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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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What exactly happens to the "Unabsorbed" Photons? [duplicate]

Things that you can assume that I am familiar with as a high school student. I know that only Photons/EM-Waves with "Specific" Frequency/Wavelength/Energy has chances/probability to --> ...
REYNEP's user avatar
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1 answer
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Destructive interference pattern perpendicular to photon's propagation direction

I'm studying the interference pattern for light. For example, the following is the interference pattern for double slit experiment: I'm curious to know if there is an existing interference pattern ...
Wael Khatib's user avatar
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How can the attraction of opposite electric charges be explained by the exchange of virtual photons? [duplicate]

Sometimes, the electric force between two electrically charged particles is explained by the analogy of two freely floating astronauts that start throwing a ball towards each other. In this analogy, ...
a_guest's user avatar
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2 answers
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Quantised optical cavities with non zero decay rate

The quantised electric field of an optical cavity can be described as a harmonic oscilator, $$\hat{H}_{\mathrm{c}}=\hbar\omega_{\mathrm{c}}\hat{a}^{\dagger}\hat{a}.$$ If the cavity mirrors are ...
Adrien Amour's user avatar
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1 answer
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Question about lasers [closed]

In a laser, how the beam is almost non-divergent? When stimulated emission happens , does it copy the photons which is used for the medium to get pumped, or does it copy the photons of spontaneous ...
Prantar Biswas's user avatar
12 votes
6 answers
6k views

How do photons have temperature?

On the internet, I found "Temperature is a measure of the average kinetic energy of the particles in an object." (source) So, this temperature should be a result of friction. Light is also ...
Shristeerupa's user avatar
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1 answer
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Does the energy of a photon in comoving space change?

Assuming a flat FLRW universe that is expanding: In comoving space, does the energy of a photon decrease or stay constant? A physical argument for this would be nice.
Matrix23's user avatar
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2 votes
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Single photon detection

Consider a thought experiment where we have a source emitting a single photon, like an atom/molecule going from an excited energy state to its ground state. We have an infinite number of point ...
spacetom's user avatar
4 votes
1 answer
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What happens if obstacles (walls) been put where destructive interference occurs in double slit experiment?

If photon doesn't have probability to be in dark (destructive interference) area, what will be the effect of adding obstacles (walls) in the dark (destructive interference) area for the double slit ...
Wael Khatib's user avatar
-6 votes
0 answers
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Einstein's Photon Theory and Special Relativity: A Comparative Perspective on the Evolution of Quantum Mechanics [closed]

Albert Einstein used the photon concept to explain the interaction of light with atoms. However, his explanation lacked the detailed rigor he applied to special relativity. Special relativity is ...
jtn's user avatar
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1 answer
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Ambiguities in optical waveguide modes

While studying concept of slab waveguide mode, I got stuck on some problems. In textbook(Yariv chapter 3 pg 112), for guided TE modes it tells that the mode function is taken as which means that the ...
photonics2024's user avatar
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Moving observer detects incoming photon: Is this the full derivation of the relativistic Doppler effect?

I have a very trivial question that I can't quite work out. Imagine an observer moving in Minkowski spacetime. In Cartesian coordinates $(ct,x,y,z)$, their 4-velocity is $$u^\mu = (u^t ,u^i )$$ where $...
P. C. Spaniel's user avatar
1 vote
4 answers
105 views

Observation in Young's double slit experiment [duplicate]

I'm new to YDSE and recently I found out that there was also an experiment done when humans were able to separate photons (What I mean is we were able to shoot photons onto the double slit) so it was ...
Neel's user avatar
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1 answer
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What is the relationship between frequency and photocurrent?

I know this question has been posted before but i'm down a rabbit hole as I keep finding different answers to it. My understanding is that photocurrent increases with intensity and decreases with ...
Pranav Borse's user avatar
1 vote
1 answer
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Photon Spin and Fermionic Fields

I have a question. It is said that the photon has spin 1. However, the spin of a fermion like the electron is 1/2. What I see is that the fermionic field has 4 fields, $$ \psi = \begin{pmatrix} \psi_{...
Julián Oviedo's user avatar
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Spinor-helicity formalism: relationship between 1 and 2 reference vector setups

The spinor-helicity formalism is usually set up so that for a massless vector boson (photon or gluon) with momentum $k$ an arbitrary reference momentum $p$ is introduced and the corresponding ...
Fetchinson0234's user avatar
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1 answer
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If particles do not have definite properties before measurement in entanglement, what are they? [closed]

It is said that in quantum entanglement, a photon does not have definite properties before measurement. I have two questions. Is this true for all interpretations of quantum mechanics? what does it ...
inquisitive 's user avatar
1 vote
2 answers
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Why do atoms absorb photons? [duplicate]

im studying spectra/spectroscopy in school in my chemistry class, and im having to resort to re-teaching myself the chapter due to my teacher's abilities (or lack thereof) i understand how they absorb ...
hannahmcdonnell's user avatar
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1 answer
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Suppose we observe a photon, what can we measure about it?

I guess we can measure spin, momentum, kinetic energy, position, time, wavelength, e.g., (I don't know much about this), are there other properties? How many degrees of freedom are there? Because for ...
Alex's user avatar
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Why do lighting snoots need to be long?

I have just been introduced to the concept of using lighting snoots to create a more focused circle of light with diffused edges. They have a tube that gradually gets thinner with a honeycomb grid at ...
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"Symmetric" photon polarization rotation

The usual linear polarization matrix/operator for a photon or classical light transforming from some combination of $|H\rangle$ and $|V\rangle$ to another is: $$ \begin{pmatrix} \cos(\theta) & -\...
user401228's user avatar
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Impact of obstacles on Single/ Double split interference pattern

What is the impact of obstacles on the interference pattern if the obstacles are arranged to be located in the destructive interference locations as follow: According to my understanding to classical ...
Wael Khatib's user avatar
3 votes
0 answers
31 views

Does conservation of Etendue mean focusing+filtering a larger led bulb into a point light in a way that casts shadows like a point light is impossible

I unfortunately never took a class on photonics but this law of Etendue seems to be very relevant to a project I am not even sure is possible. I am trying to create a lamp that does something like ...
CalebK's user avatar
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Compton's scattering

I have studied frontal elastic photon-proton collision and derived the following formulae: $$ v = \frac{2 (m^2 + M_0 m)} {2 m^2 + 2 M_0 m + M_0^2} c $$ where $m$ is the relativistic mass of the photon ...
RJurjevic's user avatar
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2 answers
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How to propagate the wavefunction of a photon in single slit diffraction?

During my undergrad, I remember writing a simulation in which the diffraction patterns which emerge from light passing through a single slit were calculated. This was done, basically, by calculating ...
Gregor Hartl Watters's user avatar
1 vote
0 answers
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Would a photon engine work?

So as far as I know for a body to move in space it needs to accelerate using it's mass because there is nothing else to push against. And as far as I understand, basically, the photon engine is an ...
bonbon's user avatar
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1 answer
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Compton scattering angle formula and drawings

The Compton formula for scattering angles $\varphi$ and $\theta$ for electron and photon, respectively, can be shown as: $$ \cot\varphi = (1+\frac{\lambda_c}{\lambda_i})\tan\frac{\theta}{2} \tag{1}\...
Leon Chang's user avatar
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Feynman trajectories of a photon from entangled pair

In a pair of Entangled photons does any of the two photons has the Feynman trajectories of the other one? Must the possible trajectories of photon 2 be taken in the Feynman's path integral for the ...
Mercury's user avatar
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Interpretation of Diffusion Coefficient in Photon Diffusion Equation

The diffusion coefficient in photon diffusion, as explained/defined here: Factor of 3 in Photon Diffusion coefficient seems counterintuitive because diffusion is inversely proportional to the ...
alain's user avatar
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0 answers
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Derivation of massive photon propagator

I'm trying to derive the massive photon propagator using the path integral formalism for a theory with $$ \mathcal{L} = -\dfrac{1}{4} F_{\mu\nu} F^{\mu\nu} + \dfrac{1}{2} m^2 A_\mu A^\nu, \text{with } ...
Gabriel Ybarra Marcaida's user avatar
2 votes
1 answer
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If helicity of photons is +1 then the light is right- or left-circularly polarized?

In particle physics, we say: a particle has +1 helicity (right-handed) if its momentum and spin are parallel, or it has -1 helicity (left-handed) if its momentum and spin are antiparellel. Now, if we ...
Atom63's user avatar
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2 votes
0 answers
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How does a photon look like in QFT? [closed]

It is very well known that in QFT the particles are excitations of the field. But how exactly is a free photon looking like in spacetime? What is it shape in free space? And what is in fact in the ...
Mercury's user avatar
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0 votes
1 answer
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Photon propagation paradox, what am I missing? [closed]

I've seen photons and EM waves be described like so "Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields." So if we have a ...
StackUser20004's user avatar
2 votes
1 answer
50 views

Does the photon emitted by a deexciting atom have directionality?

Suppose an atom deexcites and emits a photon. Will the direction in which this photon is emitted (or the direction in which the photon will be measured) be fundamentally random, or is the direction ...
QuantumQuasar's user avatar
1 vote
0 answers
39 views

Can only a collection of photons create light with a direction?

I assume that any individual photon spreads in all direction of space, resembling a circular wave. So, a single photon cannot have a specific direction of travel. Consequently, light with a direction ...
QuantumQuasar's user avatar
2 votes
0 answers
31 views

Behavior of a single emitted photon [duplicate]

Suppose you have a transmitter sending out a single photon. If you think about this photon as a wave, you will see a circular wave moving away from the transmitter. Over millennia, this wave spreads ...
QuantumQuasar's user avatar
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0 answers
15 views

Forward voltage of LEDs from quantum mechanics perspective

I have a very elementary understanding of LEDs and semiconductors altogether, but a more solid understanding of QM, and I was wondering if there was a way to explain (preferably mathematically) ...
JBatswani's user avatar
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10 votes
1 answer
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What fraction of the universe's energy is contained in photons?

From each point in the universe, the light of billions of stars, galaxies, supernovae etc. can be detected. So there seems to be a lot of energy/momentum "in flight". Is it possible to ...
2080's user avatar
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0 answers
26 views

Why the photons are deflected during the refraction? [duplicate]

From what I learnt, when photons are passing throw a denser environnement with an positive angle (from the atmosphere to water for exemple), they are slow down. But I can't understand how this ...
Jay Labarsurlakantik 's user avatar
0 votes
0 answers
32 views

What is the speed of light during reflection? [duplicate]

What is the speed of light in a vacuum when the light reflects off of a mirror?
Christina Daniel's user avatar
2 votes
2 answers
74 views

Is reflected photon the same? [duplicate]

When a single photon is reflected is the same one, or is it a new photon (emitted) while the 'original' photon has been absorbed? I'm not sure how to imagine a refleced photon - it's not a ball ...
matej's user avatar
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0 votes
1 answer
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Photons and Bremsstrahlung radiation

When fast electrons goes into a target, part of their kinetic energy is converted into electromagnetic radiation, that we call Bremsstrahlung radiation, as they change their velocity. The energy ...
Ako's user avatar
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2 votes
0 answers
51 views

Is the electromagnetic wave emitted during an atomic electron transition always sinusoidal, and why is there a spectral linewidth?

I recently engaged in a discussion regarding Planck's relation, which posits that the energy $E$ of a photon is directly proportional to its frequency $f$, symbolized as $E=hf$. The user @Sturrum on a ...
CuriousMind's user avatar
1 vote
0 answers
22 views

Encoding information on photon qubit

I study QKD which relies on the single photon as a qubit, on which information is encoded. There are many protocols which mainly differ on how information (bit) is encoded on the qubit : polarization ...
deb2014's user avatar
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3 votes
1 answer
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How is it possible that photons already move at the speed of light the moment they pop into existence? [duplicate]

This is a thought experiment and I might be horribly wrong. If we have an electron-positron annihilation a photon pops into existence. This photon is then supposedly moving at speed of light at the ...
Jurre Groenewegen's user avatar
1 vote
0 answers
18 views

In fluorescence, if you excite a fluorophore with 2 wavelengths (one closer to the resonance wavelength than the other) will the emission change?

Say my fluorophore's resonance wavelength is 500nm. I excite the fluorophore with equal energies of 500nm and 550nm. The 550nm light excites the fluorophore, but is inherently weaker than 500nm ...
sjf's user avatar
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Spin 1 operator and photon

$\newcommand{\ket}[1]{| #1 \rangle}$ The general spin formulation is the same as the cinetic momentum : \begin{equation} [S_{x},S_{y}]=i\hbar S_{z},\quad [S_{y},S_{z}]=i\hbar S_{x},\quad [S_{z}, S_{x}]...
deb2014's user avatar
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1 vote
2 answers
121 views

Analogy between the Electromagnetic Field and the Schrodinger Equation

In this answer my2cts says "The electromagnetic field is to photons what the Schrödinger or Klein-Gordon wave function is to electrons." Could someone expand on this further? Is this just a ...
psychgiraffe's user avatar
3 votes
2 answers
448 views

Physical meaning of the multipolarity of gamma radiation

During my nuclear physics class, we talked about the multipolarity of gamma radiations but without going too much into the details, and I was wondering about the meaning of that, how can the radiation ...
Lucas's user avatar
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1 vote
2 answers
57 views

Photon and qubit representation

The photon spin-1 has two states, $\pm\hbar$, just like the spin qubit ($\pm\frac{\hbar}{2}$). From a quantum information point of view, they can encode the same amount of data. However, I am confused ...
deb2014's user avatar
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