# 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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### Escape velocity at 45 degrees near a black hole

what if the formula for the escape velocity at 45 degrees compared to 90 degrees when you are near enough to a black hole for relativistic effects to become significant? I know that near enough to the ...
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### Approximation for average thermal photon number

I am currently reading the book Introductory Quantum Optics by Gerry and Knight and I am having trouble understanding an approximation they make. In the chapter on thermal fields they derive the ...
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### Impulse of a photon in water?

Im trying to calculate the impulse of the particles of a light ray in water assuming its freely traveling through and not bumping into any molecules. This i what i tried: We know that: $$E=h \cdot f$$ ...
27 views

### What's the relationship between the emission spectrum and the material properties (refractive index, thickness)?

I'm currently working on the thermophotovoltaic problem. I have known that the emission spectrum of a single-layer structure is associated with the material properties, but I'm confused that the ...
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### Using DeBroglie Waves with Compton Scattering

I am presented with the following situation: A light source $\lambda_i$ of unknown wavelength is directed at a carbon sample. An electron is scattered with a speed of $v$ (non relativistic) at an ...
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### Lorentz boosting orthogonal to the direction of a photon's momentum- what's going on here?

So while solving a special relativity problem, the following problem came to my mind, which seems to admit two inconsistent methods of resolution. I'd like to know which is incorrect, where the ...
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### Is radiation pressure wavelength dependent (does a blue photon move the solar sail more then a red photon)?

I have read this question: Where John Rennie says: Does the wavelength of the light matter? No Now in the same question, another answer, by DavePhd says: Does the wavelength of the light matter? ...
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### Is a room-temperature & pressure single-photon sensor theoretically possible?

In the realm of photonic quantum computing, single-photon sensors are required for determining the outputs of each chip. Unlike most forms of QC, photonic chips themselves may be run at room ...
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### Protons and QED

I have been researching quantum electrodynamics recently and I have found out that when electrons repel each other, they constantly exchange photons with each other. When two protons repel, do they ...
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### Why is the screening coefficient, $\sigma_K$, in Moseley's law only constant for high $Z$ and how can it be anything other than unity?

In trying to understand the screening coefficient, $\sigma_K$ and the limitations of Moseley's law (why only valid for high $Z$) I came across a section of this lab script I found online: Moseley’s ...
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### Does light (photons) exert reaction force on it's source? [duplicate]

I know that photons/ light when incident on a surface exerts forces on it (aka radiation pressure) and it is also used for applications such as solar sails or laser propulsion. This phenomenon is ...
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### Length and width of photon - check logic

so if $E = h\nu = \frac{h c}{\lambda} = pc$ where $\lambda$ is the photon's wavelength, $h$ is planck's constant, and $p$ is the photon's momentum and $L = \frac{p}{h}$ where L is length and p is the ...
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### The $D_{00}$ component of the photon propagator in the Feynman and Coulomb gauges

I am trying to understand some derivations involving the photon propagator, and I am having a lot of trouble with expressions in different gauges and also with terminology in general. Here is what I ...
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### How nucleons get excited? [duplicate]

Gamma rays can be released when nucleons at higher energy states fall down to lower energy states, but how do nucleons get that much high energy to release gamma rays?
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### How do Photons Move Solar Sails?

Ok so, this is something that I've been researching for quite some time now and, no matter where on the internet I end up at, no matter how many sources I visit and read, this still doesn't make even ...
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### Photon energy and transmit power of an optical transmitter

I have a big doubt. I have found an answer of other community a question about a relation between wavelength and transmit power. My doubt is according to the reply If you have a low wavelength and ...
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### Complex Inverse Square Law Question

I am struggling to answer a question about the inverse square law and have come here for support. The question is, how can I find the amount of light incident on a 2D plane that is perpendicular to a ...
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I am a student using CCD. I am measuring photoluminescence with ccd. When measuring, the $y$-axis comes out as count, but I want to convert it to watts, so I'm posting this. The model I am using is ...
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### How can an isolated atom remain in an excited state forever?

I have read this question: An isolated atom in a' excited state woul remain there forever. How soon does an electron emit the absorbed photon back? These solutions are time independant so an ...
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### $1/r^2$ force and "photon decay"

Background: My question is about the interpretation of the $1/r^2$ force in terms of the fundamental processes of the underlying QFT. We know that if the photon had mass $m$, then we may have a "...
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### A photon cannot exist if it travels at speed lower than $\mathrm{c}$ in a vacuum, why?

This is known in physics that photons travel only at fixed speed $\mathrm{c}$ in vacuum but also inside a medium going from one atom to the next and taking advantage of the vacuum that exists between ...
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### What evidence suggests that light exerts a force of gravitational attraction? [duplicate]

I'm aware of other posts outlining the theory of how light generates a force of gravitational attraction, cf. e.g. this Phys.SE post. I'm not so interested in this. I'm interested in what evidence ...
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### Does a photon carry "end-over-end" angular momentum to explain skyrmions and other structured light?

It is widely known that photons carry a longitudinal angular momentum of ℏ, which is readily compared with circular polarization; the angular momentum vector for right handed polarization has been ...
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### Can electromagnetic waves with long wavelengths (1 kHz to 1 GHz) also be considered as Photons?

Could we conduct the Double Slit Experiment with long wavelengths (e.g. frequencies between 1 kHz and 1 GHz) and still measure through which slit the wave/particle goes? I am sure that the ...
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### Could photons generated from the many trillions of stars be some how contributing energy to dark energy? Where do they go and what becomes of them?

What is dark energy and can photons contribute to it. When the total number of stars start to diminish will dark energy perhaps begin to diminish as well. What does happen to all those photons with ...
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### What part of the photons emitted from a star are from black body radiation and what part originate from fusion reactions?

What part of the photons emitted from a star are from black body radiation and what part originate from fusion reactions? To my understanding these are the two sources of luminosity for a star, so I'm ...
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### Is selective heating of a gas within a mixture possible? Using the absorption wavelength of the molecule

Could you effectively concentrate a particular gas in a mixture by applying only the wavelength absorbed by the target gas? Assuming that the target gas molecules heat up when absorbing the selected ...
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### How does the photon detector in this experiment work?

https://www.youtube.com/watch?v=Cg7jMXQHxvM At 5:46 to 6:37 Dr. Aspect talks about photon detectors. At 6:10 he talks about firing single photons at a time, and having two detectors. From what I have ...
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### Can we see the interference pattern in the double slit experiment if the walls of the slits are made of transparent glass?

There are other questions on this site about when we put one of the slits in glass, or cover one of the slits with glass. I am not asking about that, and none of those question answer my specific ...
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### Electrostatic interaction by exchange of virtual photons?

Although virtual photons are maybe best considered not to actually exist, it is said that electrostatic attraction and repulsion between two electrically ...
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### In this paper describing the size and shape of photons, what do "length" and "radius" mean?

https://arxiv.org/ftp/arxiv/papers/1604/1604.03869.pdf "Substituting Eq. (19) into Eq. (6) gives the photon radius ... (20) where re is the classical radius of an electron. Since the length of a ...
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I'm interested in full general relativity, not just special relativity. In natural units a photons frequency (energy) from the point of view of an observer moving with 4-velocity $u^\mu$ is given by, \...
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### Do photons emitted from different color filters leave varying size marks on a screen when fired?

I have read that photon "size" is proportion to wavelength, but I've also read that photon "size" isn't exactly a thing. So does this mean that different colors, which have ...
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### Would a black hole act as a particle accelerator between the photon sphere and the event horizon?

I'm trying to understand the behavior of photons between the photon sphere and the event horizon. Here is what I think I understand so far : the radius of the PS is 1.5 times the Schwarzschild radius ...
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### $n+n \rightarrow n+n$ scattering through photon interaction (QED Process)

If I think about the quark level process then, two u quarks can scatter through photon exchange via QED interaction by following tree-level process . This is happening because u quark is electrically ...
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### How do wave equations have an exponential solution involving wave vector and the propagation constant?

I hope you all are doing well! I was trying to derive the propagation constant for the wave vector of the surface plasmons. I have uploaded the document below, which is the following: Here , is the ...
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### Absorption rate vs. Einstein Absorption Coefficient

I am trying to understand the absorption rate (or absorption probability density), which is defined as $$p_{ab}=\frac{\sigma c}{V},$$ where $\sigma$ is absorption (transition) cross section and $V$ ...
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### Why are photons being identical particles?

Recently, I study quantum optics and deal with quantization of EM field in a cavity. We know we can express/quantize vector potential in terms of $\hat{a},\hat{a}^{\dagger}$ to get a quantized EM ...
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### Can a photon lose all of its energy due to gravitational redshift?

Imagine a photon that is emitted by a star that is at infinity with respect to me. We can observe the gravitational redshift happening to that photon with respect to my reference frame. I was ...
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### Why do some electromagnetic waves have more than one photon?

I know that the energy of an EM wave is equal to nhv, where n is the number of photons, but why/how do the number of photons in a wave vary? If a single atom emits an EM wave with an energy of 100 ...
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### Radiation pressure and specific intensity

In the book Radiative Processes jn Astrophysics (Rybicki, Lightman) a macroscopic radiation theory is described using specific intensity $I_{\nu}$. Then, the following definition for momentum flux is ...
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### When is it necessary to quantize the electromagnetic field?

I've seen both classical and quantized electromagnetic fields in quantum mechanics problems - for example, classical in linear response and the Coulomb interaction, and quantum in photon absorption - ...
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### How does the photon detector in this video work?

In this video: https://www.youtube.com/watch?v=A9tKncAdlHQ How does this photon detector at 6:58 work, mechanically speaking? Is this basically just like a camera, that beeps when it is hit by light? ...
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### (L&L vol. 4, sec. 7) Normalization of Spherical Wavefunctions of Photons

In volume 4, section 7 of the Landau and Lifshitz collection (second edition) the authors discuss spherical wavefunctions of photons. I'm having trouble understanding how the equality in (7.7) is ...
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### Does the size of the mark a photon/electron leaves on a light sensitive sheet change with distance?

This video shows an electron detection event build up. https://www.youtube.com/watch?v=hv12oB_uyFs When firing photons or electrons at a light sensitive sheet (or whatever types of detectors are used),...
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### Can monochromatic light have units of per frequency?

Plank outlines (page 209, equations 302-304) in his book that a monochromatic ray of frequency $v$ is has intensity of \begin{equation} I_{\nu} = \frac{2 h \nu^{3} F \Omega}{c^{2}} \left( e^{\frac{h\...
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### Experimental measurement of the difference of momenta of a photon

I am trying to understand how one can measure the difference of momenta $p_x - p_y$ or the sum $p_x + p_y$ of a photon where each of the quantities represents the momenta in each direction. What I ...
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### The effect black holes have on light

We all know that light loses its energy when it is moving through expanding space and time. And sense a black hole can be summed up to a super compressed space time, shouldn't that mean that a photon ...
Let us say I excite a particular mode $\omega_l$ of the electromagnetic field by means of parametric downconversion such that both of them are identical in all aspects: polarisation, direction and of ...