Tilted Forum Project Discussion Community - View Single Post

fckm · 07-28-2004, 08:14 PM

kutulu:
well, yeah, more or less. Think about it this way. I have a blue laser. It only produces one wavelength, lets say... 400nm.
Now, i have a crystal that only absorbes red light. This means that the separation of the energy levels of the crystal correstpond to red light, of say... 700nm.
So, if I shine the laser on the crystal, the light passes through with NO absorption. This happens even though the blue light is at a higher energy (lower wavelength) than the energy separation of the crystal. Classically, this makes no sense. The blue light has plenty of energy, it should be absorbed by the crystal. But, according to quantum mechanics, (well, QED, quantum electrodynamics) this doesn't happen. And indeed, experiments confir m this.
EDIT:
as for Compton, the electron is no long in a bound state, so it's energy levels are no longer discrete. I'm not too sure how this effects photon absorption/reimission.
And remember, as much as a photon is a particle, it is also a quantization of the field. In some cases, especially when there is light-matter interaction, it doesn't really make sense to talk about "the same photon". Matter takes a certain amount of energy from the field, and gives a certain amount back. The energy is quantized in multiples of plank's constant. Beyond that, whether the photon is the same or not doesn't really matter.
A photon pretty much has three properties, wavelength/momentum, spin, and polarization. If these are the same before and after an interaction, then you can think of the photon as being the "same", it doesn't really matter. (unless you're talking about entropy and information destruction)

07-28-2004, 08:14 PM	#12 (permalink)
fckm Insane Location: Ithaca, New York	kutulu: well, yeah, more or less. Think about it this way. I have a blue laser. It only produces one wavelength, lets say... 400nm. Now, i have a crystal that only absorbes red light. This means that the separation of the energy levels of the crystal correstpond to red light, of say... 700nm. So, if I shine the laser on the crystal, the light passes through with NO absorption. This happens even though the blue light is at a higher energy (lower wavelength) than the energy separation of the crystal. Classically, this makes no sense. The blue light has plenty of energy, it should be absorbed by the crystal. But, according to quantum mechanics, (well, QED, quantum electrodynamics) this doesn't happen. And indeed, experiments confir m this. EDIT: as for Compton, the electron is no long in a bound state, so it's energy levels are no longer discrete. I'm not too sure how this effects photon absorption/reimission. And remember, as much as a photon is a particle, it is also a quantization of the field. In some cases, especially when there is light-matter interaction, it doesn't really make sense to talk about "the same photon". Matter takes a certain amount of energy from the field, and gives a certain amount back. The energy is quantized in multiples of plank's constant. Beyond that, whether the photon is the same or not doesn't really matter. A photon pretty much has three properties, wavelength/momentum, spin, and polarization. If these are the same before and after an interaction, then you can think of the photon as being the "same", it doesn't really matter. (unless you're talking about entropy and information destruction) __________________ And if you say to me tomorrow, oh what fun it all would be. Then what's to stop us, pretty baby. But What Is And What Should Never Be. Last edited by fckm; 07-28-2004 at 08:24 PM..