# Tagged Questions

Photons are electromagnetic waves. They exhibit particle-like qualities in many situations and have zero rest mass.

3answers
208 views

### Hydrogen transition and photon behavior

consider a transition for an electron in the Hydrogen atom from the ground state to the 1st excited state. Let's say this transition occurs through absorption of a photon of exactly the energy ...
2answers
279 views

### Photon statistics of an incandescent light source

We usually calibrate the cameras on our microscopes by capturing 20 images of a blurry (not sharp) fluorescent particle. For each pixel in this stack of 20 images we calculate the intensity variance. ...
1answer
419 views

### Can a photon exiting from a gravity well ever reach a frequency of zero / wavelength of $\infty$?

In reading another question about gravity's effects on a photon, I wondered if it were possible for a photon to ever be redshifted to zero wavelength. I know that black holes have a gravity field ...
1answer
144 views

### How is the mechanism of greenhouse gases interacting with IR radiation?

How does atmospheric CO2 and other Greenhousgases (GHG) affect the incoming (from sun) and outgoing (from earth) radiation. I understand that at certain wavenumbers (or areas of wavenumbers) in the ...
2answers
497 views

### Tachyon and Photons

Is there a particle called "tachyons" that can travel faster than light? If so, would Einstein's relativity be wrong? According to Einstein no particle can travel faster than light.
2answers
111 views

### Photon absorption

[sorry, this way below the level of this forum -- flames are most welcome] When a photon is absorbed by a piece of matter that does not reflect it -- where does the photon "go"? Eg, one shines light ...
3answers
129 views

### Distant bodies emitting photons

This comes from a discussion forum, where a friend of mine asked the following: We can see objects in space billion of light years away, right? I started wondering about that. If you take 2 ...
2answers
235 views

### Photon emission from excited atoms

the answer given by classical quantum mechanic to the for atomica levels does not provide that an electron in an excited levels can radiate a photon and move to a lower level. How QED justifies this ...
4answers
672 views

### Why photons transfer to electrons perpendicular momentum?

Linear antenna directed along z, photons (EM waves) propagate along x. Momentum of photons have only x component. Why electrons in antenna have z component of momentum?
1answer
185 views

### If 100% of the energy from the sun is reflected back into space

100% of the energy from the sun is reflected back into space, it's just shifted from a low-entropy state to a high-entropy state, and from a high frequency (ultraviolet) to a low frequency (infrared). ...
3answers
243 views

### Action - Reaction pair, through photons

Here's an example to describe the issue Supossed a high power laser (eg a 100 kW laser, ie, electromagnetic weapons) is fired to a target, then it will receive energy and move. (and likely to burn or ...
5answers
527 views

### Particles, waves and parallel wire filters. Transmission formula?

If I think of a photon as a particle, I think a parallel wire filter should transmit proportionally to the uncovered area. (and reflect proportionally to the covered area). Obviously polarization ...
2answers
171 views

### self-antiparticles and broken symmetries

certain particles (i.e: certain bosons like the photon) do not have an anti-particle, or rather, they are they own anti-particles. lets assume that such symmetry is only approximate and these ...
3answers
631 views

### Radio waves and frequency of photon

How radio waves create the current in antenna in terms of photons? If it is Compton scattering then why is not changed the freuency of photons?
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3k views

### What do ants see?

After watching some ants in my garden today, and then looking at this very illuminating demonstration, I got to wondering, about what they would see. Not specifically ants (I understand their ...
1answer
727 views

### Light waves and Schrödinger probability waves

Ok, bearing in mind that I only have a brief understanding of quantum mechanics (no formal education, only from reading about concepts in books), so I could be way off here, I have a question ...
3answers
522 views

### Nature of Photons

Why is it that photons are emitted in bundles? My physics teacher's answer was "it's complicated"...
2answers
216 views

### Have CMB photons “cooled” or been “stretched”?

Introductory texts and popular accounts of why we see the "once hot" CMB as microwaves nearly always say something about the photons "cooling" since the Big Bang. But isn't that misleading? Don't ...
2answers
324 views

### Is energy exchange quantized?

In the photoelectric effect there is a threshold frequency that must be exceeded, to observe any electron emission, I have two questions about this. I) Lower than threshold: What happen with lesser ...
2answers
381 views

### Is spin is a conserved entity?

Suppose that an electron with spin up emits a photon in the field of an ion (bremsstrahlung). What is the spin of the emitted photon? Is it correct to say that the photon is circularly polarized if ...
1answer
383 views

### How does a particle of light reach the max speed of light? [duplicate]

Possible Duplicate: How can a photon have no mass and still travel at the speed of light? First of all I am not a professional physicist. I was curious as to how a particle of light can ...
3answers
1k views

### Photon hitting an atom with higher energy than needed to ionize

Suppose we have an atom with several energy levels (e.g. an hydrogen), and it is hit by photons. I know that in order to have the atom change energy levels, the photon must have an energy level ...
3answers
584 views

### Kinetic Energy vs. Potential energy with regards to creating particles

So I know that when you collide particles with high enough kinetic energy, (kinetic energy = at least the rest mass of the particles you are making), you get particles. How come potential energy ...
3answers
293 views

### Could a bubble of photons make a spaceship massless?

I'm not sure how theoretically possible this is but my question is... If we could somehow make a perfect bubble of photons (a massless bubble) and put a spaceship inside it, could it therefore ...
0answers
242 views

### Maxwell's Demon - laser cooling

There’s an interesting article on Scientific American that tells how to cool individual atoms can bee cooled to within a very tiny fraction of Absolute Zero. It uses two laser beams acting on a very ...
5answers
2k views

### Does a photon interfere only with itself?

I sometimes hear statements like that: Quantum-mechanically, interference pattern occurs due to quantum interference of wavefunction of a photon. Wavefunction of a single photon only interferes ...
2answers
696 views

### Does $E = mc^2$ apply to photons?

Photons are massless, but if $m = 0$ and $E = mc^2$ then $E = 0c^2 = 0$. This would say that photons have no energy, which is not true. However, given the formula $E = ℎf$, a photon does have energy ...
4answers
426 views

### When lasers are used to cool atoms or ions, etc where does the heat go?

According to the first law of thermodynamics, sourced from wikipedia "In any process in an isolated system, the total energy remains the same." So when lasers are used for cooling in traps, similar ...
3answers
578 views

### Anti-laser: How sure we are that energy is transported?

Reading this PE question can-we-transport-energy-over-infinite-distances-through-vacuum-using-light, a related question arises naturally: Is energy transported (by light)? -- (I did believed in this ...
1answer
334 views

### Does the potential energy for a given photon increase or decrease in quanta?

As a photon leaves a strong gravitational field, it loses energy and redshifts. Is the exchange in potential energy of a photon characterized by energy quanta?
1answer
247 views

### What happend with the light from all the galaxies visibles from an earth telescope?

Supposing it's possible to see some distant galaxies with an earth telescope, then, at the tip of the telescope lens there are photons comming from the distant galaxy... So, if I extend my hand in a ...
1answer
184 views

### Radio waves and frequency of photon

Is 89MHZ station emitting photons of 89MHZ frequency? (I mean $\nu$ in $E=h\nu$).
2answers
820 views

### How to count photons

How are photons counted? What is the experimental setup used to count photons from a laser or even a lamp? Of course, in the case of the lamp, I would be able to count only the photons that pass ...
5answers
2k views

### Why can't photons have a mass

Why can't photons have a mass? Could you explain this to me in a short and mathematical way?
3answers
675 views

### Reconciling refraction with particle theory and wave theory

I have searched the web for good answers to why refraction occurs when light moves from one medium to another with different density. I have limited background in physics and want to know if there is ...
7answers
10k views

### How can a photon have no mass and still travel at the speed of light?

I've read a number of the helpful Q&As on photons that mention the mass/mass-less issue. Do I understand correctly that the idea of mass-less (a rest mass of 0) may be just a convention to make ...
1answer
200 views

### Observing photons

Your replies to my question about being able to see a photon, from the side (answer, unanimous, “no”) have raised in me some additional questions. Would it reasonable to think that in consequence, ...
2answers
361 views

### Is Time Significant in the Double Slit Experiment

When doing the classic double slit experiment is the time between emitting photons significant at all? Say, a single photon is emitted, the scientist waits T seconds, then emits another photon. Are ...
4answers
388 views

### movement of photons

In a typical photon experiment the photon is depicted as moving across the page, say from right to left. Suppose we were actually able to witness such an experiment, from the side (to position of ...
4answers
1k views

### Do mirrors increase the amount of light in a room?

So if you have a light bulb in a room, and you had a tool to measure the amount of light that's in the room, then let's assume the amount of light only caused by the bulb is "1" If you place a mirror ...
3answers
2k views

### Properties of the photon: Electric and Magnetic field components

Consider an electromagnetic wave of frequency $\nu$ interacting with a stationary charge placed at point $x$. My question concerns the consistency of two equally valid quantum-mechanical descriptions ...
3answers
852 views

### How is electromagnetic wave variation distributed in space?

Imagine an electromagnetic wave (a monochromatic one for example) The electric field amplitude, and its variations travel in the propagation direction. So, if there really exists a propagation ...
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 ...
5answers
2k views

### Do photons gain mass when they travel through glass?

Please correct me if I'm wrong, but I believe that photons slow down when travelling through glass. Does this mean they gain mass? Otherwise, what happens to extra kinetic energy? I understand now ...
1answer
2k views

### How many photons per second is one Lumen?

Also the side question is how many Joules is one photon (any between 450-660nm). Thank you P.S. I am asking because I want to estimate how much thermal energy should be dissipated by LED when part of ...
2answers
654 views

### What is up and down conversion in photonics?

I have heard the terms up and down conversion in photonics/photovoltaics articles. What do the terms mean?
4answers
423 views

### What is the mechanism behind the slowdown of light/photons in a transparent medium?

So light travels slower in glass (for example) than in a vacuum. What causes light to slow down? Or: How does it slow down? If light passes through the medium, is it not essentially traveling in the ...
5answers
957 views

### How are forces “mediated”?

I hope this is the right word to use. To me, these forces seem kind of fanciful (except for General Relativity and Gravity, which have a geometric interpretation). For example, how do two charged ...
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