EPTQuantum Mechanics - Page 3
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rei
United States3594 Posts
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micronesia
United States24775 Posts
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Teoita
Italy12246 Posts
As far as structure of matter/applied quantum mechanics goes, wether a material is transparent or not depends on it's conductivity, yeah. Here's the general idea: solving Shroedinger's equation for a system means finding which energies that system can have. For some systems, this energy is discrete, so only a select, precise levels are possible. This is the example in atom orbitals: the electrons can't orbit at whatever energy level they want, they must follow precise energy levels (and orbits). On the other hand, other systems can have any energy possible, which is known as a continous spectrum. This is the case, for example, in free photons. When it comes to solids, the energy levels are organized in bands. Some intervals in energy are allowed, others aren't. Insulators are materials in which the bands are all filled completely. This means the electrons need a lot of energy to shift between one band and the other, more than visible light can give; this makes them transparent to radiation which has as much energy as the gap between one band and the other. Additionally it makes it harder to incite any kind of electric current, since you need so much energy to move the electrons. Conductors on the other hand have half filled bands, which means it's possible that the electrons in a half filled band will absorb a smaller amount of energy (for example from visible light) and be excited without jumping from one band to the other. In that sense, conductivity and reflection are dependent on each other. I'm sure there's more complex things going on as well, but that's how it was presented to me in the structure of matter class i took. If you are interested in learning more about quantum electrodynamics i recommend reading Feynman's "QED". It's a collection of the lessons he gave (i believe they might even be the same as those videos) and it's a very good read, without any math. I read it in 12th grade and was able to understand most of it. | ||
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rei
United States3594 Posts
first it depend on what kind of light. if the light source is the sun then the photons produced by the sun has a continuous spectrum, meaning it has all different kind of wavelengths. If it's a light source that produce only a specific wavelength of photons then none of the photon will get absorbed unless we have the right kind of atom as the reflecting surface. second it depend on the type of material the reflective surface is compose of, different kind of materials have different atoms, and different atoms only absorb few specific kind of wavelength photons and letting the other photons pass right through them without absorbing them. And that's because only the photons that have the right amount of energy will excite an atom and make its valence electron jump to the next orbital. Once the atom finish being excited and turn back into ground state, that energy it absorbed from the photon is released and turn back into a photon, but it is released in a random direction compare to the direction of the original photon that was absorbed a fraction of a seconds ago. and then percentage of the "reflected" photons depend on the range of absorb-able wavelength the atoms that made of the surface. thirdly, out of all those randomly re-emitted photons from the surface, because they are randomly directed, some of them could be re-emitted further into the more atoms and gets absorbed again and again until they make their way out, those that made it out through the bottom will form a refraction, and those who make it out of the surface will from a reflection. my question is about whether or not i understood what Fraynmen said about the photons that are being reflected and refracted. those 3 points i mention above all consider the photon as a particle, but to understand the reflection and refraction, we need to think of the photon as a wave. and here is what I am not sure about and want someone who know better to clarify or re-affirm, the randomly re-emitted photons also behave like waves and do go all over the place, however the waves of different photons have the same wavelength because they are all re-emitted from the same type of atom, hence if they are spaced apart with the right distance, they would ending up canceling each other out. and therefore the only place we don't see cancellation are where we will find the reflection and refraction. | ||
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Teoita
Italy12246 Posts
I don't think it has to do with interference because in reflection becuase you usually see an individual beam rather than a a interference pattern. Additionally, the light waves in a monochromatic weave woudln't necessarily cancel each other out simply because while they have the same wave length, they'd need to have opposite phases as well. | ||
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rei
United States3594 Posts
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Teoita
Italy12246 Posts
f(x) = A*sin(k*x+p); A is the amplitude, k is the weave number (which related to the frequency, which in turn is related to wavelength), x is the point in space, p is the phase. If two waves (of identical wavelegth) have the same phase at a fixed point in space they are said to be in phase (i think that's the english expression), so they produce constructive interference: the amplitudes sum each other. If they have opposite phases (for example one is 0 and the other is 180 degrees), they are said to be out of phase and the two amplitudes subtract each other, producing destructive interference. For example, these two functions (which can represent a wave) are out of phase: http://www.wolframalpha.com/input/?i=draw sin(x), sin(x+pi) If photons are emitted with random phases you will not be able to see any kind of interference pattern. The reason you don't see interference patterns for every beam of light existing is because the phases don't interact with each other at a macroscopical level, since they will be all distributed randomly. Note that this is classical physics. I don't know how things change in QED, i haven't studied any of it at a college level. | ||
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rei
United States3594 Posts
he talks about how light in nature will find the shortest path to the detector/observer, at around 40 he said something about the path that's not the shortest have their amplitude canceled out. in your equation, A is the amplitude, and only way for the A to be cancel out is to have the sin(k*x+p) to turn out to be opposite signs. | ||
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Teoita
Italy12246 Posts
2) Yeah. Basically what is happening is that all the contributions that are further away from the centered, "obvious" path have meaningless contributions, so it's as if the light was following the path everyone "experiences" at a macroscopic scale. Feynman explains it better than i do. The arrows he's talking about aren't related to wavelength though, they are phases (more or less). If you have time to either watch all of those videos or read QED (which is based on those lessons) you'll get it better than i can explain it. | ||
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rei
United States3594 Posts
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felisconcolori
United States6168 Posts
It reminds me of reading a book about black holes in high school (can't remember which one, it was the '80s). | ||
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rei
United States3594 Posts
I started my investigation about what is dark matter and dark energy, and in order to know that, I go search the history of the discovery of them, and that lead me to learn about the blue and red shift of the spectrum, and then that lead me to learn about what exactly produces spectrum, and that lead me to learn about how photon electron interaction in an atom, and that lead me to learn about how scientists can figure out the temperature of the star based on it's spectrum, and that leads me to understand how we figure out the distance of the stars based on the temperature and spectrum, and the list goes on and on, I have no one to tell me what to learn, I am just following my interest. Feynman's lectures touches on what I learned and it makes a connection when he himself said over and over that even he the one who came up with these and won a Noble prize for it does not understand these at all. | ||
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felisconcolori
United States6168 Posts
On April 02 2013 09:39 rei wrote: Feynman's lectures touches on what I learned and it makes a connection when he himself said over and over that even he the one who came up with these and won a Noble prize for it does not understand these at all. That's one of the great things about Feynman. He's an honest scientist, and excels at conveying the wonder without sounding in the least bit condescending towards anyone. | ||
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