Tag Info

New answers tagged

2

The ${\rm sech}$ pulse is, in Kerr effect nonlinear optical mediums, an Optical Soliton. This means that it is the particular time variation such that the tendency of the pulse to spread out in time owing to linear dispersion is exactly counterbalanced by the nonlinear effect that tends to confine pulses in time. This balance is a stable one in a Kerr ...


0

All the details are available on wikipedia's page on the Fizeau-Foucault apparatus


0

Space weather events, such as CMEs, can cause rapid variations (seconds to 10s of minutes) in the Earth's geomagnetic field which can induce an electric field on the surface of the Earth. This electric field induces electrical currents in the power grid and other (grounded) conductors. A consequence of this could be transformer burn out or other network ...


0

Yes. See "The Quebec Blackout", Nasa News, March 13, 2009


0

We (colleagues of mine and I at NASA) did a study recently to look into the impacts of a Carrington-like event. Fortunately, the way our power grids work leaves a great deal of somewhat unintended protections in the system. The story about the $3 trillion (USD) storm that you may be referring to assumed that these (unintended?) safe guards (i.e., if one ...


0

One thing to consider is whether the wave, by being transverse, is linearly or elliptically polarized. If it starts out as a linearly polarized wave and converts to an elliptically polarized wave (which can happen), that is different than circular to elliptical. One also needs to consider whether the wave starts in the neutral atmosphere of Earth or in the ...


1

The speed of an electromagnetic wave is indeed independent of the speed of both the source and the receiver. However, this does not mean that the relative motion between the source and the receiver has no effect on the wave's properties. The effect that is being used is called the Doppler effect, and it is the fact that the received frequency of a wave will ...


3

That is the point of a Faraday cage. Although some of the EM field can penetrate inside the shell, under various circumstances, there is an effect where a conductive shell forms a void in a pervasive EM field. To be clear, though, EM fields do not just 'stop' - they are formed of photons, which are reflected at such an interface. Consider the microwave ...


7

Your misconception is the application of Gauss's law. There are solutions to Maxwell's equations (including Gauss's law), that permit an electric field inside the shell. If there are no charges inside the shell, all Gauss's law tells you is that the number of electric field lines entering the shell must equal the number coming out. An example of a field ...


9

Very simply: just because something is true in the static case doesn't automatically make it true in the dynamic case. To reject a static electrical field from inside a shell, it is sufficient for the charges on the surface on the shell to move however slowly they want in order to arrange themselves so as to cancel the field. This is Gauss's Law. When an ...


4

The electromagnetic field itself contains energy distinct from the energy of charged bodies, the energy in a given volume of empty space can be found by integrating the energy densities $\frac{1}{2}\epsilon E^2$ and $\frac{1}{2} \frac{B^2}{\mu}$ over the region. When the EM fields increase the kinetic energy of charged particles, there is a corresponding ...


0

Blackbody radiation assumes thermal equlibrium with surroundings. For example if you have a hollow sphere heated up with a small hole in it the radiation that comes out of the hole will have blackbody distribution of intensity vs wavelength / frequency If you have a wire heated the spectrum would not look like blackbody radiation. I am afraid that I do ...


4

Acoustic waves travel through a medium (air, water, metal, etc), there is no known medium through which light travels Both the speed of sound and the speed of light have fixed values regardless of the speed of their source Acoustic waves can be longitudinal (in gases) or transversal (in solids) whereas light is only transversal. You can measure acoustic ...


1

Acoustic Wave is a wave in which motion of one atom causes motion of another atom because it is lying next to it. Light is change in electric or magnetic field which further causes changing fields.


0

Acoustic waves need a medium through which to travel. Light does not.


0

Acoustic waves are longitudinal waves. Light is a transversal wave, hence not an acoustic wave.


1

The wiki article you quote is succinct, the photon is an elementary particle in the table of elementary particles of the standard model of particle physics. It is a quantum mechanical entity which means it is described by a wavefunction whose square gives the probability of finding the photon at (x,y,z) at time t. The double slit experiment with a single ...


0

The impact on the Earth's magnetosphere causes it to fluctuate and that induces high currents in things like power lines. These are potentially so great they can cause surge arresters to trip and close down parts of the electricity grid. A Carrington Event would be so great that it could melt grid power transformers and cause massive damage that might take ...


0

There is some confusion about two points...first, independence of the speed of light on the speed of its source is nothing surprising. It is the same as independence of the speed of water waves on the speed of a boat. Nothing funny there. BUT, independence of the speed of light on the reference frame of two observers in relative motion to the same source, ...


0

Consider sound waves. If you have a car with a siren does the car's speed affect the speed of sound? No, instead sound always travels at the same speed (let's ignore for now the fact that the speed of sound can change, e.g. by the density of the medium it is travelling through). Varying the car's speed will cause a Doppler Effect where the sound waves ...


2

They control the spectrum with regulations: Below is the diagram of frequency allocations in the united states. When people think of radio, they typically think of only the FM or AM parts of the spectrum. However, any frequency of electromagnetic radiation can be used to communicate with others. There is nothing that a government can do that could ...


1

When you change the free field $A_\mu$ by means of a gauge transformation, you can easily see that it affects longitudinal and timelike degrees of feedom. Since observables are gauge invariant, those degrees of freedom cannot be physical.


0

The total field consists of the "near" field like the Coulomb one and more generally (and loosely) a retarded Coulomb field, which are always "attached" to the charge, and the photon (radiated) field with different polarization orientations. The near field is always present, its "photons" are not created and annihilated. The corresponding "photons", when ...


1

90% of Astrophysics is to do with electromagnetic phenomena. Bar neutrinos or directly grabbing stuff in our own solar system, there's not much else you can do but observe the electromagnetic radiation coming from out there. Your question is therefore massively broad. But here are some examples you could research. Rayleigh scattering observed in the ...


4

A picture is worth a thousand words. Here's how it looks as a function of space, evolving in time: Here blue is real part, and purple is imaginary part of the complex exponent $\exp(i(kx-\omega t))$. If you instead just look at $\exp(-i\omega t)$, you'll get this:


1

If you look at a wave at a moment in time, you can see how it varies spatially by plugging in different values of r: $e^{ikr}$. If you look at a point in space, you can see how it varies in time by fixing r and varying t: $e^{-i\omega t}$. If you want the behavior in both space and time, you end up with the expression you have - and you can see how the ...


2

Two parts to this answer - "near field" and "far field". When you have an parabolic antenna, or in general any source that is not a point source, you can shape the beam to follow something other than a simple $1/r^2$ relationship - as you pointed out, a laser beam can be created in a way that you can "collect" all the energy (and assuming no absorption in ...


2

There are several ways to lose signal strength: Geometric dilution As you have mentioned, if the beam spreads out, it distributes a fixed amount of intensity over an ever increasing surface, so the radiance (intensity density) decreases. The signal loss behaves like a negative power law (depending on the dimension of spreading $1/R^2$) Absorption If ...


2

Since this was not stated yet, I would just like to give my stance on it. All fundamental particles can be seen as excitations of fields. This is true for photons, electrons, neutrinos, etc. Do these fields need a medium in which they propagate? Not as far as we can tell. Everything we see and experience are excitations of these fields, a single one of ...


2

Probably the most direct example is synchrotron radiation. This is the case in which an electron is accelerating by moving in along a curved path (e.g., a helix). As it is accelerated, it emits photons in the radio spectrum: (source) Another big one would be bremsstrahlung in which an electron moving along a path is decelerated near the presence of a ...


4

It may be useful to start this explanation from the origin of a light wave: an oscillating charge. Start with the idea that a stationary charge is surrounded by an electric field, then imagine wiggling that charge up and down. Now the field lines will turn to wiggles instead of straight lines. Those wiggling field lines are the electromagnetic waves we call ...


2

The electron on an atom gets excited to a higher level when some how the energy is transferred to the electron. But I can't understand it. The way we currently understand in physics this interaction is exactly like that: a photon transfers its energy to the atom and as a consequence one of the electrons goes to a corresponding exited state. And this can ...


1

Einstein once compared the photon with a famous person (sorry I forgot the name) who changed confession at young age and returned to its initial confession before he died: Light behaves as a photon at the starting point and at the end point, and it behaves like a wave during its travel. By the way, the light wave is not going up and down, it is not a ...


3

The light we see with our eyes is electromagnetic radiation, very well modeled by Maxwell's equations. Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This 3D animation shows a plane linearly polarized wave propagating from left to right. Note that the electric and magnetic fields ...


3

The answer would appear to be "Yes", at least in theory. A "kugelblitz" is a concentration of light so intense that it forms an event horizon and becomes a Black Hole according to general relativity. It would be a BH whose original mass-energy had been in the form of light rather than matter.


0

Light waves are exactly a theoretical explanation of light radiation. Propagation of waves of electromagnetic fields is a good theory that works for low frequencies, but as Einstein showed (and was Nobel prized for) the photoelectric effect can only be explained if electromagnetic radiation is emitted as directed quanta of energy. I guess that experienced ...


1

The intensity that you talk about can be represented bt the Poynting vector. $$ {\bf N} = {\bf E} \times {\bf H}$$ The units are Watts per square metre. The power produced by an oscillating electric dipole can be derived by integrating an expression for the Poynting vector over a surface area enclosing the dipole. $$ P = \oint {\bf N} \cdot d{\bf S}.$$ ...


0

It depends on your definition of the Fourier transform. Sometimes it is written with $1/(2\pi)$ for each one-dimensional variable; sometimes the coefficient is chosen $1/\sqrt{2\pi}$, and sometimes it is just $1$. The "inverse" Fourier transform has also some coefficient at the integral and it is a matter of taste which direct/inverse transform's ...


2

According to Maxwell's theory of electromagnetism, a light pulse (or generic electromagnetic wave) carries momentum, which can be transferred to an absorbing surface hit by the pulse. This momentum transfer is known under the name 'radiation pressure'. Despite carrying momentum, light carries no mass. Yet a light pulse does carry energy. For a light pulse ...


-3

The total energy of a photon, the carrier of the electromagnetic force, is given by $ E=hf $ where $ h $ is Planck's constant and $ f $ is the frequency of the light. So yes, if two EM waves have the same energy, they will have the same frequency and wavelength, meaning they have the same colour. Photons have no rest mass, but they do have a relativistic ...


2

Your single photon pulse wave function is an element of the first Fock layer (the zeroth is the vacuum layer) of the quantised Maxwell field Fock space. The electric field is still an operator but you can obtain its expectation value as $<E>=<ψ|E|ψ>$.


1

Paper is paper. Airport security won't find a piece of hand-written paper inside a book because they aren't looking for it. They are almost exclusively concerned with weapons, and paper isn't a weapon. They don't even want to find things like narcotics - not their job and it requires lots of police paperwork. Similarly, they are spectacularly uninterested ...


3

Optically speaking, and very simply: An opaque material permits no light to pass through it. A material which passes light but does not pass image detail is called translucent. A material which passes light AND image detail is called transparent. "Passing light" technically means ANY light, but in practice some materials pass so little light that they ...


0

Opacity is a description of the scattering of light in a certain material: the more opaque a material is, the higher the scattering of light and conversely, the less opaque a material is, the lower the scattering of light. With a higher scattering of light, the probabilities that light is transmitted through the material diminishes. For a more intuitive ...


5

There are two questions here. The first - "what is the definition of opaque" is terribly broad and depends on the field / context. I will focus on the second: when and how does a body let radiation through? We should really ask the converse question: by what mechanisms does a body stop radiation from going through. I will answer this for different parts of ...


5

There are several terms with precise meaning in physical sciences that have been pulled into common language and misused. Opaque however does not appear to be one of them. According to several etymology sources I just looked up (like Etymology Online for example) it appears to have rather mushy origins in Latin and French meaning darkened or shady. For the ...


0

First of all, Huygens's principle is a very basic principle that explains expansion of waves in space and their interference when they meet. Notice, however, that it refers to waves of the same type, e.g. light of the same polarization and frequency. Two waves of the same frequency, but different polarization, can be joined into one (e.g. by a polarization ...


0

A very good review article is by Brian Butters: "Chaff" in the IEE Proc. vol 129, pt F, June 1983, and then two theoretical analyses by Peebles: "Bistatic Radar Cross Sections of Chaff", IEEE AES-20, March 1984 and "Bistatic Radar Cross Sections of Horizonatlly Oriented Chaff", IEEE AES-20, Nov 1984


1

TEM waves do exist in multi-conductor waveguides such as coaxial guides, in fact they exist in any homogeneous waveguide with more than one conductor. There are no propagating TEM, TE or TM modes if the cross section is inhomogeneous but at cutoff frequency the hybrid modes degenerate to the respective transversal modes.


1

People may say things such as "... we do not have a theoretical justification for the constancy of the speed of light... ", or say " These things simply are, therefore there is no deeper, more fundamental, explanation. ". Thus such people in general accept the effect, yet they have no desire to find the cause. However, it is to be noted that if you analyze ...



Top 50 recent answers are included