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What is the shortest MEASURED gamma ray wavelength?

In February 2024 researchers at China's Large High Altitude Shower Observatory reported the detection of a gamma ray with an energy of $2.5$ PeV. This corresponds to a wavelength of about $5 \times 10^...
gandalf61's user avatar
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Absorption and emission spectrum

If the substance is opaque you need a very tiny amount of it to get an absorption spectrum in its absorbing region. Otherwise we would just block the detector from the light completely and see nothing....
Matt Hanson's user avatar
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Ray separation in waveplates

Quarter and half wave plates are arranged in such a way, that the axis of the uniaxial birefringent crystal is perpendicular to the direction of travel of light, and parallel to the surface of the ...
Sebastian Riese's user avatar
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Why don't the delocalised electrons in a metal emit light when they hit an atom and change their velocity very quickly (i.e. accelerate)

Why don't the delocalised electrons in a metal emit light when they hit an atom... And why don't the electrons emit light when they hit other electrons? The reason is because we neglect the electron-...
hft's user avatar
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-1 votes

Why isn't the original EM wave in a beam of light in a medium not still detectable from a distance as if it were moving at the speed of light?

Light interference is a convenient method at the high school level to explain the double slit experiment and other phenomenon ... it works fairly well and with the Huygens principle there are many ...
PhysicsDave's user avatar
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2 votes

At what point does a line spectrum become a continuous one?

The spectrum plots you show in your question are claimed to represent stars of different temperatures. Stars do indeed show a continuous spectrum and the reasons for this depend on the effective ...
ProfRob's user avatar
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3 votes

At what point does a line spectrum become a continuous one?

The easy (and somewhat oversimplified) answer is: you get a continuous spectrum once you have so many lines that you can no longer tell them apart. The more precise answer is that the electrons in a ...
paulina's user avatar
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At what point does a line spectrum become a continuous one?

Line spectrum comes from single atoms in gases , band spectrum from molecules and continuous spectrum from heated solid bodies.
trula's user avatar
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1 vote

Can a region of electromagnetic waves alone be considered a thermodynamic system? Can work and heat interactions be made sense for them?

EM field in general does not easily conform to the standard idea of system in thermodynamics, because EM field often permeates matter walls, spreads to infinity, also carries energy to infinity, and ...
Ján Lalinský's user avatar
3 votes
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Can a region of electromagnetic waves alone be considered a thermodynamic system? Can work and heat interactions be made sense for them?

The thermal noise of a resistor (Nyquist-Johnson noise), a form of black body radiation that has temperature and entropy, can be filtered, amplified, frequency converted, etc., and be transmitted by ...
hyportnex's user avatar
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1 vote
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In a light wave what part of the electromagnetic wave is the light?

Excellent question. Usually electromagnetic waves are absorbed, reflected and diffused through the so-called electric dipole coupling. What we see is proportional to E$^2$. Optical phenomena are ...
my2cts's user avatar
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1 vote

Angular velocity vs Angular frequency

They are synonyms. Frequency is number of turns per second. Since we know that one turn is $2\pi$ radians, we can extrapolate angular frequency $\omega = 2\pi \nu$. Which then will be measured by $rad/...
Agnius Vasiliauskas's user avatar
1 vote

Angular velocity vs Angular frequency

Yes, these are the same. Angular velocity and angular frequency both have the units rads * s^-1. rads is a unitless scalar, ...
elfeiin's user avatar
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3 votes

If helicity of photons is +1 then the light is right- or left-circularly polarized?

For historical reasons, the optics convention and the helicity convention are opposite. The historical reason is that we didn't originally know that these two things were related, and so the initial ...
rob's user avatar
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1 vote

Photon propagation paradox, what am I missing?

Photons/EM waves only add/subtract when they cross but otherwise pass through without changing their own characteristics. Correct. You can't cancel them like you can with sound. Incorrect. The ...
The Photon's user avatar
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0 votes
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Radiant intensity of a (non-Lambertian) emitter, given its radiance function

To calculate the radiant intensity of the source, you need to integrate $L$ over the area of the source: $$ I_{\mathrm{source}}=\iint_{\mathrm{source}}L\left(\theta,\phi\right)\mathrm{d}a=L\left(\...
tush's user avatar
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2 votes

Intensity wavelength graph random peaks

Your image sketches a typical X-ray spectrum. (image from schoolphysics - X-ray spectra) The peaks are the so-called characteristic X-ray radiation. They got this name because they are characteristic ...
Thomas Fritsch's user avatar
2 votes
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Intensity wavelength graph random peaks

Since I do not know what exactly you are doing I can not be sure. But from what you described I would assume the following is happening: You have a device producing free electrons, you accelerate them ...
Zaph's user avatar
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1 vote

Effect of incident angle on wavelength of transmitted wave for normal polarisation?

In a homogneous, non-conducting, linear material then the speed of light depends only on the scalar refactive index. And, since the frequency of the wave is unchanged either side of the interface, ...
ProfRob's user avatar
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Intensity of EMR and time dependence

Your idea is basically correct. It all depends on what type of measurement is being done. If you have an instrument with a pointer that moves left or right in proportion to the electric field ...
Andrew Steane's user avatar
2 votes

In magnetrons, is it the accelerating electrons or alternating fields within the anode that produce the microwaves?

A magnetron shoots a stream of electrons past a series of cavity resonators in a circle. A DC magnetic field bends the stream into a circle that passes by the resonator openings over and over again. ...
niels nielsen's user avatar
17 votes
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What fraction of the universe's energy is contained in photons?

See (Fukugita & Peebles 2004), which estimates that $10^{-4.3}$ of the energy density of the universe is in the CMB photons, and $10^{-5.7\pm 0.1}$ in stellar and poststellar radiation.
Anders Sandberg's user avatar
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Is sky still blue when the line-of-sight is perpendicular to the sunlight ray?

First of all, using the symbol $\theta$ for the angle between the line of sight and the direction of polarization is potentially misleading here as this symbol is normally used for the scattering ...
Thomas's user avatar
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2 votes
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Is sky still blue when the line-of-sight is perpendicular to the sunlight ray?

No it doesn't because sunlight that enters the atmosphere is unpolarised. Thus it effectively consists of a mixture of two perpendicular polarisation states of equal amplitude with random phases ...
ProfRob's user avatar
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1 vote
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How is wavelength defined when it's changing continuously?

Those are good ways. Though you can't make the time interval infinitesimal because you then can match any sine wave. If the function is slowly varying, the frequency doesn't change fast. You get a ...
mmesser314's user avatar
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Is the curvature so extreme at the event horizon, that you could see curved laser beams?

Yes, this is true. Whilst laser light differs from ordinary light in being coherent, it is still light and so the effect of black hole still holds. Its worth noting that close to black hole we will ...
Mozibur Ullah's user avatar
4 votes

Is the curvature so extreme at the event horizon, that you could see curved laser beams?

Yes, the curvature is enough to bend lasers. In fact, there is an interesting feature in Schwarzschild spacetime that might answer your question well. If a non-rotating, uncharged black hole (i.e., a ...
Níckolas Alves's user avatar
1 vote

Geometrical question about solid angles

$dA_{jp}$ is the projection of the vector $\mathbf{\hat n_j}dA_j$ onto the unit normal vector at $p$ on the unit sphere. So, you take the dot product: $$dA_{jp}=\mathbf{\hat n_p}\cdot \mathbf{\hat n_j}...
Albertus Magnus's user avatar
2 votes
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Emission spectrum of a fluorescent lamp

the emission lines you detect are only those that make it all the way through the phosphor coating on the inside of the CFL tube. Those phosphors are designed to absorb as much of the spectral output ...
niels nielsen's user avatar
0 votes

Photons and Bremsstrahlung radiation

Classically, each electron makes an electromagnetic pulse. This isn't a bad model. Quantum mechanically, the Fourier power of the pulse in a band $\Delta\omega$ at frequency $\omega$ gives you the ...
John Doty's user avatar
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What exactly qualifies something to be a transverse wave?

Statement D is not true because there are transverse waves that require a medium to be vibrated and so do not travel in vacuum. Transverse waves on a rope is an example. By a process of elimination I ...
gandalf61's user avatar
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Cooling at a Distance

Maybe instead of using light to cool the pizza you could have the heat transfer to something like a precooled gas or maybe some sort of precooled solid. if you want to do it uniformly though, you can ...
Lawrence sinistra's user avatar
3 votes

Are all recursive interactions between electric and magnetic fields always orthogonal to each other?

There is not a causal relation between electric and magnetic fields. According to Jefimenko's Eqs (https://en.wikipedia.org/wiki/Jefimenko%27s_equations), $\vec E$ ($\vec B$) is caused by charge, and ...
JEB's user avatar
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Are all recursive interactions between electric and magnetic fields always orthogonal to each other?

Not necessarily. Consider a waveguide, the TM and TE modes it presents have $\mathbf{E}$ and $\mathbf{H}$ in general not orthogonal to each other. The functional form of the fields depends on the ...
agaminon's user avatar
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1 vote

Can the magnitude of the electric field or magnetic field in a plane electromagnetic wave be equal to zero?

Let your EM plane waves be described by $$ E_y(x,t) = E_0 \cos (\omega t - kx) \quad\quad B_z(x,t) = B_0 \cos (\omega t -kx), $$ then we have that $\dfrac{E_y}{B_z} = \dfrac{E_0}{B_0} = c$ at all ...
Gabriel Ybarra Marcaida's user avatar
2 votes

What's the difference between the different kinds of EM waves?

As was already pointed out, there is essentially no difference between different types of EM waves except their wavelengths (or, equivalently, the energy of the photons). However, there are certain ...
paulina's user avatar
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4 votes

What's the difference between the different kinds of EM waves?

X-rays...can exist at any given wavelength. That's not what "X-ray" means. In most cases,* when people say "X-ray," they are talking about a specific range of wavelengths/photon ...
Solomon Slow's user avatar
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1 vote
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Why do we see objects with a given color?

When light hits an object, it may be reflected, transmitted, or absorbed. Each photon undergoes one of these. There is more than one way to change the fraction of incident light that gets each outcome....
mmesser314's user avatar
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