? It ...
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Does a single photon have a wavelength or not? [duplicate]

I have read this question where anna v says: The photon is an elementary particle in the standard model of particle physics. It does not have a wavelength. What exactly is meant by the wavelength of ...
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Center of momentum frame for two dimensions

I have been investigating the scenario of two photons traveling in space (in a lab frame of reference) of different frequencies are about to collide with an angle $\theta$ between them. I set their 4-...
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If space is continuous, then frequency and photon energy too? [duplicate]

Photon is the quantum of light, i.e., energy comes in discrete level. But EM energy is a function of frequency ($E=hf$). And if frequency has a continuous spectrum, then energy level is continuous, ...
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Continuous electromagnetic radiation

What I believe (I am not a physicist) would be called the "classical" description of electromagnetic radiation is that it is an oscillation of magnetic and electric waves, each producing the ...
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What happens to the energy of photon red shifted by expansion of universe?

There are many similar questions but the answers don't satisfy me. I mean it as asked in the title. Some answers given before give the explanation based on gravitational potential energy but consider ...
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Momentum of a relativistic atom

I have been solving a problem where you should derive the formula for doppler effect when a source of light is an atom moving at a relativistic velocity v. I understood everything in the solution ...
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Why are electromagnetic field modes considered a continuum of states (e.g. in the Fermi Golden Rule calculation)?

When we consider a state transition e.g. from 2p to 1s in the hydrogen atom, the energy gets emitted in the form of a photon. As an assumption underlying the Golden Rule application, we expect an ...
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Why is the carrier diffusion neglected as a consequence of a greater penetration depth of the photoexcitation light?

"Since the diffusion lengths are considerably smaller than the penetration depth of the photoexcitation light (...), we can neglect carrier diffusion from our considerations" - THz article ...
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Is it possible to confine a photon in less than its* wavelength?

*(Its, or associated. That is somehow the question). I can think of, at least in principle, a perfectly reflecting optical cavity with dimension comparable to the wavelength of the electromagnetic ...
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Number of electron-hole pairs produced by each photon in depleted silicon

What is the distribution of the number of electron-hole pairs produced by a single photon in depleted silicon? Can you point me to bibliography on this? I am currently interested in 1064 nm photons ...
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Force exerted by light on a differential surface

Note: This is not a homework question I simply would like to understand how force is exerted on a differential surface and the question rose up in my mind while solving this problem. Suppose a ...
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How to interpret the value of the Purcell factor?

I am a little bit confused regading the Purcell effect. If someone tells me that the Purcell factor of a system is 4? then does it means that the spontaneous emission is enhanced by 4 times the ...
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Laser Physics: Question Concerning SPAD

In a Laser Physics lecture, we had had the following Figure for a SPAD (a single photon APD): The reasons for which the detection efficiency, which is plotted on the y-axis, goes down as a function of ...
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Is my understanding of thermal equilibrium of radiation correct?

In the description of a container with a hole as a blackbody, in Peter Atkins | Julio de Paula - Physical Chemistry, It's given that "the container with pin hole emits radiation that can be ...
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Atom in superposition: is it an atom-photon system, and what is the role of the environment?

Let's assume an atom would get from its ground state $|g\rangle$ to its first excited state $|e\rangle$ by absorbing a single photon. Now, let's assume the atom gets from $|g\rangle$ to $\frac{1}{\...
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Why do gamma ray and x-ray photons which have significantly higher energy travel at the same speed as photons of visible light? [duplicate]

Photons which constitute X-rays and gamma rays have a significantly higher energy than those which constitute the visible light. If they have so much more energy, why do they travel at the same speed ...
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Is it physically meaningful to combine a photon's position state space with its polarization state space?

Mathematically, we can take the tensor product of a photon's position state space $\mathscr{H}_{pos}$ with that of its polarization state space $\mathscr{H}_{pol}$. Then, in the resulting Hilbert ...
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Signal to noise ratio with noise equivalent power

I am reading this document about the noise equivalent power (NEP). I want to calculate the signal to noise ratio for a photon detector. I denote the signal power as $P_s$. The width of the signal is $\...
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Photon - Neutrino interaction

Suppose we have a laser source, that means a coherent laser beam formed by 'in phase' photons. Is it possible to calculate how many photons can change their wavelength in neutrino - photon interaction ...
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Physical meaning of the photon propagator using gauge fixing term and path integral

I am considering the following generating functional in terms of functional integral for electromagnetic gauge fields \begin{equation} Z[J] = \int\mathcal{D}A_{\mu} \exp{\left(i\int d^4x \left(-\frac{...
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Have anyone seen an unbound electron absorb/emit a photon via Bremsstrahlung process?

I know free electron cannot absorb or emit photon as it cannot satisfy both the conservation of energy and momentum at once but how about in Bremsstrahlung process? The presence of a positively ...
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What, really, makes glass transparent? [duplicate]

I don't mean to be cheeky with the question, but it reflects the myriad of often seemingly conflicting answers I've seen around this. And that's not surprising of course given the dual nature of light ...
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Does a photon have an amplitude of motion at a speed less than the speed of light?

In the book QED: The Strange Theory of Light and Matter, Richard Feynman writes that for a photon there are nonzero amplitudes of motion with a speed that is greater or less than the speed of light. ...
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Regarding the radial motion of photons

Photons move on null geodesics and the equation of motion on equatorial plane after some algebra can be written as $$e^{\nu}\dot{t}^2-e^{-\nu}\dot{r}^2-r^2\dot{\phi}^2 = 0$$ $\phi =0$ for the radial ...
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Wheeler delayed choice experiment : why the mirror is not an interaction collapsing the wave?

I came across the Wheeler delayed choice as it is described in Wikipedia : To summerize, the individual photon has many paths and some path leads to detectors that reveals the path taken and some ...
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Why would a semiconductor with a fixed bandgap have an absorption curve instead of an absorption spike?

Wouldn't the semiconductor only be able to accept photons at one wavelength?
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If the sea surface were absolutely calm should the Sun reflection be the area of a circle instead a ribbon?

Although waves produced on the sea can cause different points of the sea surface to reflect sunlight towards the same observer, how is that kind of ribbon image produced? Why isn't the reflection ...
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How come electrons (spread out like a wave) can be ejected by a photon?

The photoelectric effect suggests that photons (of frequency more than the threshold value) collide with the electrons and they get ejected. But knowing the Uncertainty principle which suggests that ...
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Can the energy of a photon be increased, thereby decreasing its wavelength and increasing its frequency?

I know about Inverse Compton Scattering, but is it theoretically possible to take photons with frequencies so low they cannot be detected, increase their energy and hence make them detectable?
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What happens to a photon when it loses all its energy?

My understanding of Compton Scattering is that when a photon collides with a free electron, it will lose energy to the electron, and this loss of energy translates to an increase in the wavelength/...
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Coherent state (or “poissonian process”) with uncorrelated phases = thermal state?

Thermal states don't have poissonian statistics. But here @A.P. says: If the phases of the emitted photons were uncorrelated the random interference would lead to a thermal state, as explained in ...
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Can you see a photon from the side?

I'm trying to understand the shape and size of individual quantum particles. According to the book Tales of the Quantum the quantum wave of a photon in the visible range is a few centimetres wide. The ...
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All photons incident upon an object are ultimately scattered somewhere, right? Either elastically or inelastically?

From Wikipedia: Light has a certain probability of being scattered by a material. When photons are scattered, most of them are elastically scattered (Rayleigh scattering), such that the scattered ...
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What is the simplest model for the creation of a coherent state from stimulated emission in a laser?

I am interested in explicitly doing the math for what is decribed in this answer. Specifically, I am interested in the simplest model for the creation of a coherent state from stimulated emission in a ...
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Need help to prove how equation 2 is obtained from equation 1 (electromagnetism)

Im a undergradute student, and I have little expirience with vector calculus. Can someone help me to obation equation 2 from equation 1: where $\vec{k}$ is the wavenumber vector and $\vec{r}$ the ...

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