# Tag Info

## New answers tagged photons

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• 624
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### Why does a laser beam not diffract unless there is a grating in front?

If we consider light as expanding in every direction by default, with every wavelet as a source of a secondary wave, This is a wrong model for classical electromagnetic radiation. There are no "...
• 222k
2 votes

### Why does a laser beam not diffract unless there is a grating in front?

I surmise the focus of your question is the following: If it is assumed that Huygens' Principle in its 17th century form is applicable then any propagation of light should be diverging all over the ...
• 15.3k
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### Why does a laser beam not diffract unless there is a grating in front?

You are right. The opening of the laser should cause diffraction. In fact as Why does a laser beam diverge say, it does. The leading answer says diffraction is a quantum effect, originating in the ...
• 26.9k
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### Is the frequency of photon invariant?

Photons move along lightlike geodesics regardless of the observer, so although energy $E=h\nu$ and momentum $\boldsymbol{p} = (h/\lambda)\boldsymbol{\hat{n}}$ depend on the observer, the following ...
• 561
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### Is the frequency of photon invariant?

Is the frequency of photon invariant The photon is an elementary quantum mechanical point particle of the standard model of particle physics. As it has no space dimension, it is a point in (x,y,z,t), ...
• 222k
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### Is the frequency of photon invariant?

A light-signal has a lightlike 4-momentum $\tilde \omega$ where $\tilde \omega \cdot \tilde \omega=0$. So, its energy-component is equal to the magnitude of its momentum-component. For simplicity, ...
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### Is the frequency of photon invariant?

Time dialation/length contraction is the reason. This is called the relativistic doppler effect
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### Frequency and intensity in photoelectric effect

Whatever you have said is right but only one thing wrong. "But according to Einstein's theory number of photoelectrons is affected only by the intensity of light not its frequency." That's ...
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### Is there any real world "double slit experiment" where the which-path measurement made?

There are many example’s of which path testing. In a double slit experiment the particles go one path or the other And overtime create an interference pattern. If you block one of the slits you have a ...
• 3,772
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### Compton scattering by proton

I refer to my MIT phd thesis (1962). The energy dependence of the scattering cross section shows a big bump at an energy that excites the proton to a higher energy level.
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### Can phonon follow similar curve as photons follow in blackbody radiation?

I assume that you're asking if phonons can "radiate" with a distribution of energies like those for blackbody radiation. If so, the answer is yes. This is often assumed in models of thermal ...
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### Can phonon follow similar curve as photons follow in blackbody radiation?

Phonons are vibrations in a material. Usually we look at phonons in the context of a crystal that has a specific arrangement of atoms. This structure is periodic and like electrons going through a ...
• 574
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### If photons have no mass, how can they have momentum?

Something that hasn't been mentioned is the concept of electromagnetic momentum and the Poynting vector. The Poynting vector is defined as $${\bf S} = \frac{1}{\mu_0}\,{\bf E}\times{\bf B}$$ and &...
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### Can you make photons without momentum?

The momentum of a photon is given by: $$p = \frac{h}{\lambda}$$ so all photons have a momentum and there is no way to make the momentum zero. You can make the momentum arbitrarily small by making ...
• 331k
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### What would happen if I were to shine light on a metal for a long time?

If the metal were not grounded then there would indeed be a build up of positive charge on the surface of the metal. As a result of the electron depletion, the work function of the material would ...
• 624
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### If the range of a laser depends on its crossection perimeter does it mean that a wider beam would reach much longer distances?

Light from any source can in principle reach any distance. It is just that the light spreads out and causes the intensity to decrease. So what you want to know is how fast the light spreads out with ...
• 12k
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### Distribution function for squeezed light as a function of energy

I'll write $\epsilon=\hbar\omega$ the energy of the boson. In general, it is in a mixed state described by the density matrix $\rho$ and the distribution of boson number is given by:  p(n) = \langle ...
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### Why is the photon sphere spherical, but the accretion disk not?

the accretion disk gets its shape from angular momentum conservation ... and from interactions between the elements. There are multiple different orbit collections that are possible that have the ...
• 34.3k
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### What is the Feynman diagram associated with Bremsstrahlung?

Actually, the Feynman diagram for Bremsstrahlung is that of Compton scattering with the little difference that the incoming photon at the "first" vertex is a virtual one that is generated by ...
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### How does a photon interact with a conduction band electron?

Let's talk about how a radio wave is created. Then we will be able to imagine how this kind of EM radiation affects the surface electrons on the receiver rod. To create a radio wave, the electrons in ...
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### Diffraction of photons exhibiting random walking motion

From your link: Once you know, or assume, a typical distance between collisions, you also have to figure out how many steps the photon has to take to travel from the core to the surface. Photons are ...
• 222k
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### Very old photons from the Sun

In the interior of the star the electrons are stripped from the hydrogen atoms, so absorption shouldn't be occurring, but scattering where the photons change direction would still happen. The photon ...
• 574
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### Partial single photon reflection

I didn't go though the whole video, but looking at your comments, a couple of things that might help clear things up. Feynamm was tilting the back surface of the glass so it wouldn't bounce directly ...
• 574
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### How does a photon interact with a conduction band electron?

Quantum mechanics is the underlying basic theory of matter for the mainstream models of physics. All other theoretical models can be proven mathematically to emerge from this quantum level . The ...
• 222k
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### If the black hole attracts photons do they do the same with respect the black hole?

A black hole does not lose energy because of gravity (at least, not unless we count extreme scenarios like the merging of two black holes). Instead, infalling photons and matter exchange potential ...
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• 8,528
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### Why is light with higher frequency more destructive, if it emits the same amount of electrons?

For a complex mixture of chemical compounds, a lot of things happen. Photochemical reactions may occur without ejecting electrons from the compounds. They are, however, similar to the photoelectric ...
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### Why is light with higher frequency more destructive, if it emits the same amount of electrons?

That "high enough" conditional can be a big deal. Bodies are not made of a uniform material with a single response to a single frequency. Ejecting electrons (ionization) is harmful to many ...
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### Electrons in Atom in different energy states

Bohr's atomic energy model is outdated at present by the correct mathematical theory of quantum mechanics. Bohr's model was one of the stops on the way to the final theory , but it is an ad hoc model, ...
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### Electrons in Atom in different energy states

When an atom absorbs a photon and changes to an excited state that photon is destroyed and no longer exists. It is not stored in the atom in some way. The photon is destroyed and its energy goes into ...
• 331k
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### UV & IR radiation are mostly absorbed by water. Our eyes are predominantly water. If our eyes were made of another liquid, could we see slight UV/IR?

No. Over distances of order $1cm$ the absorption by water of near IR varies from about 1/1000 nearest the visible spectrum to near complete absorption at the low frequency end of near IR. For near UV ...
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