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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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2answers
65 views

When did scientists first experimentally measure wavelengths of EM radiation? How?

When did someone first discover that short wavelength light has higher energy than long? And can gamma ray wavelengths be measured, even today?
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1answer
37 views

About blocking of microwaves

When a phone is inside microwave oven and if we call it using wifi the phone doesnt recive call as frequency of wifi is same as frequency of wave generated in oven as there is a mesh like thing which ...
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0answers
10 views

What is the energy of Evanescent waves in Total Internal reflection?

In TIR, all energy is reflected back as the reflection coefficient becomes 1. Since all energy is reflected back , how come evanescent waves possess energy?
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Do sidebands corresponds to real photons at that frequency?

Say I have a carrier laser (optical) frequency $\omega_c$: $E=E_0 e^{i\omega_c t}$. I propagate it through an electro-optical modulator that modulates the phase by $\beta \sin\Omega t$: $E = E_0 e^{i\...
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1answer
35 views

What happens to absorbed light?

I was wondering what happens to a ray of light after it is completely absorbed by a surface (making it appear black). I mean, obviously, the energy should be conserved. So does the energy change into ...
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2answers
55 views

In which situation does the electric field vary sinusoidally with position?

If I have a point charge q at the origin then the electric field will be $$ \mathbf{E} = \frac{1}{4\pi\epsilon_0} \frac{q}{r^2} \hat r $$ that is to say that electric field will diminish by the square ...
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1answer
22 views

Planck theory of discrete energy of photons and electromagnetic spectrum

Planck said that "energy of photon is quantized. That means energy of a photon cannot be any random value but a certain values only". If this is true how electromagnetic spectrum is continuous. For ...
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0answers
23 views

Radiation from collision of two charged particles

Consider a collision between two electrons. Since each particle is accelerated during the collision, I assume each radiates energy according to the Larmor formula. Question 1: How much energy is ...
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1answer
47 views

Why violet light bends the most? [duplicate]

I have read about refraction through a triangular prism,rainbow formation and other natural phenomena.They all told about the strong bending of violet light . I know that it has a shorter wavelength ...
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1answer
71 views

Will a rotating disk contraction emit light?

The Ehrenfest paradox suggests an object will become smaller at the outer rim due to contraction, When near light speed. Or actually, any object near light speed will contact and appear smaller in its ...
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1answer
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Black body radiation and line spectra

Firstly, Blackbody radiation and line spectra are both a phenomenon of energy emitted(EM), but why is the blackbody radiation spectrum continuous while line spectra is not continuous? Secondly, ...
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1answer
19 views

Can the UV degradation of polymers be calculated?

Is it possible to calculate the mass lost due to the UV degradation of polymers (like PET or LDPE)? I have found no information regarding equations or formulas which would provide an answer. Any ...
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2answers
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Standing Wave in a transmission line and Resonance

let's consider a transmission line of length L closed on a mismatched load. So, there will be a travelling wave and a reverse travelling wave: V(z) = V+(z) + V-(z) My question is: will there always ...
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1answer
28 views

What is the difference between scattering and absorption/emission?

As far as I know, scattering occurs when light excites the atoms or molecules to their higher energy state(virtual state for scattering) followed by emitting photons corresponding to energy ...
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Why should $E$ and $B$ both contribute $(1/2) kT$ in equilibrium radiation?

At high temperatures/low frequency, energy distributes itself by equipartition. There's a factor of $\frac{1}{2}kT$ for each degree of freedom that contributes quadratically to the energy. So, for ...
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25 views

What are the deciding criteria for occuring of reflection, absorption, Emission, refraction and burning? [closed]

Why does some objects reflect, refract, absorb, emit and burn, when electromagnetic wave is thrown on them? I want to know all these things at atomic and molecular level.
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Retarded Coulomb's law and EM waves; Feynman texts

In this chapter, Feynman writes down the retarded Coulomb's law, $$\begin{equation} \label{Eq:II:21:1} \mathbf E=\frac{q}{4\pi\epsilon_0}\biggl[ \frac{\mathbf e_{r'}}{r'^2}+\frac{r'}{c}\,\frac{d}{dt}\...
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How does a single satellite like the upcoming BIOMASS use interferometry?

The future satellite BIOMASS, from the ESA, will supposedly use p-band frequency interferometry to detect changes in forests. By itself? How?
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Relation between AC Current and electromagnetic wave in transmission lines?

I m so confused we started by learning about electromagnetic waves in the vacuum , then we went to transmission lines and there the professor mentioned the em waves are traveling through them. I ...
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1answer
19 views

Collective dipole oscillation and specular reflection

Let's say I shine light upon a surface and observe a specular reflection. If I look with a electron microscope, I see tiny particles much smaller than the wavelength of light (~10 nm) and also closely ...
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2answers
98 views

What does it actually mean for an electron to be excited? [duplicate]

I've been confused on the connection between photons and electrons for a very long time. Some examples of questions I"ve asked are linked here: What produces higher frequency light? Frequency of ...
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3answers
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Why doesn't hot charcoal glow blue?

I was learning about black body radiation and the explanations given by Max Planck and Albert Einstein when a thought crossed my mind. When we heat an iron piece, its color changes gradually from ...
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4answers
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Frequency of light versus frequency of electron vibration

I've been trying to understand photons and light, although entirely conceptually (layman with not much of a background here, but I really want to understand this a bit better) and there's a couple of ...
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0answers
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Will a magnet diffuse/defocus a laser?

I have read through hundreds of questions here and we discuss why light is not bent by an electric or magnetic field (at least not at easily visible levels in the classical sense), but that there is ...
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0answers
25 views

How to do colour map in MRI?

For this question, I assume if we do a colour map, it is based on the magnitude of the magnetic strength? After that how do we know where each coordinates belong to? Like for example at p= 0 degrees,...
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4answers
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Can a magnet bend light? [duplicate]

As light is electromagnetic radiation. Then why I don't see any magnet bending light wave? Or why light doesn't diffract whenever it passes by a live wire?
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1answer
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Why don't phosphorescent molecules just immediately re-emit the original UV photon?

The energized electron of a phosphorescent molecule is said to be 'trapped' in its excited state, because it has to undergo one (or more?) 'forbidden' transitions to get back down to it's original ...
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1answer
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Interpretation of real and imaginary parts of the Poynting vector in a vacuum

It is known that the flux of the Poynting Vector through a certain surface represents the total electromagnetic power flowing through it. Then, its real part is the active power, while its imaginary ...
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1answer
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What is the average energy of visible light photons, of what we perceive to be white light? (i.e. visible spectrum of 5500K radiation)

Visible light is anything between 1.65eV and 3.10eV, so the answer is somewhere between these values. Naively we could average this to 2.375 eV, but that would not be actual average of full spectrum ...
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2answers
87 views

Deriving the speed of light from Maxwell's equations?

Relationship between speed of light and EM force? Can it be said that Maxwell used measurements of the "strength of electric force and strength of magnetic force", to derive the value for the speed ...
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3answers
136 views

Why are speeds of different EM waves in vacuum not EXACTLY equal?

It is said in my textbook (reference below) that the different waves of the Electromagnetic Spectrum have velocities almost equal to each other. (Variations are within a few m/s according to my ...
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0answers
47 views

Sensing different forms of energy [closed]

Can someone indicate whether electromagnetic radiation can be felt by the human body? And if so at what frequency? And what are the effects of exposure to it. Looking forward to your reply.
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How to justifiy that $\rho_{\text{rad}} \approx \rho_{\text{mat}}$ at recombination time?

In standard cosmology, the recombination time is estimated to be $t_{\text{rec}} \approx 380~000~\mathrm{years}$ after the Big Bang, when matter and electromagnetic radiation becomes decoupled. It's ...
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3answers
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Are electromagnetic waves only caused by accelerating charges?

Like the title says, are electromagnetic waves only caused by accelerating charges? Are there any other methods for the generation of photons? If electromagnetic waves are ONLY able to be created by ...
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25 views

Electromagnetic waves highest frequency and beyond? [duplicate]

If an electromagnetic waves frequency is generated by the change of voltage / current in a wire or a atom decaying to get gamma waves. What is the highest frequency possible give the faster the ...
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1answer
26 views

Is the falling back to ground state of an electron what makes the electromagnetic wave, or is it the action of absorption AND falling down?

When an electron absorbs a photon, there it accelerates and thus it creates a kink in its electric field. Then, when it falls down to its normal state, it creates another kink. Here are my questions. ...
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1answer
18 views

Conductors and non-optical photons

While there seems to be plenty of information available about the photoelectric effect and the emission and absorption of photons by conductors (metals) at optical frequencies, I’ve been searching for ...
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0answers
28 views

A travelling field; Feynman lectures on Physics Volume 2

In Feynman's Lectures on Physics, volume 2, Chapter 18 (18-5), we look at the creation of a electromagnetic field, due to a moving infinite sheet of charge, By looking at the Maxwell equations ...
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1answer
148 views

Black body radiation curve

See images about black-body radiation. How does Planck's quantum theory explain the low intensity of radiation for high frequencies? I.e why does the black body curve become lower at the high ...
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3answers
42 views

Are interference patterns the basis for all other measurements of light frequencies and wavelengths?

The wavelength or frequency of monochromatic electromagnetic radiation is determined indirectly by calculations on the interference or diffraction pattern. These patterns are generated by gratings, ...
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2answers
62 views

Is it possible for a radiant heat transfer to create higher temperature in the sink compared to the source?

Assume we concentrate the rays of the sun onto an object (in the vacuum of space) that has very low emissivity - The energy coming in is higher than the amount being emitted as radiation - hence it ...
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3answers
25 views

Electromagnetic Waves in a Dielectric [duplicate]

I read that when an EM wave enters a dielectric material, it keeps the same frequency, while its wavelength is reduced, so that their product is a quantity less than c = 3 × 10^8 m/s. Why does it ...
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2answers
34 views

Skin depth; EM wave and AC

When I google skin depth, I get the following definition, Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current ...
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1answer
64 views

Does an electromagnetic wave have any spatial extent transverse to its direction of propagation?

Electromagnetic waves (classical, non-quantum conception) seem to be typically depicted as mutually orthogonal plane waves with amplitudes varying orthogonal (transverse) to the direction of ...
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1answer
54 views

Is a Microwave oven without a faraday cage a health hazard?

Microwave ovens have something of a Faraday-cage/shield to keep from leaking (emitting) EM radiation. What would happen if the shield or the door was removed? No doubt it would be less efficient as ...
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1answer
33 views

Alternating current radiates?

Does a wire carrying alternating current release electromagnetic radiation in accordance with Maxwell's equations?
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0answers
48 views

Is every kind of energy quantised?

I know this is been asked on this site before but I felt unsatisfied by its one answer. I’m familiar with the concept of quantisation, I keep reading that “energy is quantised” but what do they mean ...
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Detecting particles that do not make it through double slits, do observations match those after the slits?

I suppose we have 4 conditions to examine : { single particle | beam } * { with eraser | without eraser } I suppose we have to get the same results that those ...
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0answers
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How to calculate the RCS (Radar Cross Section) of a sensor tag?

The equation for finding the RCS of sensor tags is given in the following paper. where $S_{21}^{tag}$ stands for the $S_{21}$ measurement taken for the tag in front of a reader. $S_{21}^{isolation}$ ...
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1answer
31 views

Irradiation at high frequency

I read this statement in Feynman Lecture on Physics (see here for more details): In low-frequency circuits the connection is made with wires, but this method doesn’t work very well at high ...