# Tagged Questions

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

1answer
99 views

### Commercial Infrared lights

I purchased an infrared light. It's a 100 W Philips infrared lightbulb. Says it's infrared, but I haven't done any spectrum analysis so I don't know for sure if it's just red or really infrared. As I ...
2answers
197 views

### Radiated power and energy density for a black-body

I am having an hard time trying to understand why the radiated power per unit area $P$ of a black body is given by $$P=\frac{c}{4} u$$ in terms of the energy density $u$ and the velocity of light. I ...
1answer
110 views

### Spectral radiance unit conversion [closed]

I have spectral radiance data in SRUs (spectral radiance units), as a function of wavelength: $$a = \mu W cm^{-2} sr^{-1} nm^{-1}$$ However, I am working with software which requires my data in the ...
3answers
370 views

### Intensity of light

If we have 2 beams of light with equal intensities, but with different frequencies, wouldn't the one with the higher frequency generate more power? If so, how come the intensity, which is in $W/m^2$, ...
2answers
197 views

### Jones vector and matrices

With Jones vectors and matrices one can describe the change in polarization of a EM wave. What is the convention of the reference coordinate system; Is it fixed or does it change whenever the ...
1answer
72 views

### Can I find the frequency generated by tapping a battery on a coin?

According To How Stuff Works , if you tap a 9 volt battery on a coin it will transmit Radio waves. I was wondering, because I don't have a radio with me at the moment, Is there a way I could discover ...
3answers
760 views

### Is there something special in the visible part of electromagnetic spectrum?

I always wondered how much information we get from color. Things we see have different colors; edible products change color when began to spoil so we have a notion what color a fresh product should ...
1answer
123 views

### RF WiFi Radiation

I would like to extrapolate my current router wifi radiation from my phone. If I know that my router is transmitting at 300mW and my phone displays the strength in -dbm (from 0 to -100 scale) if I ...
1answer
66 views

### Signal emission and detection with certain parameters; Electrical Engineering [closed]

Could a micro-transmittting device, smaller than, say, a golf ball be employed to emit an ultra-narrow detectable beam, (e.g. 1mm) with range of one or 1.5 decimeter to 2 or less centimeter? The ...
0answers
52 views

### Maximum acceleration [closed]

Assume that a tiny and shiny (perfect conductor) Au metal in free space has a shape of a thin cylinder of thickness 1 m (like a tiny coin of thickness 1m) and end cap area of 500 m2. A ...
3answers
195 views

### Magnetic fields and gravitational waves. How far do they reach?

I read that magnetic fields perpendicular to a current shoot out and expand all the way to infinity. Additionally a gravitational wave, no matter how small will also expand to infinity at the velocity ...
3answers
712 views

### Amplitude of an electromagnetic wave containing a single photon

Given a light pulse in vacuum containing a single photon with an energy $E=h\nu$, what is the peak value of the electric / magnetic field?
1answer
226 views

### Optical constants of noble metals: the Drude model for microwave modelling

I have a question regarding the optical constants of noble metals. According to Johnson and Christy's paper Optical Constants of Noble Metals (Phys. Rev. B 6, 4370–4379 (1972), ...
1answer
232 views

### How EM waves are produced by accelerating charged particles?

How the electro-magnetic waves are produced by the accelerating charged particle? Graphical explanations are most welcomed. Is the explanation given by the below mentioned article correct regarding ...
1answer
318 views

### How do EM waves get detached from an antenna?

How does an electro-magnetic waves get detached from an antenna and spread to the space? While an antenna receives an EM wave, which quantity of the EM wave (electric or magnetic) is used for ...
0answers
76 views

### How to calculate radiative transition rate of exciton in a quantum dot with specific dimension?

I am writing rate equations for a nanophotonic system including three quantum dots. I need to calculate that radiative transition rates of exciton in ground state in those quantum dots. In the paper ...
2answers
240 views

### What is the physical meaning of retarded time?

Consider this figure Now, when I measure a field produced by the charge $e$ at the point $\mathbf r$, at the time $t=t_1$, it means that the charge sent the signal field at the time $t=t_r$, where ...
1answer
65 views

### Idea of precursors of the electro-magnetic waves

The idea of the material Maxwell equation is almost clear. But I'm curious about the idea that except for material equation the pure Maxwell equation should work, but in harder sense: more currents ...
0answers
57 views

### Formula for transverse decrease in signal strength?

Alice is transmitting an electromagnetic beam to Bob. Assume it is a well collimated beam, like a laser beam, or a maser beam. As Bob's distance from Alice increases, his signal strength decreases ...
2answers
216 views

### Why does the electric field dominate in light?

I read a book on the wave property of light where the author mentioned that the electric field, instead of magnetic field, dominates the light property. I don't understand why. In Maxwell's theory, a ...
1answer
531 views

### Phase and Group Velocity of Electromagnetic Waves

Moving charges produce oscillating electric and magnetic fields -we have an electromagnetic wave. In terms of moving charges or at the level of charges, what is phase velocity and group velocity of ...
1answer
75 views

### Units of azimuthally integrated radiant intensity

If the radiant intensity $I(\phi,\theta)$ of electromagnetic radiation is given in spherical coordinates ($\phi$ the azimuth, $\theta$ the polar angle, indenpendent of $R$) with units $Js^{-1}sr^{-1}$ ...
1answer
119 views

### Finding the energy levels of an electron in a plane perpendicular to a uniform magnetic field

Suppose we have an electron, mass $m$, charge $-e$, moving in a plane perpendicular to a uniform magnetic field $\vec{B}=(0,0,B)$. Let $\vec{x}=(x_1,x_2,0)$ be its position and $P_i,X_i$ be the ...
3answers
156 views

### Difference in velocity of light in change in medium

It is often seen that according to physics the light changes it's velocity according to the medium through which it is traveling. So can it be explained that why so happen?
1answer
64 views

### Getting a given wavelength radio signal given an antenna with real-world constraints

Supposing you are given a transmitting antenna of whatever type of metal is most commonly used these days, and supposing that you are applying an AC current with the intent of transmitting a 1 m ...
2answers
2k views

### Why is Near Field Communication (NFC) range limited to about 20cm?

Near Field Communication (NFC) operates at 13.56 MHz. Near Field is the region situated at a distance r << λ λ = c/f ...
2answers
268 views

### Can electromagnetic fields be used to deconstruct and reconstruct atoms?

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 ...
1answer
394 views

### Force on Earth due to Sun's radiation pressure

I have been asked by my Classical Electrodynamics professor to calculate the force that the Sun exerts in the Earth's surface due to its radiation pressure supposing that all radiation is absorbed and ...
2answers
277 views

### why dosen't a charged particle radiate energy in circular motion in a uniform magnetic field?

I have studied in my Physics course that one of the drawbacks of Rutherford's atomic model was that when an electron will revolve around the nucleus, it is undergoing acceleration and so it should ...
1answer
49 views

### Range of electromagnetic waves

I was reading this article. There is a statement "It is a well-known fact that the telecom towers mounted with antennas in the lower frequency bands can cover far greater areas than those using the ...
1answer
149 views

### Does a magnetic field have gravity?

Re-reading http://physics.stackexchange.com/a/33156/5265; I find myself thinking if light, being EM in the humanly visible spectrum, may possess gravity - does a magnetic field also possess gravity?
1answer
120 views

### What is the ion drag mechanism in dielectric heating?

While reading about dielectric heating on Wikipedia, I read about the ion drag mechanism but there wasn't enough information about. I know there is another Phys.SE question talking about the ion drag ...
1answer
64 views

### How does the specific frequency of EM Radiation relate to displacing electrons from their orbits?

I've only a general grasp on how all this works, so it could be I'm asking this poorly or misunderstanding what happens. With that said: The energy of EM radiation is a function of its frequency. ...
2answers
191 views

### Radio communication and antennas

I'm trying to understand how an antenna is able to understand, but have a few quesions: I understand that for an antenna to distinguish between 2 (or more) signals, they have to be in different ...
1answer
158 views

### Does $\lambda\nu = c$ hold for all the waves in the universe?

Are all waves in the universe the same as electromagnetic waves? Basically, my question arises from an equation I found in my chemistry textbook: $$\lambda \nu ~=~ c.$$ This states that the ...
10answers
3k views

### Why is light called an 'electromagnetic wave' if it's neither electric nor magnetic?

How can light be called electromagnetic if it doesn't appear to be electric nor magnetic? If I go out to the sunlight, magnets aren't affected (or don't seem to be). And there is no transfer of ...
0answers
84 views

### Wireless signal strength

My question is possibly somewhat misplaced, but I'll try to explain as best as I can. Suppose I have a transmitter with a frequency of 2500MHz and a power of 1W. It radiates uniformly in all ...
1answer
61 views

### Does every wavenumber of IR result in a different kind of vibration?

Does every wavenumber of IR result in a different kind of vibration? If that is true, what if a molecule absorb 2 different wavenumbers (which cause different rocking and symmetrical stretching for ...
2answers
106 views

### Power due to dipole radiation and time reversal symmetry in classical E&M

The dipole formula for the power loss emitted by a time varying electric dipole is (in natural units) $P = \frac{\dot d_i^2}{6 \pi}$. This is clearly even under time reversal symmetry $T$, but a ...
0answers
110 views

### Antenna Power and gain calculation [closed]

I have a wireless security related question, the second part confused me: Your wireless network usually has a range of 100 feet. However you are having a (confidential) meeting in a 10’ x 10’ x ...
1answer
58 views

### Could we really charge metal plates using microwaves?

While skimming through Dielectric heating, I read that they use microwaves to charge the plates. How do they do that?
3answers
64 views

### What kind of “camera” and “light” source should I use to detect the path along which the light moves?

I would like to have an image ( in any kind of space ), where I see the path of a "light" source. In my understanding the most common, directed source would be a laser pointer. ...
1answer
158 views

### Velocity of electron in electrostatic field. Does radiation matter?

There's a voltage difference of 1000 Volts between two points 2 meters apart. An electron starts at the point of lower potential and is left to travel alone in a straight line until it reaches the ...
0answers
20 views

### Why can radio wave travel through walls while imfrared can't? [duplicate]

Possible Duplicate: If both radio waves and gamma rays can travel through walls Does it have anything to do with the materials used to build the wall? What determines if a material can ...
0answers
51 views

### Electromagnetic charge [duplicate]

Possible Duplicate: If electromagnetic field give charge to particles, does photon carry charge? Is it possible to charge photons I was wondering if light when it's in wave form (seeing ...
2answers
105 views

### Xray compression of secondary in hydrogen bomb

Accounts of the "secret" of the hydrogen bomb describe Xrays from a primary fission explosion reflecting off of the bomb case (occasionally passing through polystyrene foam) and compressing and ...
3answers
159 views

### What if $\gamma$-rays in Electron microscope?

I was referring Electron microscopes and read that the electrons have wavelength way less than that of visible light. But, the question I can't find an answer was that, If gamma radiation has the ...
0answers
128 views

### Transmission formula for two near field polarizing filters?

Consider two parallel wire "comb" filters oriented at 45 degrees relative "tooth" angle. The comb tooth spacing spacing is much less than one wavelength, and the comb tooth length is much more than ...
2answers
328 views

### How objects have different color in different lights?

As everybody knows that Black objects are black because almost all the light that falls on them is absorbed into the material. Little or no light is reflected back toward our eyes, therefore we see ...
1answer
64 views

### 1MeV ray hits a band gap of 2eV : trying to understand X-rays

I am trying to understand why radio waves pass though the human body, light does not, but X-rays still pass. In my mind I imagine that radio waves don't supply enough energy and don't excite any ...
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