Vis Viva Posted May 13, 2007 Share Posted May 13, 2007 Ok, the way I get it is this: One can work out Maxwell's equations in conductive or nonconductive media, and find a slower characteristic wave speed for electromagnetic waves, than the value for this speed in vacuum. I'm having trouble however to see past these equations; how does this work on a subatomic scale? What does this picture look like? Is there constant absorbtion/reemission going on, slowing down the passage of the photon, or does the photon interfere somehow with the electrons wizzing around in the medium, or...? Any advise is welcome. (First time poster, please be gentle ) Link to comment Share on other sites More sharing options...
ajb Posted May 13, 2007 Share Posted May 13, 2007 You have to be careful trying to apply equations that govern classical physics to the (sub)atomic scale. So indeed, Maxwel's equations won't be able to explain what is going on inside a material at the quantum scale. The correct langauge to use is that of quantum mechanics. Light is slowed down in a medium, because it interacts with the electrons in the medium, so that the effects of this interaction shorten the mean free path and so the photon will on averege travel slower in the medium. I am sure you can find out more by using google. Link to comment Share on other sites More sharing options...
YT2095 Posted May 14, 2007 Share Posted May 14, 2007 IIRC, the process of absorption and re-emission takes a little time. Link to comment Share on other sites More sharing options...
Klaynos Posted May 14, 2007 Share Posted May 14, 2007 IIRC, the process of absorption and re-emission takes a little time. That would be a photon - electron interaction Link to comment Share on other sites More sharing options...
swansont Posted May 14, 2007 Share Posted May 14, 2007 Well, photon/atom interaction, technically. Free electrons don't absorb photons, bound systems do. Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted May 14, 2007 Share Posted May 14, 2007 Well, photon/atom interaction, technically. Free electrons don't absorb photons, bound systems do. Can a free electron interact with and absorb some of the energy and momentum of a photon? Link to comment Share on other sites More sharing options...
swansont Posted May 14, 2007 Share Posted May 14, 2007 Can a free electron interact with and absorb some of the energy and momentum of a photon? It can scatter a photon and get some energy and momentum, but it can't just absorb the photon — you couldn't conserve energy and momentum, which is a standard relativity exercise to work out. At some point, though, you might invoke Heisenberg to tell you how long the scattering might be able to delay the photon. Compton scattering always changes the photon direction, but this is not what is being described here. (I'm sure of one were to discuss what happens in a plasma there would have to be interactions with electrons) Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted May 14, 2007 Share Posted May 14, 2007 It can scatter a photon and get some energy and momentum, but it can't just absorb the photon — you couldn't conserve energy and momentum, which is a standard relativity exercise to work out. At some point, though, you might invoke Heisenberg to tell you how long the scattering might be able to delay the photon. Compton scattering always changes the photon direction, but this is not what is being described here. (I'm sure of one were to discuss what happens in a plasma there would have to be interactions with electrons) Thanks. I assume these two (that I bolded above) are related. Interesting. Link to comment Share on other sites More sharing options...
swansont Posted May 14, 2007 Share Posted May 14, 2007 Yes, they are. Link to comment Share on other sites More sharing options...
Vis Viva Posted May 18, 2007 Author Share Posted May 18, 2007 IIRC, the process of absorption and re-emission takes a little time. You say photons are being absorbed and reemitted. Does'nt this affect their general direction? That can't be inside plain glass for example... Link to comment Share on other sites More sharing options...
Klaynos Posted May 18, 2007 Share Posted May 18, 2007 It does, get a glass of water and put a straw in it, the straw looks bent. Have a look online for refraction, and snells law. Link to comment Share on other sites More sharing options...
swansont Posted May 18, 2007 Share Posted May 18, 2007 You say photons are being absorbed and reemitted. Does'nt this affect their general direction? That can't be inside plain glass for example... The absorption and re-emission by virtual states still has to obey conservation of energy and momentum when all is said and done. The light changes direction at an interface whaere the index changes, but not in the bulk material. Link to comment Share on other sites More sharing options...
waitforufo Posted May 18, 2007 Share Posted May 18, 2007 I recommend that you read "QED, The Strange Theory of Light and Matter" by Richard P. Feynman. This lecture was written by Feynman to explain difficult concepts to the general public. It's a very enjoyable read from a Nobel laureate. You can read through in a few hours. By the way Feynman goes out of his way in the book to explain that physics and physicists are in the business of studying and understanding “how” things behave not “why” they behave that way. I know it’s a bit of a nit, but science is easier to except and understand when you give up on the notion that there is cognitive reason behind it. Link to comment Share on other sites More sharing options...
swansont Posted May 18, 2007 Share Posted May 18, 2007 By the way Feynman goes out of his way in the book to explain that physics and physicists are in the business of studying and understanding “how” things behave not “why” they behave that way. I know it’s a bit of a nit, but science is easier to except and understand when you give up on the notion that there is cognitive reason behind it. The homonym substitution (should be accept vs except) aside, this is something that needs to be stressed more, especially to posters that show up with philosophical objections to scientific theories, or purely metaphysical underpinnings to their own ponderings. Not a nit in my view. Link to comment Share on other sites More sharing options...
waitforufo Posted May 18, 2007 Share Posted May 18, 2007 The homonym substitution (should be accept vs except) aside ... I know better and yet I still get it wrong. Thanks! Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now