Propagating solutions to Maxwell’s equations in classical electromagnetism and real photons in quantum electrodynamics. A superset of thermal-radiation.
71
votes
7answers
25k 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 ...
35
votes
7answers
1k views
Cyclist's electrical tingling under power lines
It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I ...
24
votes
4answers
2k views
Why doesn't light kill me?
I was attending my philosophy class and in the middle of student presentations, I found myself mentally wondering off and thinking about light. After a few minutes of trying to piece together how the ...
23
votes
8answers
13k 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 ...
22
votes
3answers
1k 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 ...
21
votes
2answers
413 views
Why does laser light not affect glowing materials?
I have this childrens rubber ball which glows in the dark after it's exposed to light. I "charge" it with a flash light then play with my dogs at night. I thought to try a very intense green laser, ...
20
votes
2answers
2k views
Why is the sky not purple?
I realise the question of why this sky is blue is considered reasonably often here, one way or another. You can take that knowledge as given. What I'm wondering is, given that the spectrum of ...
18
votes
3answers
1k 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?
18
votes
3answers
3k views
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. ...
17
votes
3answers
2k views
15
votes
5answers
1k views
Home experiments using wireless LAN or mobile phones about electromagnetism?
Are there any nice experiments using wireless LAN access points or routers or mobile phones to demonstrate physical features of electromagnetic fields, especially em-waves?
More precisely I am ...
14
votes
4answers
4k views
Why glass is transparent?
Once I asked this question from my teacher and he replied "because it passes light", "and why it passes light" I asked and he said "because it is transparent".
Same question again, Why glass is ...
14
votes
3answers
710 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?
14
votes
4answers
545 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 ...
14
votes
3answers
549 views
Distorted colors of Google StreetView photographs near electric power lines
This is a followup to my question:
Cyclist's electrical tingling under power lines
Some users presented a convincing picture that the electric shocks under power lines are primarily from the ...
13
votes
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 ...
13
votes
3answers
1k views
Newton's rings: What causes the other rings?
This is from an experiment we did in physics class. We shone a sodium light at a convex lens on top of a sheet of glass - and this image was captured by a USB microscope. I know what causes the main ...
13
votes
3answers
504 views
Can light exists in $2+1$ or $1+1$ spacetime dimensions?
Spacetime of special relativity is frequently illustrated with its spatial part reduced to one or two spatial dimension (with light sector or cone, respectively). Taken literally, is it possible for ...
11
votes
6answers
597 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 ...
11
votes
6answers
2k views
Why is electricity not transmitted wirelessly?
Why is electricity not transmitted wirelessly such that we don't need to span cables on the earth's surface? As in: electricity is transmitted wirelessly from the power plant to the household.
11
votes
2answers
608 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, ...
10
votes
3answers
1k views
Batman spotlight in the sky
I have noticed that obstructing a spotlight typically results in a blurred shadow unlike the crisp batman symbol in the comics of batman. Is there a way to create a spotlight with a crisp batman ...
10
votes
2answers
3k views
How do Optically Active Compounds Rotate Plane Polarized Light?
I am not sure if this is more of a Chemistry or a Physics question, but in my Organic Chem class we discussed that chiral molecules will rotate plane polarized light. However, my professor did not ...
9
votes
2answers
262 views
Effect of gravitation on light
Einstein predicted that the gravitational force can act on light. This was verified in one solar eclipse that light from a star near to the sun's disc bent due to Sun's gravity as predicted. Since ...
9
votes
4answers
568 views
Why does the speed of light $c$ have the value it does?
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 ...
9
votes
2answers
451 views
If you run an electric current through a wire loop, do the accelerated charges radiate?
Does an accelerated charge always radiate?
For example the current electrons in an electric circuit when moving through a turn they are accelerated, do they radiate because of that acceleration?
If ...
9
votes
4answers
783 views
What causes polarised materials to change colour under stress?
Our physics teacher showed the class a really interesting demonstration. He used two polarised filters in opposite orientations, then he took some antistatic tape and stretched it under the two ...
9
votes
5answers
505 views
Superposition of electromagnetic waves
The superposition of two waves is given by
$$\sin(\omega_1 t)+\sin(\omega_2 t)=2\cos\left(\frac{\omega_1-\omega_2}{2}t\right)\sin\left(\frac{\omega_1+\omega_2}{2}t\right).$$
For sound waves, this ...
9
votes
1answer
96 views
Wave Physics - can a dynamic waveform be constrained to a specific geometry by signal processing alone?
Suppose that you have a signal source, a set of point-transducers, and a handful of moderately powerful DSPs. Is it possible to construct an arrangement of the transducers such that the original ...
8
votes
5answers
865 views
What happens to light after it enters an eye
What happens to the light [energy] after it enters an eye and hits the rods and cones? I presume the energy becomes electrical, and it must be near 100% perfect, else our eyes would heat up? Or am I ...
8
votes
5answers
590 views
Could cell-phone radiation cause cancer?
It is very crucial that I ask whether it ``could'' and not whether it does. I do not mean to be the least controversial.
To my surprise, having read ``Physics for Future Presidents'' by Richard ...
7
votes
5answers
770 views
Why is Light invisible?
Why can't we see light? The thing which makes everything visible is itself invisible. Why is it so?
7
votes
3answers
719 views
Can radio waves be formed into a pencil beam?
Laser beams are said to have high "spacial coherence". This means that the beam is highly concentrated even at long distances (low spread).
Can this be achieved with radio waves (much longer waves) ...
7
votes
2answers
217 views
Can light waves cause beats?
My question is pretty brief. When two sound waves of nearly same frequencies interfere, we get beats.
But, I have not observed something like that happening in the case of light. In fact, most of the ...
7
votes
3answers
625 views
Can we transport energy over infinite distances through vacuum using light?
I know that light (or electromagnetic radiation in general) attenuates in intensity as the square of the distance it travels.
Why does it attenuate?
Are the photons being scattered by the medium ...
7
votes
2answers
144 views
What exactly is the difference between a sychrotron light source and a free electron laser?
Cyclotron radiation is radiation emitted by a charged particle being bent by a magnetic field. The only difference between cyclotron and synchrotron radiation is that the latter occurs at relativistic ...
7
votes
3answers
2k views
Phase shift of 180 degrees on reflection from optically 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 it ...
7
votes
5answers
476 views
Why Does Light Not Become Polarized In A Magnetic and/or Electric Field?
I am familiar with the Faraday Effect, but I remain confused as to why the electric and/or magnetic components of light do not naturally align themselves with a magnetic or electric field (in a ...
7
votes
2answers
276 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 ...
7
votes
2answers
2k 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 ...
6
votes
3answers
154 views
Where does the light of the Big Bang come from?
I'm wondering whether the residual light of the Big Bang comes from one particular direction and what possibilities do we have to detect its position?
6
votes
3answers
3k views
Why do lightbulbs continue to glow after the light is turned off?
I've noticed that whenever I turn the lamp off in my room at night, the lightbulb seems to continue to glow for a minute or so after that. It's not bright though; the only way I even notice it is if ...
6
votes
7answers
3k views
Electromagnetic fields vs electromagnetic radiation
As I understand, light is what is more generally called "electromagnetic radiation", right?
The energy radiated by a star, by an antenna, by a light bulb, by your cell phone, etc.. are all the same ...
6
votes
1answer
628 views
If microwave ovens and WiFi both operate on the same frequency, why doesn't WiFi cook things?
If we ignore 5GHz WiFi, then both microwaves and WiFi create photons at ~2.4GHz but one of them will boil water in a few seconds but the other doesn't have any effect. So what's the difference?
Is it ...
6
votes
1answer
219 views
Why is there a photon every millimeter on earth coming from a star millions of light years away?
I've been having this confusing thought for so long now it would be amazing if someone could answer me.
Imagine this asterisk * . As you see, from the center point, lines go outwards, just like a ...
6
votes
1answer
298 views
Where does the energy go, when light is blocked by polarisation
I've been looking around about LCD monitors, and how they polarise light. When a pixel needs to be black, the light is "twisted" so that it can't go through the polarising sheet in front.
What happens ...
6
votes
6answers
2k views
Why does light change direction when it travels through glass?
This was explained to me many years ago, by a physics teacher, with the following analogy:
"If someone on the beach wants to reach someone else that is in the water, they will try to travel as much ...
6
votes
3answers
790 views
Why is it necessary for an object to have a bigger size than the wavelength of light in order for us to see it?
I keep hearing this rule that an object must have a bigger size than the wavelength of light in order for us to see it, and though I don't have any professional relationship with physics, I want to ...
6
votes
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
...
6
votes
2answers
321 views
Is it true that any system of accelerating charges will radiate?
I was recently told by a physics teacher that "any system of charges in which at least some of the charges are executing some sort of accelerated motion, will radiate and lose energy". This refers to ...
