Tag Info

New answers tagged

0

I wonder why Poynting vector can be used to describe the intensity of standing EM wave. It's because a standing wave isn't really standing. Hence the photon in the cavity is off like a shot when you "open the box". It didn't accelerate from zero to c in an instant, it was always propagating at c. The Poynting vector denotes the wave motion even when it ...


1

The standing wave solution you quote is a solution to Maxwell's equations - as is any linear superposition of travelling waves. The relationship that $E = cB$ is really only applicable to travelling transverse electromagnetic waves in vacuum, however I note that in this case $E_0 = cB_0$. The Poynting vector (in vacuum) is defined by $$ S = \frac{\vec{E} ...


2

In short, no. It's not possible to generate a 600THz electrical signal. The sole reason for this is that electrons could not physically oscillate (vibrate) fast enough using the same methods as when generating radio waves (100MHz.) One way to have electrons (which make up electricity) have that high of a frequency, is to accelerate them to high speeds and ...


0

Actually, this is a nice question $-$ why do the dimensions of the slit or a hole (which are transverse to the direction of your incident plane waves) limit possible range of wavelengths (which are longitudinal, between two EM planes) of the transmitted wave. Without deriving the behavior from wave equations which would do the job, one may say that it's due ...


0

I do some work with magnetic fields in tissue as well for wireless power applications, though we don't typically deal with fields that strong hopefully I can help. First of all human tissue is largely magnetically transparent at low frequencies. While modeling the electromagnetic properties of tissue is very difficult problem (this is why you largely see ...


2

Photons (radio waves, "light", gamma-rays, etc.) and neutrinos contribute negligibly to the total energy density of the Universe. By far, most of the photons that exists today are the cosmic microwave background, with 450 photons per cm$^{3}$ (e.g. Hobson et al. 2006). The number density of neutrinos is similar, 330 per cm$^{3}$. In total, the energy density ...


1

I tried answering this by going to the XCOM database where you can get a calculation of the stopping power of elements and compounds. First - pick a few likely candidates. I found a table of elements with density which is a good place to start. The highest density elements are also among the highest Z ones: proton density ...


1

To start with a particle loses kinetic energy, and therefore momentum, when radiating electromagnetic energy in some electric field, it is the basic reason why the planetary model of an atom cannot work. Brehmstrahlung, "braking radiation" or "deceleration radiation") is electromagnetic radiation produced by the deceleration of a charged particle when ...


0

The likelihood of a photon scattering off a particle in its path is proportional mostly to the amount of mass per unit area in the path of the photon beam. This means that, though higher atomic mass is important, it is really density that makes a good gamma shield. Wikipedia cites lead as being only 20-30% better at shielding than a similar shield made of ...


0

Note that light (in one of its interpretations) is a Electromagnetic Wave. Now, there is a huge difference between the oscillating magnetic field in an EM wave and a magnetic field generated by a permanent magnet or an electromagnet.\ One of the differences is that magnitude of the magnetic field in an EM wave happens to be very small. In fact, the ...


-1

This is a really good question and it puzzled me for many years. Physics is composed of many different theories, and as history has progressed, physicists have refined them or created new ones. My fav example is Newton's theory of gravity, which is great at explaining gravity for most applications; however, Einstein's are much more accurate and a more ...


1

Yes, all electromagnetic radiation shows this 'dual nature' - which is to be expected since there is nothing really separating light from the other radiation in the spectrum apart from the arbitrary boundaries we have decided for it (i.e. visible light is just defined by what humans can see). As you'll discover as you learn more about quantum mechanics, ...


2

Yes, everything shows wave-particle duality to varying degrees because "wave-particle duality" is just a name for a certain behaviour of quantum objects, and everything is believed to be a quantum object. In particular, all electromagnetic radiation can be conceived of as being made of photons, which exhibit particle- and wave-like properties.


0

Graphene is not a superconductor. It's just better than any other element eg Silver. Given how thin a single sheet is, I would expect almost no alteration to the antenna's characteristics. That would, however, depend on the frequency.


1

Graphene is composed of a single atomic layer of graphite whose carbon atoms are very tightly bonded and organized into a hexagonal lattice. The carbon to carbon bonds between the atoms in graphene are so tiny and so strong that they prevent destabilization due to thermal fluctuations. Due to its ability to absorb large and varied amounts of light without ...


0

The problem is how waves interact with matter. Thickness of walls is not to be taken into account, since basically a wall is (partially) not metallic, therefore attenuates e.m. waves but doesn't reflect them. You cannot generalize on the material too, because air is in fact not very different from concrete. What changes? Epsilon and Mu of the medium in which ...


2

The first 2-D image you posted is a typical simplification for teaching purposes. In it, they use the height of the sine wave to represent magnitude, and the directions of the sine waves to show how the fields point relative to each other. The light itself however is not itself at all cone-like. You have to imagine this sine wave existing at multiple points ...


0

As light spreads out the density of photons per area goes down. However the intensity can never drop below one photon. Hence we can see galaxies from billions of light years away.


1

Electrons accelerating due to EM fields in the presence of gravity field radiate - examples are cyclotron radiation, antenna emissions. In the absence of EM field, whether the electrons radiate in the presence of gravitational field is theoretically problematic question, because Earth is not an inertial system, so Maxwell's equations should not apply ...


0

I am here because of the bright spots on Ceres. It does not have the dense atmosphere of the Ionsphere, so I will quote you a reference on the phenomena of Reflection. This is covered by Philip M. Morse "Chpt 7 Hndbk of Physics Vibrations of Elastic Bodies; Wave propagation in Elastic Solids Sec 4. Reflection from a Plane Interface, Surface Waves" '(if the ...


4

You're not missing anything. You are right, $k=\omega/c$. The argument $\sqrt{\frac{\omega ^2}{c^2}-k_z^2}$ in the Bessel function is the projection of the wavevector onto the radial direction. The use of Bessel functions beclouds what's going on a bit. Recall that a plane wave with wavevector $\vec{k}$ has the functional variation $\psi(\vec{r}) = ...


-3

In a classical way excited particles emit the received energy in discrete quanta, later named photons. As a good description of photon emission one can take the process of stimulated emission. The imagination, that the emission of energy from an excited electron is distributed all other the space is wrong. The best way to predict processes inside the ...


3

In the classical picture, an incoming wave excites a (damped) oscillation in an atom. We imagine that the oscillation is of bound electron(s). The oscillating electron(s) then re-radiates electromagnetic waves, but importantly, these waves are emitted in a continuum of directions, following the spatial distribution emitted by an oscillating electric dipole. ...


0

The attenuation of a waveguide is minimal when it is excited with the correct mode, but it isn't infinite for field configurations that aren't, so you can also send field configurations trough that don't fit the mode patterns, they will simply be attenuated very strongly for low frequencies. How strong that attenuation is depends on how much of the ...


2

A "ray" in geometric optics is a locus of continuous propagation of light. Think of it as mapping where the energy is going in space. In principle there are an arbitrarily large number of them, but we draw a manageable number for visualization purposes. The various [letter]-rays were so named when people didn't know what they were beyond being things that ...


-1

How do EM waves propagate? Like other waves propagate. IMHO the best way to appreciate this is to shake a rubber mat. When you do this you stretch a portion of the mat, and then the elasticity of the material contracts it back to its original size, but in doing so the rubber is stretched further along. What you then have is a shear wave with speed v = ...


0

What Maxwell derived was from radio waves. Radio waves are modulated photon radiation. Electrons in an antenna rod get accelerated at once and emit photons. The density of this photons distributes in space. Detecting this radiation with a receiver one get a sinus wave form. The frequency of this wave has to do with the frequency of the antenna generator and ...


2

Individual photons are not considered rays. Because of the wave and particle nature of photons, they are much more complicated than what they are generally thought of: a projectile of light. In fact, they do not have an exact measurable position, but do travel in straight line trajectories. What we consider rays are lines perpendicular to the wave front of ...


0

Are the EM fields really moving. My textbooks says it's changes in field that is moving. I don't understand this part. In your post, you mention a wobbling rope. Well, each fragment of that rope does not move along the rope. They just wobble where they are, without moving forward. But the wobbles themselves do move forward. Likewise, the electric and ...


2

The ray theory of light is equivalent to the Eikonal Equation, which in turn is essentially a slowly varying envelope approximation to Maxwell's equations. If we write the electric and magnetic field vectors as $\mathbf{E}\left(\mathbf{r}\right) = \mathbf{e}\left(\mathbf{r}\right) e^{i\,\varphi\left(\mathbf{r}\right)}$, $\mathbf{H}\left(\mathbf{r}\right) = ...


2

Light has a frequency of approx. 1e15Hz. Can light be transmitted in a hollow copper tube? Yes. No need to go relativistic. Can objects move at near the speed of light in a coax cable with inner conductor? No. They can't move in there, at all, not even at walking speed. Does any of this has anything to do with photons? No. Your experiment does have a ...


2

When a wave travels through a rope, the rope goes up and down, the position of all the 'rope-particles' changes, they oscillate and this makes up the wave. With light, it is indeed the electromagnetic field oscillating, but you shouldn't think of the arrows that represent that field in your first picture of light as 'extending into the rest of the space'. ...


3

If you or one of your friends has transition lenses for their glasses, then you can test the UV blocking ability with those. The transition to darker shades in these lenses is initiated by UV-light. So hold your sunglasses over some transition lenses and see if they start to turn darker. You can try this with any photochromatic material really, but ...



Top 50 recent answers are included