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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112 views

Do radio beams self-focus in the interstellar medium?

Summary: In a plasma, electromagnetic waves create a ponderomotive force that pushes electrons and ions out of the way. Thus, in an intense laser beam, electrons tends to move away from the areas ...
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Why is the speed of light in a medium smaller than its value in vacuum?

The speed of electromagnetic waves in a medium is smaller than its value in the vacuum: $$v=\frac{1}{\sqrt{\mu\epsilon}}=c/n<c$$ with the refractive index $n=\sqrt{\frac{\mu\epsilon}{\mu_0\...
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Do EM waves transmit spin polarization?

Suppose you have a normal dipole antennae (transmitter and receiver) . Spin polarized current (as opposed to normal current) is sent into the transmitter, it emits an EM wave and the Receiver receives ...
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Detailed form of light waves in vacuum and how to test it experimentally?

Consider a light wave in vacuum. Do the $\vec{B}$ and $\vec{E}$ fields have to be orthogonal to each other? Since you can add constants to a solution to Maxwell's equation it doesn't seem neccesary ...
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Experimentally diminishing random errors for low wavelength UV observations

Part of the work that I do involves observations of solar low wavelength UV observations, specifically UV-B and UV-A II (up to 340nm). I have noticed that when I observe responses on a CCD or CMOS ...
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1answer
956 views

Analytical solution of two-level system driving by a sinusoidal potential beyond rotating wave approximation

A quantum mechanical two-level system driven by a constant sinusoidal external potential is very useful in varies areas of physics. Although the widely used rotating-wave approximation (RWA) is very ...
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Why does light not propagate backward in dense media?

It is easy to see how light propagating in a dense medium destructively interferes laterally and constructively interferes in the forward direction. This is why light will travel forward in such a ...
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1answer
739 views

Hidden momentum

I'm trying to learn about hidden momentum. After reading what I could find with a google search, I understand that it is equal to the momentum carried by radiation, calculated with the Poynting vector....
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Can ray-tracing be used to design a stealth air craft? Or is it necessary to use wave theory?

This question is inspired/kicked-off by a question over in Aviation SE asking about the design of stealth air craft. Stealth technology is usually referred to when trying to make vehicles invisible ...
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1answer
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Is Wikipedia wrong about Huygens-Fresnel Principle?

Someone posted a question about why backward waves don't exist according to the Huygens-Fresnel principle: Why doesn't a backward wave exist? In following up on this question I read the ...
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Can light (electromagnetic radiation) cause electromagnetic induction in a wire?

Can light, as an electromagnetic wave, cause electromagnetic induction in a wire by passing near the wire? Does a moving electromagnetic wave cause a varying magnetic field in the region near the wire?...
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Trying to reproduce curves with angle of CMB anisotropies as a function of distance and curvature parameter

I am looking for a way to get, by a simple numerical computation, the 3 curves on the following figure: For this, I don't know what considering as abcissa (comoving distance ?, i.e $$D_{comoving} = ...
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Consider two electrons released from rest at a distance $d$. How much energy is radiated?

Abraham-Lorentz? Two electrons d apart have potential energy. Release them, they will be repelled according to Coulomb's law. I could make an assumption about the associated vector potential, but I ...
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Does Blue Light Really Have a Dark Side?

I was reading the following article on Wikipedia which discusses the dangers of blue light: Blue-light hazard is defined as the potential for a photochemical-induced retinal injury resulting from ...
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140 views

What happens to the incident electromagnetic wave in Rayleigh scattering?

In Rayleigh scattering an incident EM wave causes an induced dipole oscillation of an atom/molecule, which in turn causes radiation at the same frequency of the incident wave. But what happens to the ...
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1answer
85 views

Are these two definitions for propagation speed be the same?

We know for a fact that for any medium with permittivity and permeability $\epsilon$ and $\mu$, the propagation speed of electromagnetic waves is given by $$v = \frac{1}{\sqrt{\mu\epsilon}}$$ We ...
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Can one distinguish between near and far field for gravitational radiation?

For electromagnetic radiation one usually distinguishes between near and far field. The wave equation for the far field are based on Maxwell equations for the vacuum and predict that the radiated ...
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2answers
179 views

Why do clothes produce sounds after being in front of a computer for a long time?

Quite often, after sitting in front of the computer not moving a whole lot for a long time my upper body clothes (usually sweaters) make soft crackling sounds when moving or specially when taking them ...
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1answer
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What does a hot, optically thin gas *look* like?

In another question I tried to answer what a sample of the Sun's photosphere or core would look like, if it could be brought into the lab. Here is a broader question - if I have a small inert ...
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1answer
374 views

Is it possible to create arbitrary shapes of magnetic fields?

I've been reading about Transcranial Magnetic Stimulation (TMS) which can activate neurons using a magnetic field, which got me thinking... what if we can pick and choose exactly which neurons to ...
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Shield gamma/X-ray/ultraviolet radiation without blocking radio waves

I was theoretically considering building an airtight enclosure that shielded the insides from gamma/X-ray/ultraviolet radiation but did not block radio waves (so that communication would not be ...
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1answer
98 views

Why does hot (molten) glass glow, while diamond does not?

I have read this question: Why doesn't diamond glow when hot? This is because of Kirchhoff's law of thermal radiation. The corollary from it is that emissivity of a material is equal to its ...
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Do photons form bound states?

Since photons do not have electric charge, they do not interact via the electromagnetic force. They are also reported to be devoid of any colour-charge, so interaction via the strong force is ruled ...
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Reason for phase change in the electric field oscillations, not the magnetic field oscillations?

Wikipedia states that when an EM wave undergoes reflection, there is a phase change only in the electric field oscillations, not the magnetic field oscillations. It also suggests that the opposite is ...
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How is magnetic flux viewed in QED?

I know that QED is formulated in terms of Lagrangian density. So when we solve for a physical system we ultimately ends up finding $\psi$ and $A_{\mu}$. I'm not interested in $\psi$ for now. Since I ...
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Polychromatic propagation of EM wave

Say I have a continuous light source emitting a flat light spectrum $\hat G(\nu)$ centered at $\nu_o$ with bandwidth $\Delta \nu$ (i.e $\hat G(\nu)=rect((\nu-\nu_o)/\Delta \nu$) ) and want to see its ...
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2answers
101 views

How are photons of the electrostatic and magnetostatic fields different from electromagnetic radiation?

I understand that a photon of electromagnetic energy (light or radio) has both wave and particle characteristics and is best modelled using quantum field theory. I also understand that static and ...
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Poynting vector flux in a tube (geometrical optics)

Let's consider the following tube (defined as the solid described by two arbitrary surfaces dA1 and dA2, and by all the rays which connect their boundaries): I read in a textbook that, because of ...
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2answers
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What is electromagnetic radiation (when it is not a 'wave' of EM fields or interacting locally as a photon)?

My questions are related to the question asked at Are EM radiation and EM waves the same thing?. My background is in math (my Ph.D. thesis was in geometric analysis), and I have only taken basic ...
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166 views

Can the Taylor expansion of Energy/Intensity be explained more in-depth?

In reference to the question, "Why is energy in a wave proportional to amplitude?" Ben Crowell answered with the answer attached. The original post is linked here. I did not respond to the original ...
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If I move one electron does the rest of the Universe react to that change from QFT perspective?

I apologize if this is something super trivial: I couldn't find any answers myself so decided to finally ask my first question on Physics.SE. Classically speaking, if I move one electron from A to B (...
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Frequency spectrum of bremsstrahlung

I have tried to find a method to derive the spectrum of the emitted radiation of accelerated relativistic charged particles but I've never found such a method in any books. Does anyone know a method ...
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66 views

Transfer of an EM wave's momentum according to Lorentz Force

Momentum density in electromagnetism is \begin{equation} \vec{p_{EM}}=\frac{\vec{E} \times \vec{B}}{4\pi c} \end{equation} so, when dealing with electromagnetic waves, momentum seems to be ...
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1answer
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How does a electric quadrupole oscillate?

I know that in static a electric quadrupole is made of two positive charges and two negative charges, distributed as in the following figure: What I don't understand is when it oscillates and ...
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What are the mechanisms that cause the vacuum to be nonlinear to EM excitation?

Consider a electromagnetic excitation that is confined in time, and locally looks like a plane-wave traveling in a vacuum: $$ \mathbf{E}(\mathbf{r},t) = \mathbf{\hat{x}}E_0 \cos(\mathbf{k}\cdot\...
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Circular vs Sinusoidal Cyclotron Radiation

Why is the wavelength of circular cyclotron radiation different than that emitted by a non-relativistic electron passing through an undulator? From what I understand, the wavelength of circular ...
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121 views

Is frictional heat just electromagnetic interactions creating infrared radiation?

Example: Rub your index finger on a sheet of paper and you will feel that it will get noticeably hotter Is the reason you feel this increase in heat (technically an increase in kinetic energy of the ...
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165 views

Will the photon wavelength fluctuate in the presence of a gravitational wave?

The microwave background is due to the expansion of the Universe where the wavelengths of radiations are stretched by spacetime. As in the LIGO experiment, in the presence of gravitational wave, ...
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1answer
669 views

Why must this boundary condition be met? (Electromagnetic wave at interface between two mediums)

My textbook says that The laws of Electromagnetic Theory (Section 3.1) lead to certain requirements that must be met by the fields, and they are referred to as the boundary conditions. ...
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98 views

Estimating temperature with Boltzmann relation with split emission lines

I'm trying to estimate the temperature of a plasma through the use of hydrogen lines, $H_{\alpha}$ and $H_{\beta}$ using the Boltzmann relation: $$ \frac{ n_{2} }{ n_{1} } = \frac{ g_{2} }{ g_{1} }e^...
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Higher order multipolar second harmonic generation in centrosymmetric materials

As is pointed in this question, second harmonic generation is forbidden in the bulk of the materials possessing centrosymmetry. In some papers it is said that in the dipolar approximation the SHG ...
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0answers
247 views

Free charge movement in an electric field - including bremsstrahlung

Let us imagine a free, negatively charged object that is in rest and placed in an elecric field of a point positive charge. The positive charge has a huge mass and cannot move, so we consider only the ...
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1answer
78 views

Mathematical Formulation for Accelerating Charges Generating Light?

I haven't been formally taught the fact that accelerating charges generate light, and I have seen the picture of an accelerating charge sending ripples down its field lines, but is there a ...
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Questions about Michelson interferometer

I have been doing experiment on Michelson experiment, but I don't quite understand why white light results in an interferogram with very few fringes, and why are they necessarily Gaussian? I know that ...
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0answers
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What is the attenuation spectrum of radio waves in pure water?

Here is a plot of microwave attenuation in atmosphere: (source) I'm looking for a similar plot, but for pure water, and a frequency range covering at least 100 MHz to 2 GHz, hopefully more.
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1answer
91 views

An accelerating charge emits EM waves, but how can this be explained in terms of photons?

I was reading this response to a question involving EM radiation due to an accelerating charge. A charge's oscillations disturb its electric field, and this effect propogates at the speed of light. If ...
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Lossy electromagnetic wave and reactive power

Consider an electromagnetic wave in a lossy medium. This medium has a complex permittivity $\varepsilon_r=\varepsilon_r' +j \varepsilon_r''$ and a wave impedance given by $$Z=\frac{E}{H}=\sqrt{\frac{\...
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Complex Poynting vector and energy loss

Consider continuum medium (no boundaries) and the electromagnetic wave Poynting vector $$ \mathbf{S} = \mathbf{E}\times \mathbf{H}^*$$ with $^*$ the complex conjugate. When there is a phase difference ...
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1answer
29 views

Why is the impedance of an electromagnetic wave written as a ratio between the electric and magnetic fields, $E/H$?

The 'impedance of free space is written like this, too, as the absolute value of the $E$-field over the absolute value of the $H$-field. What does the ratio of the strengths of the two fields of an ...
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1answer
95 views

Is constant acceleration not enough for radiation?

A reference source: Does A Uniformly Accelerating Charge Radiate? Consider the behavior of a DC current compared to an AC current. Direct current: has locally fast moving free electrons that ...

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