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Questions tagged [electromagnetic-radiation]

Propagating solutions to Maxwell’s equations in classical electromagnetism and real photons in quantum electrodynamics. A superset of thermal-radiation.

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2 votes
3 answers
523 views

Intuitive understanding of the derivation of the Rayleigh-Jeans law

I know the Rayleigh-Jeans law and how the formula predicts UV catastrophe. Without getting into the exact derivation, I am trying to get some intuitive understanding of it by using some of the broad ...
8 votes
4 answers
1k views

How can photons interact with nuclei?

How can photons such as X-rays or gamma rays interact with the nuclei of atoms given that, as I understand it, the length scale of a nucleus is around a couple of femtometers? So, shouldn’t the size ...
1 vote
1 answer
304 views

How to get the formula of the energy of EM waves?

I am trying to get the formula for energy of EM waves: $$W = \frac{E^2 + B^2}{2}$$ calculating the work done on a test charge by the force: $$\mathbf F = q(\mathbf E + v \times \mathbf B)$$ $\mathbf ...
5 votes
2 answers
483 views

Does the Larmor Formula assume circular motion?

The Wikipedia page on the Larmor formula says that the Larmor formula makes the unavoidable assumption that the charged particle is orbiting in a circle. This quoted sentence isn't true, correct? I ...
0 votes
5 answers
228 views

Is there a filter which can directly convert sunlight to microwaves?

Is there a way to filter sunlight (visible, UV, and/or IR) and convert the energy from it directly into microwaves without putting in any energy? Barring this, is there a way to do the same but while ...
0 votes
4 answers
117 views

Why should an electron falling into the nucleus, according to the Maxwell's laws of electrodynamics, destroy the atom?

It is often said in physics and chemistry classes and textbooks that atoms must be unstable when the electron continuously loses energy and finally fall into the nucleus according to classical physics....
0 votes
1 answer
92 views

How many photons pass through us every second?

I just read this answer https://physics.stackexchange.com/a/229374, which says that, when a magnet rotates, photons are emitted with wavelength $λ=c/f$, where $f$ is the frequency of rotation. And ...
4 votes
2 answers
990 views

Why do accelerating electrons not emit electromagnetic radiation?

I've read at many places that accelerating charged particles emit electromagnetic radiation, but on this article on Bohr's hydrogen atom, at the end part, it's written under Limitations of the Bohr ...
-2 votes
2 answers
179 views

Is it possible to explain the Ultraviolet Catastrophe as a manifestation of the Riemann-Lebesgue Lemma? [closed]

Is it possible to explain the Ultraviolet Catastrophe as a manifestation of the Riemann-Lebesgue Lemma? I don't fully understand any of both topics, but reading about the Ultraviolet Catastrophe on ...
0 votes
2 answers
217 views

Can polarization states of a photon be understood in terms of spatial orthogonality/dimensions?

For example, do the terms 'horizontal', 'vertical', 'diagonal' and 'anti-diagonal' polarization have any relevance to the physical, quantum state of a photon, or are they simply descriptive of how one ...
2 votes
0 answers
30 views

Why does oxygen green (S1) emission in aurorae only occur at lower altitudes? [duplicate]

Aurorae have a red color at high altitudes caused by the excitation of atomic oxygen and the subsequent emission at about $630 \,\text{nm}$. This happens at high altitudes because at that height there ...
1 vote
1 answer
473 views

How big can solar flares get and could they boil away Earth's atmosphere?

Is there a known theoretical maximum size for a solar flare and would it mean curtains for life on Earth?
1 vote
2 answers
38 views

Is luminescence from impact of fast neutral atoms/molecules on a suitable substance known?

Cathodoluminescence is emission of photons by electrons impacting on a luminescent material. The Rutherford scattering experiments detected impacting helium nuclei on a phosphor screen. Many other ...
0 votes
2 answers
276 views

What effect does color on the upper and lower side of umbrella have?

This question isn't referring to the general use of umbrella to provide protection from rain rather it concerns the optical effects of the color on lower and upper portion of umbrella. Umbrella are ...
10 votes
3 answers
2k views

What do we mean when we say the CMB has a temperature and how do we measure it?

I have read this: An object without any internal degrees of freedom, like a single photon, can't really have a temperature. But an ensemble of photons can have a temperature. If you put an ensemble ...
4 votes
2 answers
3k views

Why are electromagnetic waves inverted when reflected at a fast to slow medium boundary?

I've been taught before that mechanical waves, when they hit a boundary between a less dense (faster) medium and a more dense (slower) medium, the reflected wave is inverted because it acts like a ...
7 votes
1 answer
714 views

Can gravity radiate?

In electromagnetism, when a charge accelerates, it emits radiation. We know this because we can write the retarded potentials, apply $\vec E=- \nabla V-\frac{\partial \vec{A}}{\partial t}$ and $\vec B=...
0 votes
1 answer
104 views

Why is the Poynting flux not conserved across the interface between two conducting materials?

This question is part of my attempt to use an answer to another question I've posted on this site. If I have two materials with complex indices of refraction, $\widetilde{n}_0$ and $\widetilde{n}_1$, ...
0 votes
0 answers
22 views

dipole-radiation in semiclassical dynamics solid state

Using the semiclassical dynamics in solid state physics (electrons on a lattice with periodic potential, constrained to a band structure), we usually obtain that in the presence of external fields (...
0 votes
1 answer
722 views

What is the physical meaning of the complex field amplitude?

I see that the harmonic field is sometimes written in exponential form. But sometimes the complex amplitude of this form is just a constant and in others (like when talking about modes) is dependent ...
2 votes
2 answers
229 views

How can we visualize electromagnetic radiation?

In so many classes I’ve been told that light is nothing more than an electromagnetic wave. While this makes sense, it’s never been nearly as clear as imagining a static charge. At every point in space ...
1 vote
1 answer
134 views

How to solve Relativistic Lorentz Force equation if $\gamma$ is not constant?

I am trying to numerically obtain the trajectory of an electron inside a periodic magnetic field $\mathbf{B}$, taking into account that the relativistic factor $\gamma$ is not a constant (the electron ...
0 votes
2 answers
386 views

Origin of electromagnetic radiation in the universe

I don't know if the total electromagnetic radiation in the universe is finite or not. But either way: what is the origin of the electromagnetic radiation in the universe? Can we say that all of it ...
0 votes
2 answers
47 views

Confusion about EM spectrum and the Fourier transform

Since courses on signal analysis and electromagnetism I have become confused about what the spectrum of electromagnetic radiation really means. I know light is when electric and magnetic fields become ...
2 votes
2 answers
108 views

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 ...
0 votes
1 answer
22 views

Dipole radiation of a non-relativistic electron in elliptic motion [closed]

I'm trying to solve the following problem: A non-relativistic electron is moving in elliptical motion inside a positively charged cylinder of homogenous charge density $\rho$. The initial radius ...
2 votes
1 answer
38 views

Is there a way to use the optical transfer-matrix method to determine the absorbance of each layer in a multilayer stack?

I've implemented the transfer-matrix method to determine the the transmittance and reflectance of a multilayer stack of thin films. I know that (ignoring scattering) the absorbance of the entire stack ...
0 votes
2 answers
316 views

Why is this question indicating a Fraunhofer Region rather than Fresnel Region?

I was thinking that because the source plane is infinitely large would be analogous to having the source plane being infinitely close, which would imply a Fresnel region rather than Fraunhofer. ...
-2 votes
1 answer
1k views

Can Amps be used as a unit of measure for the amplitude of an EM wave?

As far as I know the definition for the amplitude of a wave is the distance between the line $y=0$ and the peak of the wave which in most cases is going to be a unit of length. I also understand that ...
5 votes
3 answers
2k views

Is it possible to get an electromagnetic wave equation if the speed of light were infinite?

In an old question: How would night sky look like if the speed of light was infinite? the best answer was voted down to negative credits. I cannot understand why. From Maxwell's equations, we derive ...
0 votes
1 answer
88 views

Lambertian surface and the luminous intensity

I am currently studying the basics of photometry to better understand the rendering equation of Kajiya. One thing I'm currently struggling with is Lambert's cosine law. Let's go over the premises: A ...
-1 votes
1 answer
52 views

Do bosons exist at different energy densities?

Excuse my rudimentary (at best) physics knowledge, but I was running a thought experiment earlier (read musing to myself). I was wondering whether a photon could be considered analogous to bowshock, ...
1 vote
1 answer
228 views

Induced charge density on perfect conductor

Assuming there is a perfect conductor at $x=0$ in $\mathbb{R}^3$ and a plane EM wave $\vec{E}_i(\vec{x},t)=\vec{E_i^0}e^{i(kx-\omega t)}$ is coming from $x=- \infty$. We know, that the wave $\vec{E_r}(...
1 vote
3 answers
106 views

If quaternions are an extension of complex numbers, is there a study of EM wave theory in terms of quaternions?

I find that in standard textbooks and online resources, basic EM wave theory (such as radiation, plane wave solutions, polarization) is discussed by treating fields in terms of complex numbers. Is ...
6 votes
4 answers
2k views

Accelerating Expansion of Universe - Why Not Caused by Radiation?

As I understand it, dark matter and dark energy are used as an 'explanation' for how universe expansion is accelerating; because without it gravity would be expected to cause a long term shrinking. ...
2 votes
0 answers
53 views

The $\alpha$ particle's energy inside a nucleus is lesser than the Coulomb barrier height. Justify

The $\alpha$-decay is usually explained via quantum tunnelling. This is because the $\alpha$ particles do not have sufficient energy to climb over the Coulomb barrier. But how do we know this? We can ...
1 vote
1 answer
291 views

Interference of coherent electromagnetic waves in two dimensions?

Interference of two coherent but mutually perpendicular electromagnetic waves whose electric field vectors are given by $$E_{1} =E_{0} \cos (\omega t) \widehat{i} $$ $$E_{2 } =E_{0} \cos (\omega t ...
2 votes
2 answers
205 views

Understanding the Interference of Light

I would like to understand interference using 2 illustrations: 1) Bulbs send out light in all directions, right? If so, at a particular instant of time, shouldn't all the light waves coming out from ...
0 votes
0 answers
39 views

Spherical laser beam terminology

I am currently seeking the correct terminology for a beam that expands linearly from a fixed point, resulting in its wavefronts forming spherical surfaces. However, the beam does not expand in all ...
29 votes
5 answers
14k views

How can a point source emit spherical EM waves when they are forbidden by Maxwell's equations?

I know that there exist plane wave solutions to the Maxwell equations in free space, and I tried solving them for a spherical wave emanating from a point but could find no solution consistent with the ...
0 votes
0 answers
33 views

What is the solar radiative power on a clear sky day through a surface which is parallel to the sun's incidence on Earth?

Assuming that the solar power through the incident surface is known. How can we calculate or estimate the radiation through the parallel surface assuming a clear sky? Looking for watts per square ...
9 votes
2 answers
3k views

Why doesn't photon lose energy over distance?

In an atom, ( or even an oscillating circuit) When an electron falls from higher energy level to a lower one, it gives a photon of energy $hf$. On other hand it can be considered as an accelerated-...
3 votes
1 answer
750 views

Why is the stress-energy tensor for electromagnetic radiation traceless?

A photon gas obeys the equation of state $\rho=P/3$ and hence $T^{\mu}_{\quad\mu}=3P-\rho=0$. (Can also be seen by expressing the stress energy tensor in terms of of the electromagnetic tensor as ...
134 votes
2 answers
69k views

How many photons are received per bit transmitted from Voyager 1?

As of 2024, according to https://voyager.jpl.nasa.gov/ , Voyager 1 is around one light·day away from Earth and still in radio contact. When Voyager 1 sends messages to Earth, roughly how many photons ...
0 votes
3 answers
216 views

What is the intensity of photon before and after striking the glass prism?

during dispersion we all know that white light enters in to the prism and disperses into the visible radiation, my question is whether a glass prism can eliminate other wavelengths and allow only ...
0 votes
2 answers
39 views

Stars that have fairly high gravitational redshift and calculation of their surface temperature by Planck emition spectra?

How high can the ratio between gravitational redshift and planck emition spectra be depending on the mass of the star so by how much this gravitational redshift could elongate the Planck spectra of ...
0 votes
4 answers
266 views

How does blackbody radiation suggest the quantization of energy?

I have read about Wein's law and Rayleigh-Jeans law which were apparently based on classical mechanics and couldn't explain the radiation spectrum of a blackbody. Then Planck came up with the ...
0 votes
1 answer
347 views

Finding angles of second and third order minima given only angle of first order minimum

For the following problem we are asked to find the second and third order minimums of a single slit diffraction experiment at $13^{\circ}$. I tried using the following $$\theta_p=p\cdot\frac{\lambda}{...
1 vote
1 answer
86 views

What is light? how is it connected to electromagnetic radiation?

I am trying to understand how light works and electromagnetic radiation, from what I understand charges cause disturbances in electric fields, which triggers a magnetic field, and then the back and ...
11 votes
5 answers
2k views

How can a free electron "oscillate" in the presence of em waves?

I have been told a many times that in a region with oscillating electric and magnetic field, a free electron if left will also oscillate. But I don't think its true. I actually asked this to my ...

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