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

learn more… | top users | synonyms (1)

2
votes
4answers
115 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 ...
0
votes
0answers
14 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?
10
votes
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 ...
1
vote
3answers
58 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?
0
votes
3answers
65 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 ...
0
votes
1answer
48 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
votes
1answer
37 views

Lasers : Threshold Pump Power for Laser Oscillation [on hold]

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 ...
1
vote
1answer
35 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 ...
0
votes
0answers
11 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 ...
6
votes
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
votes
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?
0
votes
2answers
65 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... ...
0
votes
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, ...
5
votes
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 ...
3
votes
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?
2
votes
1answer
32 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 ...
1
vote
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
votes
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 ...
3
votes
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 ...
4
votes
0answers
37 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 ...
0
votes
1answer
63 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
votes
2answers
35 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 ...
-1
votes
0answers
16 views

what is this invisible undetectable substance? [closed]

It was once thought that, like all other waves, electromagnetic radiation required matter as a medium through which to pass. a) Identify the invisible undetectable substance that was proposed to ...
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?
17
votes
3answers
978 views

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 ...
15
votes
5answers
1k views

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 ...
2
votes
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 ...
0
votes
0answers
15 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
votes
2answers
595 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 ...
0
votes
1answer
46 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 ...
2
votes
3answers
960 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
votes
2answers
71 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 ...
0
votes
0answers
29 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
votes
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
votes
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
votes
2answers
110 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 ...
1
vote
2answers
189 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
votes
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 ...
1
vote
0answers
75 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
votes
1answer
48k views

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 ...
0
votes
0answers
25 views

reflecting foil on window

Assumptions: A house window acts as a cavity resonator, and so can be treated as a blackbody. If the room temperature is 20 C then the outgoing radiation is 418 W/m2 If the outside temperature is ...
0
votes
0answers
16 views

Mosotti-Clausius formula from the first principles

I would like to understand how to get the Mosotti-Clausius formula for the dielectric susceptibility of a dielectric material with spherically symmetric molecules from the first principles, where by ...
8
votes
2answers
116 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 ...
0
votes
0answers
28 views

Why do aromatic rings absorb in UV?

Why do aromatic rings absorb in UV? I think that the whole molecule absorb light which causes transition of an electron to another molecule orbital.
4
votes
1answer
138 views

Is it really possible to “discover” the speed of light with a microwave oven?

I've seen a number of sites/videos online that describe a method for measuring the speed of light, using a microwave oven and a chocolate bar. For example, this video on youtube. The basic idea is to ...
0
votes
1answer
81 views

How does Huygens Principle explain interference?

How exactly does Huygens theory about the propagation of wavefronts account for interference?
2
votes
5answers
858 views

Radio antenna producing waves in the visible spectrum

If a radio could produce waves in the visible light spectrum, what would the result be? This is a thought experiment that I've pondered for a few years now. I realize there are a few/many real-world ...
13
votes
2answers
713 views

Why is the bottom part of a candle flame blue?

What’s the explanation behind the bottom part of a candle flame being blue? I googled hard in vain. I read this. I don’t understand how it’s explained by the emission of excited molecular radicals in ...
4
votes
2answers
97 views

Is there something equivalent to a diode for light?

In electronics a diode is a component allowing current passing in only one direction, and blocking the other side. I'm wondering if something similar exists for visible light or other EM waves, like ...
2
votes
1answer
113 views

Probability density of detection of collinearly emitted photons in two detectors

Update: As proposed by @dmckee, I added equation numbers and improved the display of some equations. The answer by @Trimok inspired me to look at coordinate systems which are not specific to the ...