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

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Why is it hard to detect a black hole

I've read in some texts that we can't directly observe a black hole in space because not even light can escape from its gravity. Some of the indirect observational methods mentioned are, gravitational ...
3
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2answers
76 views

Do light particle/waves have a frequency? [duplicate]

I sought the answer to the question about amplitude of light waves first, but I was actually thinking about whether the wavelength is the only property of a single quanta of light. I suppose direction ...
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2answers
187 views

Can we draw analogy between em power flow through free space and ac power flow through a transmission line?

Knowing that the free space has a characteristic impedance (which is purely resistive, measured in ohms) I was wondering if I can model the free space as an infinitely long transmission line- ...
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2answers
28 views

Is the screen in CRT connected with a positive pole?

I don't understand this passage would you clarify it? "These electrons are then freed (liberated) from the metal and are then picked up by the screen, which is connected to a positive pole called the ...
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5answers
43 views

Light's oscillation in time

Electromagnetic waves have electric (and magnetic) fields that oscillate spatially and with time. But light, moving at the universal speed limit, is a "space-like" object according to relativity since ...
6
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1answer
171 views

How is extremely low frequency (ELF) radiation collected by a submarine antenna?

The U.S. Navy Project ELF managed to generate extremely low frequency (ELF) radiation at down to $\approx 76$ Hz (implying a wavelength of $\approx 3,945$ km!). I was curious, what kind of receiving ...
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0answers
31 views

Is this fine to think of light as the following? [on hold]

Is light quantums (increments [photons]) of the electromagnetic waves which are synchronized by oscillations of electromagnetic fields
2
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1answer
71 views

How to remember the Electromagnetic Spectrum?

This may sound off-topic but I am in a severe need of remembering the following shown Electromagnetic Spectrum along with the frequencies and wavelengths. So far I have looked at several mnemonics but ...
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3answers
140 views

Would a rotating magnet emit photons?

If a magnet is rotating, around an axis perpendicular to the axis north-south axis of the magnet (which I assume to be cylindrical symmetrical), in space (so no-gravity/freefall or friction), should ...
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6answers
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What are the various physical mechanisms for energy transfer to the photon during blackbody emission?

By conservation of energy, the solid is left in a lower energy state following emission of a photon. Clearly absorption and emission balance at thermal equilibrium, however, thermodynamic equilibrium ...
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1answer
53 views

Why does a non-contact voltage detector light up when you touch a plasma ball with the other hand?

I am doing a science experiment and we decided to try holding a non-contact voltage detector up to plasma ball. We were surprised that it would light up when it was 3 ft away from the plasma ball. I ...
2
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0answers
171 views

Fourier Transform of ribbon's beam Electric Field

I have a monochromatic ribbon beam with $E(x)e^{i(kz-\omega t)}$ being the electric field's amplitude. I want to show that the lowest order approximation in terms of plane waves is ...
2
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4answers
116 views

Faraday's Law - recursive?

So we know that the EMF is induced by change of flux. The thing that was always confusing me is the following: we start changing the magnetic field which in turn induces electric field which makes ...
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0answers
15 views

Why are certain opaque objects so biased?

Why is it that most of the opaque objects only block em waves in the visible spectrum but fail to block waves with frequency higher and(or) lower than the visible spectrum?
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8answers
5k views

Why no longitudinal electromagnetic waves?

According to wikipedia and other sources, there are no longitudinal electromagnetic waves in free space. I'm wondering why not. Consider an oscillating charged particle as a source of EM waves. Say ...
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3answers
63 views

What causes the disturbances in fields that produce electromagnetic waves?

I know that electromagnetic radiation is synchronized by oscillations of electric and magnetic fields, but what causes the disturbance in the fields to create the waves in the first place?
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3answers
69 views

Induction and electromagnetic fields

I've got a few questions on induction and electromagnetic fields. My current understanding of induction and electromagnetic fields is that, when electricity/current flows through a wire, it creates an ...
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1answer
50 views

Can electromagnetic fields be used to deconstruct and reconstruct molecular bonds?

I was thinking one day and came up with a theory after reading about how scientists were studying anti-matter by using electro magnetic fields to separate matter from the anti-matter they made. It got ...
0
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1answer
39 views

Lasers : Threshold Pump Power for Laser Oscillation [closed]

I was working my way through some basic laser problems , when I cam across this one : Consider the ruby laser for which we have the following values of the various parameters: $N =$ $1.6$ x ...
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1answer
38 views

What is a lineshape function $g(\omega_0)$ in a Laser?

I am a newbie to the world of lasers and was working my way through some basic problems, when I encountered this one: Optical Electronics, A.K. Ghatak and K. Thyagarajan (Cambridge University ...
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0answers
14 views

ratio between conduction current and displacement current

First, recall that Maxwell displacement current for a plane wave is $$ \vec j_D = \epsilon \partial_t \vec E = \epsilon \partial_t (\vec E_o cos(\vec k \cdot \vec r - \omega t)) = \epsilon \omega ...
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2answers
2k views

How do mirrors work?

My physics professor explained to me that electromagnetic waves are consisted of two components - electric and magnetic - which cause each other. Which part of the mirror actually reflects the wave? ...
6
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5answers
1k views

Why are electromagnetic waves called waves even though they don't travel through a medium?

If waves are defined as the oscillation of a medium, why are electromagnetic waves called waves as they do not need a medium to travel through?
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2answers
68 views

What can happen when 2.3*10^28 positrons collide with 2.3*10^28 electrons? [closed]

I'm interested in this question after a writer friend asked me what happens when a human gets bombarded with positrons. Didn't want to post this under scifi because I want more "scientific" answers... ...
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0answers
15 views

Can a patterned microwave beam with alternating frequencies be created?

Is there a way to create a patterned microwave beam with alternating frequencies such that, in the far field, from top to bottom of beam, there is repeated pattern of Wavelength one, Wavelength two, ...
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4answers
4k views

Real and imaginary parts of dielectric constant vs refractive index?

So for a complex dielectric constant $\epsilon = \epsilon_a + i\epsilon_b$, the wave vector and index of refraction are related to it through $k = \frac{\omega}{c}n$ and $n = \sqrt{\frac{\mu ...
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5answers
7k views

Is all kind of light same speed?

Is there any speed difference between blue or red light? Is there ever a speed difference? Or does all types of light move at the same speed?
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1answer
33 views

Does array gain violate the laws of physics or not?

I am a bit disturbed lately since I don't know the answer this basic problem. Say we have a standard isotropic antenna with some fixed parameters (load impedance, etc), and we feed this antenna with ...
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0answers
16 views

Energy conservation if photon absorbed below resonance

Suppose I have some quantum system (like atom) with excitation energy $E_{exc}$ which is homogeneously broadened due to finite lifetime. I shine light with narrow spectrum centred around energy ...
6
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2answers
698 views

Microwave oven + water: dielectric heating or ion drag?

When you place a water or food in a microwave oven, it heats. Which process commits more energy to that: dielectric heating, or ion drag i.e. resistive heating? AFAIK, in distilled water (which is a ...
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1answer
83 views

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 ...
0
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1answer
64 views

Momentum around an accelerated electron

Assume that an electron is accelerated along the +x-axis. The electron will radiate electromagnetic energy and momentum in every direction. But it seems to me that the EM momentum it radiates in ...
0
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2answers
36 views

What happens to electric field of a bar magnet?

Electromagtic waves say that Magnetic field and electric field exist orthogonal to each other. Also all electric field has some magnetic field and vice-versa (As I understand). In that case what is ...
3
votes
1answer
79 views

Earnshaw Theorem for ionic solids

Is a single molecule of sodium chloride (say) or a cluster of molecules of NaCl unstable, although macroscopically NaCl is in fact, stable? How can I reason this based on Earnshaw's theorem?
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Detectability of interstellar messages

Recently a debate started whether it is a good idea to send more messages into space in the hope of having alien civilizations receive them. There are some predecessors, most notably the 1974 Arecibo ...
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5answers
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Can a photon get emitted without a receiver?

It is generally agreed upon that electromagnetic waves from an emitter does not have to connect to a receiver, but how can we be sure this is a fact? The problem is that we can never observe non ...
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4answers
2k views

Some questions about car radio and cellphone antennas

1-Why the antenna of the radio of cars is located outside the car and not inside? 2-If the answer to 1 is because that cars are like Faraday cages then how come my cell phone can receive signal ...
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0answers
18 views

Phased non linear array antenna - First Sidelobe

I have a problem I cannot seem to solve and I REALLY need some help. It's about phased-array antennas whose dipoles are not equally spaced, not equally phased, not equally fed (amplitude). Let's ...
8
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2answers
610 views

If light rays obey to the wave equation, why can they be thought as straight lines?

I'm a newbie with physics but I'm wondering how a ray of light can essentially be represented. I have always known that a ray of light proceeds in a straight line until it encounters another object ...
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1answer
47 views

Why isn't a metal pot a faraday cage?

Someone left their cell phone here, it was ringing like crazy. I stuck it in a metal pot with a metal lid to shut it up, it still rang. I later put it in a safe, it still rang, but so muffled as to ...
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3answers
975 views

Why does a dielectric have a frequency dependent resistivity?

This question has come about because of my discussion with Steve B in the link below. Related: Why is glass much more transparent than water? For conductors, I can clearly see how resistivity ...
0
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2answers
73 views

How does a radiometric infrared camera estimate an objects temperature?

Say we have an infrared camera which measures some amount of radiation, in a spectral bandwidth which is given, between wavelength $\lambda_1$ and $\lambda_2$ from a perfect black body. How is it ...
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0answers
30 views

What do you call this “asymmetric polarization”?

I am considering this unusual polarization of EM waves: which travels in the x-direction and has magnetic and electric fields as shown. This can be produced by non-oscillatory currents. What name ...
2
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0answers
27 views

Why don't electrons return to their ground state immediately after photoexcitation? [duplicate]

In terms of photoluminescence, why don't the electrons, which have been excited by photons earlier, immediately fall down to their ground state and reemit a photon? In other words, why does ...
7
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2answers
1k views

What does an electromagnetic wave look like at a fixed moment in time?

I am curious what the electric and magnetic field's of light look like when time is stopped. A "photograph" or illustration/description of these fields at a moment in time is what I desire. Also, ...
2
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2answers
113 views

Energy in Electromagnetic Waves

Looking at diagrams of Electromagnetic Waves, it would appear to me that at certain times the waves have zero amplitude, and consequently zero energy. Indeed, substituting in the sinusoidal terms into ...
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2answers
192 views

Why doesn't an electromagnetic wave violate conservation of energy?

I'm starting to study electromagnetic waves and as i understand, an electromagnetic wave projects a varying electric field. This electric field can in turn give forces of repulsion/attractions to the ...
3
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3answers
2k views

Is there any example where electric and magnetic fields are not perpendicular?

Perpendicular electric and magnetic field creates light or other electromagnetic waves. Is it a necessary property to have a perpendicular fields? If not what would happen when the fields are not ...
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0answers
79 views

Proof that electric and magnetic fields in a EM wave are perpendicular

Is there a general proof, that for electromagnetic waves the magnetic and electric fields are perpendicular? The only ones I can find only focus on plane waves.
114
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1answer
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Why does NASA use gold foil on equipment and gold-coated visors?

I've read several websites about equipment covered with gold foil and astronaut helmet visors are coated with gold. However, their explanations are devoid of almost all physics content. Can someone ...