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Offline syhprum

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Reflections
« on: 24/10/2007 11:27:35 »
What happens when a Photon is reflected from an aluminized mirror?.
Is it adsorbed by an Aluminium atom on the surface and then re-emitted? if so with what delay?


 

lyner

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Reflections
« Reply #1 on: 24/10/2007 12:20:17 »
Reflection on a mirror  surface doesn't involve interaction with a single atom. We all learn about atomic theory in the context of gases, where the atoms are all independent but, in a solid, there is a very different situation.
Forget atoms, for a minute, and think of a radio wave hitting a metal surface. The electric field next to the metal surface causes electrons to move backwards and forwards - a current  (AC) flows across the surface. The surface now behaves like a transmitting antenna - the induced current produces a wave traveling away from the surface. The detailed calculations tell you that the wave emerges at an angle to the surface, symmetrically to how it arrived. (The angle of reflection equals the angle of reflection - remember?)
Somehow, it seems more reasonable to talk of radio waves than light, because we are all obsessed with photons and it isn't always the best way to look at a problem.
The same thing happens with light - electrons are made to vibrate on the surface of the aluminium  and a beam of light emerges. The wave that is produced is caused by a lot of electrons moving - if it were only one electron / atom involved, the  light would spread out in all directions (simple atomic theory with gases) . The most likely direction for an isolated photon to go is to follow the laws of reflection , but it may not, due to the statistics involved. With millions of them, the beam will follow that law. If the mirror is very small, though, there will be a finite amount of diffraction and the effect will be to spread the beam out, around the direction that simple geometry gives you.
As for how long it takes. A single atom 'absorbs' a photon at a very precise frequency and this is a form of resonance. It is a narrow - band resonance, so it takes time to build up and to decay - the delay would be many many cycles at the particular frequency  of the light.
The reflection at the surface of a metal is a wideband effect and takes very little time for the absorption and re-emission.  A fraction of a cycle of the light frequency. (In the case of a radio reflector, you would say  a slight phase shift (90O)
 

Offline syhprum

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Reflections
« Reply #2 on: 24/10/2007 14:09:54 »
Very interesting Sophie, this was a follow on from the question about the passengers in the falling elevator with the light beam passing across.
I was going to suggest that if there was a delay at the surface of the mirror they would gradually overtake the light beam.
 

lyner

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Reflections
« Reply #3 on: 24/10/2007 14:13:12 »
Nope - I think only about 90degrees per reflection..
Anyway, this is a THOUGHT  experiment! Stop bringing in fancy practicalities.
 

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Reflections
« Reply #3 on: 24/10/2007 14:13:12 »

 

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