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Questions tagged [electromagnetic-radiation]

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

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112 votes
9 answers

What is the relation between electromagnetic wave and photon?

At the end of this nice video (, she says that electromagnetic wave is a chain reaction of electric and magnetic fields creating each other so the chain of wave ...
Xtro's user avatar
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204 votes
10 answers

If photons have no mass, how can they have momentum?

As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects ...
david4dev's user avatar
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62 votes
8 answers

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 ...
Douglas B. Staple's user avatar
56 votes
13 answers

Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?

I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them. How and why ...
SMUsamaShah's user avatar
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32 votes
4 answers

Explain reflection laws at the atomic level

The "equal angles" law of refection on a flat mirror is a macroscopic phenomenon. To put it in anthropomorphic terms, how do individual photons know the orientation of the mirror so as to bounce off ...
yrodro's user avatar
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56 votes
4 answers

Why do prisms work (why is refraction frequency dependent)?

It is well known that a prism can "split light" by separating different frequencies of light: Many sources state that the reason this happens is that the index of refraction is different for ...
Brandon Enright's user avatar
73 votes
7 answers

Does a charged particle accelerating in a gravitational field radiate?

A charged particle undergoing an acceleration radiates photons. Let's consider a charge in a freely falling frame of reference. In such a frame, the local gravitational field is necessarily zero, ...
Sergio's user avatar
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96 votes
5 answers

How and why do accelerating charges radiate electromagnetic radiation?

Let's consider it case by case: Case 1: Charged particle is at rest. It has an electric field around it. No problem. That is its property. Case 2: Charged particle started moving (it's accelerating)....
claws's user avatar
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111 votes
6 answers

Why is glass transparent?

Once I asked this question from my teacher and he replied "Because it passes light.". "And why does it pass light?" I asked and he said, "Because it is transparent.". The same question again, Why ...
SMUsamaShah's user avatar
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55 votes
6 answers

Phase shift of 180 degrees of transversal wave on reflection from denser medium

Can anyone please provide an intuitive explanation of why phase shift of 180 degrees occurs in the Electric Field of a EM wave, when reflected from an optically denser medium? I tried searching for ...
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37 votes
5 answers

Redshifting of Light and the expansion of the universe

So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light ...
QEntanglement's user avatar
58 votes
5 answers

Do two beams of light attract each other in general theory of relativity?

In general relativity, light is subject to gravitational pull. Does light generate gravitational pull, and do two beams of light attract each other?
Jakub Narębski's user avatar
39 votes
3 answers

Is it really possible to break the speed of light by flicking your wrist with a laser pointer?

Minutephysics has a popular YouTube video called "How to break the speed of light". In the video it states that if you flick your wrist while pointing a laser that reaches the moon, that the spot of ...
miguel.martin's user avatar
23 votes
4 answers

Why don't electromagnetic waves require a medium?

As I understand it, electromagnetic waves have two components which are the result of each other, i.e., when a moving electric charge creates a changing magnetic field at point X then a changing ...
Ryan's user avatar
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16 votes
6 answers

Electric field associated with moving charge

I have recently started to learn about the electric field generated by a moving charge. I know that the electric field has two components; a velocity term and an acceeleration term. The following ...
PiccolMan's user avatar
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59 votes
7 answers

Does a constantly accelerating charged particle emit EM radiation or not?

The Abraham-Lorentz force gives the recoil force, $\mathbf{F_{rad}}$, back on a charged particle $q$ when it emits electromagnetic radiation. It is given by: $$\mathbf{F_{rad}} = \frac{q^2}{6\pi \...
John Eastmond's user avatar
6 votes
2 answers

Why one should follow Snell's law for shortest time?

whenever two media and two velocities are involved, one must follow Snell's law if one wants to take the shortest time. Why snells law must be followed to travel diffrent media in shortest time? ...
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26 votes
4 answers

Is the electromagnetic spectrum discrete?

I'm just starting to learn physics and I have a question (that is probably stupid.) I learned that energy levels that the bound electron can have are discrete. I also learned that when an electron ...
Andrej T.'s user avatar
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47 votes
8 answers

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?
Andrey S's user avatar
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40 votes
4 answers

Why does wavelength change as light enters a different medium?

When light waves enter a medium of higher refractive index than the previous, why is it that: Its wavelength decreases? The frequency of it has to stay the same?
ODP's user avatar
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12 votes
3 answers

Explain how (or if) a box full of photons would weigh more due to massless photons

I understand that mass-energy equivalence is often misinterpreted as saying that mass can be converted into energy and vice versa. The reality is that energy is always manifested as mass in some form,...
Alan Rominger's user avatar
40 votes
4 answers

Is there an infinite amount of wavelengths of light? Is the EM spectrum continuous?

The electromagnetic spectrum is a continuum of wavelengths of light, and we have labels for some ranges of these and numerical measurements for many. Question: Is the EM spectrum continuous such that ...
toothandsticks's user avatar
13 votes
2 answers

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? ...
jcora's user avatar
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55 votes
5 answers

Scattering of light by light: experimental status

Scattering of light by light does not occur in the solutions of Maxwell's equations (since they are linear and EM waves obey superposition), but it is a prediction of QED (the most significant Feynman ...
Keenan Pepper's user avatar
5 votes
2 answers

Black body vs. Thermal radiation

I am seeking clarification of the following terms: Black body radiation Thermal radiation Thermal light source At the first glance, the first two are the same thing, and they are the radiation ...
Roger Vadim's user avatar
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141 votes
6 answers

If you view the Earth from far enough away can you observe its past?

From my understanding of light, you are always looking into the past based on how much time it takes the light to reach you from what you are observing. For example when you see a star burn out, if ...
JD Isaacks's user avatar
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1 vote
1 answer

Are magnetic fields just modified relativistic electric fields?

Feynman's Lectures, Volume 2, says that the electromagnetic force is invariant in any reference frame, and the magnetic force in one frame becomes the electric field in another. And Wikipedia says: ...
A. Remorov's user avatar
12 votes
1 answer

Why does the refractive index depend on wavelength? [duplicate]

Why do different wavelength get impeded more or less when in different materials? Moving with the same speed, but a longer physical distance would imply that the fields oscillate less times in the ...
user avatar
20 votes
2 answers

What is the minimum wavelength of electromagnetic radiation?

As a first approximation, I don't see how a wavelength of less than 2 Planck distances could exist. The question is: Are there any other limits that would come into play before that? For example: ...
BCS's user avatar
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16 votes
6 answers

Have we directly observed the electric component to EM waves?

For example, has anyone has directly observed charges oscillating due to standing EM waves? I am particularly interested because it'd demonstrate that radiation has a transverse electric component to ...
Maximal Ideal's user avatar
54 votes
5 answers

Are there any theoretical limits on the energy of a photon?

Is there any lower or upper limit on the energy of a photon? i.e. does the mathematical framework we currently use to study photons blow up when a photon surpasses a certain upper limit of energy? (or ...
Hritik Narayan's user avatar
18 votes
1 answer

Relation between radio waves and photons generated by a classical current

Several questions have been posted on Physics SE regarding the relationship between photons and electromagnetic waves, and several good answers have been given. Some of those questions are listed ...
Chiral Anomaly's user avatar
14 votes
3 answers

Why don't absorption and emission lines cancel out in our Sun?

I was looking at this answer on why absorption lines and emission lines don't cancel out: An experiment shining light on the material and looking at the reflected spectrum will see absorption ...
macco's user avatar
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6 votes
5 answers

What exactly does the *frequency* of a photon mean?

If you emit a single photon, wait an interval, emit a second photon, wait the same interval, and keep repeating, is the resultant stream of photons describable by the frequency at which you emitted ...
argonaut's user avatar
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24 votes
1 answer

What is the relationship between Faraday cage mesh size and attenuation of cell phone reception signals?

This is related to the question how can electromagnetic waves reach a cell phone in faraday cage?, where in the answer it was stated that the holes (=size of the mesh) would need to be smaller than ...
user avatar
17 votes
2 answers

How does reflection work?

In Newton's model of light as being composed of particles, it's easy to imagine reflection as being the rebounding of individual corpuscles off a surface. However, since light can also behave like a ...
voithos's user avatar
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8 votes
1 answer

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 ...
Inquisitive's user avatar
43 votes
4 answers

Why is Huygens' principle only valid in an odd number of spatial dimensions?

Apparently Huygens' principle is only valid in an odd number of spatial dimensions: Huygen's principle in curved spacetimes Why is this? [EDIT] This is ...
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35 votes
2 answers

How can wifi penetrate through walls when visible light can't?

I did search the question on Physics S.E considering it would be previously asked. I found this How come Wifi signals can go through walls, and bodies, but kitchen-microwaves only penetrate a few ...
Shashaank's user avatar
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30 votes
1 answer

Why is black the best emitter?

Why are emitters colored black better emitters than other colors? Why is white a worse emitter?
Brinn Belyea's user avatar
29 votes
5 answers

What exactly is a quantum of light?

I am currently trying to learn some basic quantum mechanics and I am a bit confused. Wikipedia defines a photon as a quantum of light, which it further explains as some kind of a wave-packet. What ...
Dejan Govc's user avatar
20 votes
2 answers

How does the grid on the microwave oven window prevent microwave radiation from coming out?

If I look through the microwave window I can see through, which means visible radiation can get out. We know also that there is a mesh on the microwave window which prevents microwave from coming out. ...
Revo's user avatar
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18 votes
5 answers

How do electromagnetic waves travel in a vacuum?

This is perhaps a total newbie question, and I will try to formulate it the best I can, so here it goes. How does an electromagnetic wave travel through for example, the vacuum of space? I usually ...
jotadepicas's user avatar
11 votes
4 answers

Understanding the diagrams of electromagnetic waves

I'm having trouble understanding the diagrams of elctromagnetic waves. I have no problem with any concept in classical mechanics, and I think this can be answered without any relativity (which I don'...
fiftyeight's user avatar
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6 votes
5 answers

Existence of monochromatic pulses?

Why there can not be a monochromatic pulse? My physics professor told us that we can't generate a monochromatic light pulse and I was wondering what are the physical limitations causing this.
Mac Sat's user avatar
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4 votes
5 answers

Radio antenna producing waves in the visible spectrum [closed]

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 ...
Austin A's user avatar
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301 votes
2 answers

What is Chirped Pulse Amplification, and why is it important enough to warrant a Nobel Prize?

The 2018 Nobel Prize in Physics was awarded recently, with half going to Arthur Ashkin for his work on optical tweezers and half going to Gérard Mourou and Donna Strickland for developing a technique ...
Emilio Pisanty's user avatar
80 votes
10 answers

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 ...
makerofthings7's user avatar
41 votes
4 answers

If both radio waves and gamma rays can travel through walls

and they are on opposite ends of the electromagnetic spectrum, then why can't light travel through walls which is right in the middle of the spectrum? This question has already been asked here. ...
morpheus's user avatar
  • 548
18 votes
7 answers

Why does the speed of light $c$ have the value it does? [duplicate]

Why does light have the speed it does? why is it not considerably faster or slower than it is? I can't imagine science, being what it is, not pursuing a rational scientific explanation for the speed ...
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