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)

34
votes
8answers
25k 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 ...
7
votes
3answers
839 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 ...
10
votes
6answers
5k 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 ...
19
votes
3answers
2k 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 ...
31
votes
4answers
2k 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 ...
15
votes
6answers
1k 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 ...
13
votes
4answers
8k 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 ...
27
votes
4answers
2k 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?
19
votes
6answers
8k 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 ...
16
votes
3answers
3k 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?
9
votes
5answers
678 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 ...
4
votes
2answers
1k views

How do mirrors work?

Apparently, light is just a certain wavelength, or "the visible spectrum" of electromagnetic waves. If I recall correctly, my physics teacher explained to me that electromagnetic waves are basically ...
85
votes
6answers
32k 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 ...
21
votes
4answers
2k 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
votes
3answers
1k views

What happens to light in a perfect reflective sphere?

Let's say you have the ability to shine some light into a perfectly round sphere and the sphere's interior surface was perfectly smooth and reflective and there was no way for the light to escape. If ...
43
votes
7answers
3k 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 ...
5
votes
2answers
31k 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?
7
votes
5answers
4k views

Interference of polarized light

Does polarized light interfere?
4
votes
3answers
2k views

How can I create hindrances to radio waves?

How can I create hindrances to radio waves?
5
votes
2answers
263 views

How do you calculate power at the focal point of a mirror?

I'm a Mechanical Engineering student and I'm working on my senior project, so I need help. My project is about designing a solar dish having a diameter of 1.5 meters and a focal length of 60cm. so at ...
3
votes
1answer
1k views

Can you see yourself in a mirror when you are riding on top of a light stream?

What happens if you would ride on top of a light stream and you would look into a mirror that is in front of you, could you actually see your own face? I am asking this because I heard that nothing ...
0
votes
1answer
184 views

Accelerating electrons via microwaves

In Synchrotrons I think they use microwaves to accelerate the electrons bundles that fly through-how does putting a microwave through a cavity accelerate an electron? I know that the Electric and ...
24
votes
4answers
5k 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. ...
6
votes
3answers
711 views

If photons carry 1 spin unit, why does visible light seem to have no angular momentum?

Spin 1 silver atoms have a definite spin axis, e.g. up or down along an axis labeled X. This in turn means that they carry angular momentum in an overt, visible fashion. However, spin 1 photons do ...
4
votes
2answers
1k views

Make a semi transparent mirror with copper

The question: How would you make a semi transparent mirror (50% reflection, 50% transmission) with glass with a layer of copper. For light $\lambda$ = 500nm Try to be as realistic as possible What ...
7
votes
2answers
3k 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
votes
2answers
5k views

How is the speed of light calculated?

How is the speed of light calculated? My knowledge of physics is limited to how much I studied till high school. One way that comes to my mind is: if we throw light from one point to another (of known ...
4
votes
4answers
4k 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 ...
3
votes
2answers
300 views

Electromagnetic Momentum

My book says : The fact that electromagnetic radiation of energy carried momentum was known from classical theory and from the experiments of Nichols and Hull in 1903. This relation is also consistent ...
15
votes
4answers
1k 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, ...
19
votes
2answers
618 views

Gravitational slingshot of light using a black hole/massive object

Wikipedia has this page on gravity assists using planets. In some cases this effect was used to accelerate the spacecraft to a higher velocity. This diagram shows this in a very oversimplified manner. ...
15
votes
4answers
637 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 ...
8
votes
6answers
6k 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 ...
2
votes
1answer
208 views

Physical meaning of Maxwell's equations and origin of EM waves

Is it possible to describe the physical meaning of Maxwell's equations and show how they lead to electromagnetic wave, with little involvement of mathematics ?
7
votes
1answer
2k 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 ...
7
votes
1answer
813 views

What’s the relationship between thermal radiation and Johnson thermal noise?

All objects above absolute zero emit radiation due to random collisions between the atoms they are made of. The spectrum of radiation emitted varies according to the temperature of the object, I ...
5
votes
3answers
794 views

Radio antennas that are much shorter than the wavelength

From my limited experience with ham radio when I was a kid, I expect transmitting and receiving antennas to have lengths that are on the same order of magnitude as the wavelength, and in fact I recall ...
5
votes
1answer
954 views

What IS reflection?

How does quantum electrodynamics actually explain HOW reflection occurs on a microscopic scale? Note that Feynman's QED lecture series/book is not sufficient, as he only assumes that light DOES ...
5
votes
3answers
1k views

What other shielding material than lead is effective against gamma rays?

As the question in the title states I am wondering what material can be effectively used to shield gamma rays apart from lead? I believe concrete is often used, but it is nowhere near as effective as ...
2
votes
3answers
1k views

Producing photons with same frequency, different amplitude wave

I don't understand how two photons of the same frequency can have different amplitudes, neither how to produce them. I know that classically the square of the amplitude is proportional to the energy, ...
8
votes
2answers
308 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 ...
3
votes
2answers
2k views

The energy of an electromagnetic wave

The intensity of an electromagnetic wave is only related to its amplitude $E^2$ and not its frequency. A photon has the same wavelength as the wave that's carrying it, and its energy is $h f$. So ...
3
votes
2answers
277 views

Big Bang and Cosmic microwave background radiation?

One of the experimental evidence that supports the theory of big bang is cosmic microwave background radiation (CMBR). From what I've read is that CMBR is the left over radiation from an early stage ...
2
votes
3answers
326 views

Difference in velocity of light in change in medium [duplicate]

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?
1
vote
2answers
65 views

Why doesn't the magnetic field polarize when polarizing light?

If the magnetic field doesn't polarize does it follow the electric field path of propagation? or does it vanish?
1
vote
4answers
317 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 ...
1
vote
1answer
310 views

How is it possible that we see light from shortly after the big bang?

How can astronomers see light from shortly after the big bang? How did we get "here" before the light that emanated from our "creation"?
-3
votes
4answers
1k views

Electro Magnetic Waves can cause matter displacement?

After watching this video: http://www.youtube.com/watch?v=j1jIjx0XF_U The experience is made with a speaker that generates a sound wave or mechanic wave. Can you use this to establish a link to ...
46
votes
7answers
6k views

Why does a remote car key work when held to your head/body?

I was trying to unlock my car, but I was out of range. A friend of mine said that I have to hold the transmitter next to my head. It worked, so I tried the following later that day: Walked away from ...
24
votes
3answers
2k views

Can I use an antenna as a light source?

Can I use a normal metal antenna to emit visible light?