An electron  absorb a photon and jumps to the last energy level.Now when it goes done,How does it choose which level it will step down to?.Also I've come to now that it can omit one step(e.g from 5th level to the 3rd level without going by the 4th) is that true and how come ?!.
53800
I was watching the Cosmos documentary where Neil deGrasse Tyson explained how certain energy photons get absorbed by an atom, which causes the electrons of that atom to climb into a higher energy state.  He then says that an atom produces a photon when those electrons drop to a lower energy state, but that we don't know why this happens. I understood that as 'we don't know what triggers this to happen'.  Is that true? And if so, are there any feasible theories that explain this phenomenon?
46410
I get confused on the low-level physics of electricity from time to time. It came up in "," and I don't totally get it.  How fast does electricity flow? Is the speed of an electron different in say a resistor than in a wire? Does it matter? Or are the effects of the electron the only important thing, with lower levels of abstraction not useful in practice?  I know there are already materials on this topic, and I have read some of them. I think having the question on this site might inspire some interesting answers to the age-old question.  Bonus points for:   Identifying and clearing up common misconceptions Explaining in a way that someone with a high school diploma could understand, without oversimplifying it so much that its incorrect
32604
Why do electrons not fully fill up all the orbitals sequentially? By this I mean: why don't the electrons fully take up the previous orbit first and then move onto the next orbit? Take calcium for example, it's electron config is 2, 8, 8, 2. Why isn't it 2, 8, 10? Why does another orbit need to be created when there are still many spots to fill up?  (I know that this is a chemistry question in a physics forum, but I really could not find a chemistry or even a broader science type forum here )
50508
Suppose I have an atom in its ground state $|g⟩$, and it has an excited state $|e⟩$ sitting at an energy $E_a=\hbar\omega_0$ above it. To excite the atom, one generally uses a photon of frequency $\omega$ equal (or sufficiently close to) the transition frequency $\omega_0$, and this will stimulate a transition.  One thing that is often left by the roadside,* though, is the fact that the incoming photon has momentum as well as energy, and that if the atom wants to swallow the energy it also needs to swallow the momentum. So, in the nuts and bolts of states and operators,     how does one describe the transfer of momentum during an atomic transition?   In addition to this, the fact that this recoil momentum is rarely mentioned is a good indication that it is also rarely an issue. Why is it that in most circumstances we can safely ignore the photon's momentum when describing electronic transitions?  *Apart from treatments of Doppler cooling, which simply take the momentum transfers for granted and do not explain how and why they happen.
21899
The question itself is pretty self explanatory. I asked this to my chemistry teacher when he was doing the photoelectric effect while teaching atomic structure, and he just shrugged it off.  One possible mechanism that I thought up, is an atom with a unpaired electron (say hydrogen) to absorb one photon, and jump to a higher energy level, and then just immediately be hit by another photon with a different energy which raises it to an even higher level. Then finally the electron falls back into its proper non exited state releasing not two but only one photon which has the energy of both the initial photons, ideally combining photons.  Is this mechanism correct? Does it occur in nature? Do photons have other ways in which they merge? or do they not merge at all?
67533
I know that the light is reflected from a object to my eyes, but I don't understand exactly how. The photons appear from the light source and disappear in my eye! Can someone explain the phenomenon of where the photons go and do to allow us to see?
79481
There are a lot of questions about this topic on this site, none of them answer my question specifically. I have read this question:   I think of the emitted photon as a point particle (but with a polarization vector) traveling in a straight line from the atom to the measuring device.    For some people, a photon is a dimensionless point traveling on a world line (Eugene Wigner's definition of a particle).    The photons do not have a well defined trajectory. The diagram shows them as if they were little balls travelling along a well defined path, however the photons are delocalised and don't have a specific position or direction of motion.    because photons take all paths, but since their underlying physics is &quot;wavey&quot; different paths can &quot;interfere.    This calculation assumes that light simultaneously travels over all possible paths. To what extent this is just a calculational device and to what extent it reflects an underlying physical reality is a matter of opinion.    Indeed, nothing in nature moves around on all possible paths, in reality it's a field that permeates the vacuum which has quantized solutions.  There are mainly two thoughts:  the photon travels in a straight line, and that explains why it only interacts with a certain atom, meaning, that the photon can only be detected once, and the photon will not interact with the other atoms because they are out of its trajectory  the photon takes all paths, and that is why it truly goes through all possible avenues, explaining experiments like interference     Now these lead to two different propagation pictures. On the top, you can see the photon (EM wavepacket) spreading spherically in all directions. On the bottom, you can see a photon traveling from A to B, taking all possible paths. But which picture could be experimentally proven to be correct? The picture on the top cannot explain why the photon misses all the other atoms, that is, why it is not interacting with other atoms, because all atoms are in its way basically (it spreads spherically). The picture on the bottom cannot explain diffraction, because the photons is shown not to spread like a wave (cannot interfere), but just like a billiard ball on different paths. After the question was closed, I am editing to clarify (to reopen), that (as I understand), the question was closed due to the word &quot;really&quot; and what it physically means for the photon to take all paths, or whether the picture on the top (spherical spread) physically describes what is happening. I now revise these words, and the &quot;physically&quot; is meant here to say &quot;experimentally provable&quot;. Question:  Do photons take all paths or not?
92568
I'm curious about QM and spent the last 3 days thinking about Delayed Choice Quantum Eraser (DCQE) experiment, but I couldn't solve this issue:   Assume we do the DCQE experiment so that for the whole experiment (let's say, 1000 particles) finishes before the first particle reaches the detectors 1-4 (using the notation from ). Assume also we're using a switch to control if the idler particle goes to which-way-detector or to the eraser.  shows what we should see in D0, even though we don't know yet R01-04 Now, knowing D0, use the switch to send the 100 particles that reached the most-left side of D0 to the eraser detectors (D1 and D2). It seems much more likely that they reach D2 than D1, because it's on peak on D2 (and on rest on D1) even though it's going through a half-silvered mirror. This looks like a contradiction to me.   So, am I missing something or QM is just that weird?
61607
Say a photon hits some atom.  What determines whether there will be a photoelectric effect (photon is absorbed, electron is released) or whether there will be a Compton scattering (the photon is  scattered at some angle, and the electron is released with another direction)?
37535
I was wondering how light or any electromagnetic radiation can be "absorbed" if it consists of electric and magnetic fields. For example if there is a charge at point A, and the  light reaches point A, it will cause that charge to move depending on the value of the electric field. However, why wouldn't this electric field continue on and influence others particles everywhere else? For example, if you have a static charge creating an electric field, then if you put a charge at point A it will feel its effects and then if you bring a charge at another location B it will also feel its effects, namely it will not have been absorbed. Please shed some light on this. Thanks  edit: Does this mean the electron that absorbs this light moves in such a way that it creates an electric field of its own canceling the initial one? Or am I way off?
58234
Apologies if this has been asked before (I did check and I believe it wasn't). I have a question about the  of photons (or other particles). Depending on what and how we measure the photon turns out to be either a wave or a particle. Recently I saw some web page (and I can't remember where) that maybe neither is true. Both the wave perception and the particle perception are just that: perceptions, strengthened by our perhaps limited ability to observe. What if the reality of the photon is something else, something "on top" of our two perceptions?? Could someone direct me to a site that could tell me more about it (or maybe debunk the whole idea?)?
74471
Why do we use the ?  Why did it come to be the electron volt and not, say, just a prefix of the joule, like the nanojoule?  Does the electron volt represent anything particular as far as the mathematics goes? I am guessing that it does, and if so, what is it that the electron volt exactly represents in terms of the mass of a particle, as I have seen it used for both the energy of a photon and the mass of subatomic particles?
71193
If photon energies are continuous and atomic energy levels are discrete, how can atoms absorb photons? The probability of a photon having just the right amount of energy for an atomic transition is $0$.
84847
My teacher told me that in an atom (assume hydrogen), as soon as the single electron absorbs a photon with the right amount of energy, it jumps up to another energy level, and instantly falls back down to the initial one, releasing a photon with the same wavelength as the one it absorbed. So, if I place a source of white light, and in front of it a sample of hydrogen, and then a prism, according to what I have read so far, the screen should show an absorption spectrum, but why would it? Since the electrons are falling back down in all the hydrogen atoms almost instantaneously, they should give out the specific wavelengths of light which form the emission spectrum, which, I think, would combine with the previous one (absorption spectrum) to give a continuous spectrum? TL;DR: If a sample of hydrogen atoms is excited by white light, why don't the electrons getting excited and coming down instantly give absorption and emission spectrums respectively, which would combine to give a continuous spectrum? **NOTE:**I would have the same question regarding emission spectrums, but I assume all the atoms in the discharge tube would be excited (so no room for further absorption and production of an absorption spectrum), which would give out just the emission spectrum of hydrogen.
113898
When incident light passes through a hydrogen gas, for example, does it have 50% chance (since it's a 1:1 ratio of protons to electrons) of getting absorbed by the proton? Any chance at all? If no, why not? Does a proton have a "bandgap"? If yes, what happens when a photon is absorbed by a proton?
110807
OK, lets formulate it differently and say water works as a blue passing / red restricting filter.  It is actually observable. Just do a dive in a swimming pool with white light (maybe even at night) and a pool with a metal casing or white tiles. The further you look, the more blue it is.  So why is this so? Why is water not transparent?  And how does it happen on a molecular, subatomar or electron shell level?  Do 'red' waves become absorbed and retransmitted as 'blue' light waves or do only the 'red' waves get absorbed and the 'blue' can pass?  Can Maxwell help?
48919
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 reflect ('turn around and go back') in order to expound his path integral theory. My question is why does light have the propensity to turn around in the first place.  Is it just the absorption and re-emission of photons, and if so, why does it happen so uniformly (i.e. on a shiny thing, entire scenes are reflected near-perfectly). In essence, why are flat things shiny? Are all the molecules arranged at exactly the same angle?