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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Is it possible for an electron to absorb a photon of lower energy and emit a photon of higher energy?

One electron can absorb more than one photon at a time but it is very less likely to happen. But why cannot it emit photons of more energy than it absorbed? The conservation of energy is still valid ...
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How do retinal cones see color when the cones pickup photons and not waves?

I am wondering how cones (photoreceptors in the retina) can see color if colors are wavelengths but cones send signals when they absorb photons. Maybe by using the energy level (or only being ...
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Failure of Newton's corpuscular theory and success of photon theory of light

Corpuscular theory of light States that: Light is made up of small discrete particles called "corpuscles" (little particles) which travel in a straight line with a finite velocity. ...
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Does a photon have a minimum energy level, below which it disappears? [duplicate]

As photons travel through expanding space they become red shifted, resulting in the CMB. Eventually they will no longer be detectable as they continue to redshift. Is there a point in the distant ...
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What is the difference between photoelectric effect and photo-ionization?

Photoionization, ionization by a photon, and the photoelectric effect aren't they identical? If not then what is the phenomenological difference between them?
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Degeneracy of Photons

The density of states for a photon gas is defined by, $$D(\epsilon)=\frac{g}{2\pi^2}\frac{\epsilon^2}{(\hbar c)^2} $$ where g is the number of independent internal states for a photon. The question is ...
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Doubt on photoelectric effect

One failure of wave nature of light in photoelectric effect was that increasing the intensity of light did not increase the kinetic energy of electrons. I don't understand how this is true. We know ...
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Calculating divergence and flux of geodesic word lines

Given a family of neighbouring geodesic word lines, is there a way of calculating properties such as their divergence or flux? maybe by converting the tangent vectors of the world lines to a vector ...
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Is light's momentum $0$?

From Paul Dirac's extended mass-energy equivalence equation, we know that, E2 = (mc2)2 + (pc)2 We also know that E = mc2 So, if we write mc instead of ...
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Reducing the filament voltage has no effect on the maximum photon energy produced by characteristic or Bremsstrahlung radiation. TRUE - why?

I'm studying for my radiology exams and I don't understand the answer to this question. It states that the following sentence is true: Reducing the filament voltage has no effect on the maximum ...
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2 answers
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Phonon, photon have chemical potential equal to zero

Hi I know the chemical potential of the phonon and photon is equal to zero. And I know the reason is the number of these particle isn't conserved. But I don't know why the number of the phonon isn't ...
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Does light intensity affect transparency

The reason for the transparent nature of glass is that the photons of visible light don't have enough energy for the electrons of glass to absorb and jump to higher energy levels. As a result, the ...
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What does it exactly mean when we say that a photon or light is "frozen" at the event horizon?

Objects on this site are said to "freeze" at the event horizon, but what about photons themselves? I have read this question: At the event horizon $v_{eff} = 0$ and the light beam is frozen ...
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Are gamma rays the limit of the frequency photons can attain, and if yes, why? [duplicate]

Recalling that the Planck constant is $6.62607015 \times 10^{-34} m^2kg/s$ and taking into account the formula $E=hf$, for the energy of photons, we can rapidly derive the energy of gamma rays, which ...
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How does a single slit produce coherent light?

I was wondering why some apparatus seem to place a single slit right before a double slit. Then I saw this this question. The answer explained it quite well; but I still don't quite understand how a ...
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Why Photon gas's Equation of State Diverges?

We know that, for Photon gas, the equation of state is given by $pV=\frac{1}{3}U$; where $p$ is the Pressure, $V$ is the Volume and $U$ is the Internal Energy of the Photon Gas. (see Equation of State ...
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Shouldn't the speed of each photon in vacuum depend on its energy? [duplicate]

The speed of anything is dependent on its kinetic energy. why is that not the case with photons? photons with different energies have the same speed in vacuum, their speed does not depend on their ...
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Why is Equation of State of Photon gas different from the Equation of State of Boson gas?

We know that, for Bosons, the Equation of State is given by $pV=\frac{2}{3}U$; where $p$ is the Pressure, $V$ is the Volume and $U$ is the Internal Energy of the Boson Gas. (see Equation of State of ...
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Is it really true that time stops for matter moving at the speed of light? [duplicate]

If time stops when you achieve the speed of light, then how can photons ever change frequency, or how are Green lasers possible through additive synthesis in a crystal, if photons move at the speed of ...
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How exactly can quantum interference inhibit the creation of spontaneous emission (via the purcell effect)?

What is the simplest model for understanding how spontaneously emitted radiation from an atom is inhibited by a cavity? Considering an off-resonant cavity, it is said that there is some quantum ...
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Ensemble associated with a system of Photons

In the book "Fundamentals of Statistical and Thermal Physics" by F. Reif it is said that : Photon Statistics is a special case of Bose-Einstein Statistics with no restriction on the total ...
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Where can I find photon absorption cross-sections for various isotopes?

I am looking for absorption cross-sections for photons. Usually, I am dealing with neutron absorption where I utilize https://www.nndc.bnl.gov/endf/ for my data. This is a really handy website that ...
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Is photon drive reaction less propulsion? [duplicate]

If photon dont have mass how can produce thrust, it seems that violate Newton 3. law? If light dont have mass how can push solar sail?
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What gives particles mass? Also why are particles like photons considered massless when they have energy and momentum? [closed]

Interested in knowing the quantum explanation for what gives particles mass, and why particles like photons are considered massless when they have energy and momentum.
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Does electromagnetic field have two types of spin: one is electric and another is magnetic?

A particle like an electron has a single spin. However, owing to de Broglie's hypotheses a matter wave is associated with all microparticles. The photon is an electromagnetic wave composed of electric ...
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Will 'free' photons exert gravitational force [duplicate]

From my general understanding, it is widely accepted that light will bend around a massive object, as it will experience gravity, is predicted by GR, and was experimentally verified. Also, the energy ...
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3 answers
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Can light exist if it never interacts with anything? [closed]

I've heard some physicists mention that photons do not experience time. From the perspective of the photon, it begins and ends instantaneously. With all the photons shooting out into empty space, it ...
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Why doesn't the Weinberg-Witten theorem forbid collinear photons?

The Weinberg-Witten theorem tells us that any theory that has an effective graviton, i.e. a massless helicity-2 particle as a state in the free-particle Fock space, cannot have a gauge-invariant and ...
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Why do photons: "have to give all their energy or none at all", in photon-electron excitation interactions? [duplicate]

Consider the following A-level physics question: The transitions between energy levels shown in Fig. 1 can occur when electrons in the atom are excited. This can happen when a photon interacts with ...
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How can recombination lead to photon decoupling if scattering can still occur with neutral particles?

During the recombination era, two things happened: Electrons and protons bonded to form neutral hydrogen atoms. As a result of #1, Compton scattering is no longer efficient enough to keep photons and ...
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What Happens When Three Photons Enter a Non-Polarizing Beam Splitter?

When a single photon enters a non-polarizing beam splitter, in any of the two inputs, it has 50% chance of leaving by each of the two outputs. When two photons enter this beam splitter, each by one ...
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Why does Quantum Harmonic Oscillator treatment to Blackbody Radiation result in Ultraviolet Catastrophe?

Planck, in his treatment to solve Blackbody Radiation, considered that the Electromagnetic Waves inside the Cavity of the Blackbody are Standing Waves due to Oscillating charges on the wall of the ...
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Photons in magnetic field vs photons in electric field

I am trying to draw intuitions from different sources to better understand Lorenz force, in particular why the magnetic field generates orthogonal effects where electric fields generate parallel ...
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Photon-wise justification of chosen compensation crystal length in SPDC

I know a compensation crystal in SPDC schemas serves the temporal walk-off erasment purpose. However, I am not sure why half the length of the original crystal is opted for. I read an article that ...
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2 votes
1 answer
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The effect of the non-existense of longitudinal polarisation mode of the photon on equipartition theorem

Massless vector bosons like photons only have 2 independent polarisation degrees, unlike massive vector bosons. For a spin 1 boson with mass $\mu$ and with $k^λ = (ω, 0, 0, k)$ the longitudinal mode ...
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Why photocurrent doesnt increase after saturation limit in Photoelectric effect on increasing Voltage

As per what i learn that current(i) is rate at which charge flow through a cross section. Let assume we have 10 balls and it reaches another end while passing through some cross section, if 10 balls ...
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Is annihilation a smooth process that requires time?

There are a lot of questions on this site about annihilation and quantum processes (and whether they are instantaneous), but none of them specifically give a satisfactory answer to my question. None ...
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13 votes
1 answer
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If frequency of photons is a continuous spectrum, wouldn't the chance of a photon having the exact right frequency to excite an electron be zero?

As far as I'm aware, the energy needed to excite an electron to a different orbital is discrete. Since the frequency of light is continuous, wouldn't it be impossible for a photon to have the exact ...
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Cavity decay rate dependence on frequency

Photons inside an imperfect cavity will decay outside with a rate proportional to the peak width $\delta \omega$. A spectrum of such a cavity (with different quality factors) is shown below My ...
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Can Compton scattering occur between a photon and a neutrally charged particle?

Compton scattering occurs because of the particle-like nature of light, but does it occur between photons and charged particles only? Is it impossible for Compton scattering to occur between a photon ...
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Why does light behave differently when interacting with multiple particles?

When light hits an atom (I will use a carbon atom for simplicity), if it is not in the absorption and/or emission spectrum of carbon, it will simply pass through without interacting with the atom. ...
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Why does light not pass through opaque objects? [duplicate]

Since light is massless it does not interact with particles unless that specific wavelength is on the emission and absorption spectrum. The emission and absorption spectrum of an atom indicates the ...
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2 votes
2 answers
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What prevents a "beam combiner" from being able to cancel out energy?

I understand that normally when two waves combine destructively this happens only in certain places; the energy doesn't disappear but is redistributed to other places where the interference is ...
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If light spreads spherically, then why do we see these light sources spread in just four (perpendicular) angles on images?

Many times on this site it is said, that light spreads spherically, in all spatial directions. Incoherent light can be made coherent making a point source but the spread is still spherical. Why is ...
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Linear independence of Electromagnetic Field polarization

In electromagnetism, we are free to choose a gauge such that $\partial_{\mu}A(x_\mu) = 0$. This, along with the mass on-shell condition for a massless gauge field, implies that there are only 2 ...
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Energy of light across different media

As a high school student, I was studying the dual nature of matter and radiation and wave optics. According to Planck's theory, the energy of a particle is given as hf where h is the Planck constant ...
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Minimum distance between photons after attenuation

Light from a 1 mW He-Ne laser is attenuated by a factor of $10^{11}$. Estimate the minimum distance between the photons. What does it mean by the minimum distance between photons? As I understood, the ...
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If I split coherent state light in half (by a beam splitter) are the 2 output intensities correlated?

From what I have been told and understand, if I use a simple beamsplitter to split coherent state light, I will obtain 2 coherent states as a result. Are the intensities of these two output states ...
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Modified photon propagator with a fluctuating current

I am studying a paper on the effects in bound-state QED coming from nuclear dynamics, and I am struggling to understand one basic derivation. The authors introduce a modified photon propagator $$ \tag{...
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Is sub-atomic particles (quarks) bunch of compressed photons? [closed]

According to theoretical physics,protons,neutrons and electrons are made up of sub particles called quarks which are the building block of matter (baryons). what if the quarks are bunch of compressed ...
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