Questions tagged [electromagnetic-radiation]
Propagating solutions to Maxwell’s equations in classical electromagnetism and real photons in quantum electrodynamics. A superset of thermal-radiation.
967
questions
117
votes
9
answers
66k
views
What is the relation between electromagnetic wave and photon?
At the end of this nice video (https://youtu.be/XiHVe8U5PhU?t=10m27s), she says that electromagnetic wave is a chain reaction of electric and magnetic fields creating each other so the chain of wave ...
- 1,681
209
votes
10
answers
271k
views
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 ...
- 2,774
67
votes
8
answers
11k
views
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 ...
- 1,526
60
votes
13
answers
42k
views
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 ...
- 5,387
34
votes
4
answers
21k
views
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 ...
- 697
61
votes
4
answers
32k
views
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 ...
- 12k
80
votes
7
answers
11k
views
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, ...
- 2,635
107
votes
5
answers
109k
views
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)....
- 7,335
116
votes
6
answers
115k
views
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 ...
- 5,387
38
votes
4
answers
5k
views
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 ...
- 3,871
60
votes
6
answers
82k
views
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 ...
WhizKid
18
votes
7
answers
23k
views
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 ...
- 741
60
votes
5
answers
9k
views
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?
- 935
42
votes
3
answers
16k
views
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 ...
- 551
24
votes
5
answers
64k
views
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 ...
- 493
63
votes
7
answers
12k
views
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 \...
- 5,925
6
votes
2
answers
4k
views
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?
...
user102917
27
votes
4
answers
4k
views
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 ...
- 373
49
votes
8
answers
23k
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?
- 1,056
12
votes
3
answers
6k
views
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,...
- 21.1k
41
votes
4
answers
192k
views
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?
- 4,607
40
votes
4
answers
13k
views
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 ...
- 597
13
votes
2
answers
9k
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?
...
- 2,119
55
votes
5
answers
6k
views
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 ...
- 7,737
13
votes
1
answer
26k
views
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 ...
user24082
5
votes
2
answers
1k
views
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 ...
- 60.4k
143
votes
6
answers
98k
views
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 ...
- 1,853
1
vote
1
answer
1k
views
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:
...
- 53
22
votes
3
answers
13k
views
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:
...
- 1,269
12
votes
4
answers
2k
views
How is light emitted by an incandescent lamp?
I am looking for better understanding of how light is produced in an incadescent lamp. More specifically: how is the kinetic energy of electrons converted to light?
Are we dealing with interband ...
- 60.4k
18
votes
6
answers
5k
views
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 ...
- 8,650
8
votes
4
answers
1k
views
How can things around us have different colours if they have specific emission spectra?
Objects appear in different colours because they absorb some colours (wavelengths) and reflect or transmit other colours. The colours we see are the wavelengths that are reflected or transmitted.
As ...
- 1,064
15
votes
3
answers
2k
views
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 ...
- 2,005
54
votes
5
answers
9k
views
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 ...
- 7,319
18
votes
1
answer
2k
views
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 ...
- 54.5k
8
votes
2
answers
3k
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 ...
- 872
7
votes
6
answers
4k
views
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 ...
- 173
311
votes
2
answers
30k
views
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 ...
- 134k
45
votes
4
answers
6k
views
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:
https://mathoverflow.net/a/5396/21349
Huygen's principle in curved spacetimes
Why is this?
[EDIT] This is ...
user4552
25
votes
1
answer
17k
views
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 ...
user60063
17
votes
2
answers
9k
views
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 ...
- 3,439
8
votes
2
answers
2k
views
Principle of Reflection on atomic level
This well-observed phenomenon has, besides several others, always been a fascination to me. We are well aware of several theories, experiments, and practical applications of this well-known phenomenon,...
- 5,481
156
votes
1
answer
15k
views
What is an "attosecond pulse", and what can you use it for?
The 2023 Nobel Prize in Physics was announced today, and it was awarded to Pierre Agostini, Ferenc Krausz and Anne L’Huillier, for
“experimental methods that generate attosecond pulses of light for ...
- 134k
38
votes
2
answers
29k
views
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 ...
- 2,797
31
votes
1
answer
68k
views
Why is black the best emitter?
Why are emitters colored black better emitters than other colors? Why is white a worse emitter?
- 425
29
votes
5
answers
35k
views
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 ...
- 495
21
votes
2
answers
74k
views
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.
...
- 17.1k
19
votes
5
answers
51k
views
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 ...
- 306
12
votes
4
answers
21k
views
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'...
- 1,075
6
votes
5
answers
2k
views
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.
- 69