Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 13/11/2008 09:42:08
-
At the level of individual atoms and electrons, what is going on when a light ray hits certain surfaces (such as a mirror) and is "reflected"?
Chris
-
There are two mechanisms for em reflection.
1. The more straightforward one is what happens at the surface of a good conductor (shiny metal). There are many, loosely bound electrons on the surface (Metallic Bonding). They can be made to move very easily, which is why metals conduct electricity and heat so easily. The incident em fields cause the electrons to move, transferring energy to them. Once they are moving (oscillating) they radiate the energy again. If the surface is flat (plane), the way the re-radiated waves combine (the phases and the positions of these tiny radiators) is to produce a reflected wave which emerges at a symmetrical angle to the surface. (Laws of reflection, which we all did at School).
All metals have some resistance so there is always some energy loss with this type of reflection.
2. Transparent insulators (air / glass / pure water) can still produce reflection due to the induced movement of electrons but it is more complicated and harder to understand and explain.
A wave going through glass will stimulate the electrons to vibrate but they are tightly bound to their molecules and they move as if 'on springs'. Also, they interact much less with the wave, so it penetrates easily, merely being slowed down by the lag involved with the electrons absorbing and re radiating. The more dense the material, the slower the wave will travel, as a rule. The speed change is described by the refractive index and it causes change of direction when the light hits obliquely. It's easier to treat this 'macroscopically' but what I said above describes the 'microscopic' connection.
When the em wave goes from a slow medium to a fast medium at an oblique enough angle, the light is 'bent so much' that it can't, in fact, emerge from the interface and all the energy is reflected. This is 100% reflection and the effect is used in many optical instruments where right angled prisms are used instead of mirrors to make the image brighter. Gemstones and cats' eyes are so bright because this Total Internal Reflection is used.
-
Concerning the first mechanism, why are the photons re-emitted at the same angle as they arrived? Why aren't they emitted in random directions? My understanding was that when an atom absorbs a photon an electron jumps to a higher energy level momentarily, then re-emits the photon on the way back to its original energy level in a random direction. Or am I thinking in particles when I should be thinking in waves?
-
Huygen's principle says that you can predict the progress of any wave in terms of 'secondary wavelets'. You take your wavefront (it works for any shape) and 'cover it' with a new set of point sources which radiate omnidirectionally but in the same phase as the incident wavefront. The vector sum of all these wavelets gives you the shape of the wavefront in the near future.
This is, effectively, the same as treating the problem as a diffraction calculation where you integrate over the aperture you are studying.
The re-emitting electrons can be looked on as a set of Hugens sources on the surface of the mirror. They will be phased such as to give a 'beam' in the right direction (because of the phase of the incident wave as it stimulated each electron. Waves in other directions will cancel out (destructive interference.
If your reflector is small enough (say a dipole antenna or a light-scattering air molecule), the resulting wave will, indeed, be spread out.
In a metal, the available energy levels are very closely spaces - you have, effectively, a continuum of possible states.
-
Madidus:)
Ahh, can I shorten it too Mad:)
A very interesting question that one.
If you look at as waves it seems very reasonable that it will reflect back .
But if you see it as 'particles' what happens to the momentum?
As the 'photon exchange' takes place?
(Btw: it goes for the 'waves' too i think?:)
Sophie,:) you explain it beautifully as waves, but if treated as particles?
How would you describe it then, it should be possible or..
Or Max?
(I have a very low state of humor
Or if you like.
My humor is in a very sad, although permanently changing, state of affairs:)
-
Treated as waves you still get a momentum exchange of the same value. It's all there in classical em theory books.
As for the particle approach:
If you insist then you could say that each photon interacts with a particular electron which re radiates a photon. The phase of the wave associated with each new photon relates to the phase of the original photon - so the Huygens idea takes over.
Where and how did the 'stream of incident photons' come about? You really need the wave approach for that, unless you take a lot for granted - such as "Why are they all going the same way?" and similar questions. . . . .
Unless you have doubts about duality then why not choose the easier route to the solution to any particular situation?
It can always be done but it's sometimes very cumbersome.
The mirror IS pushed backwards, btw, by the amount you'd expect.
-
Ok Sophie.
Would that mean that in this case (reflection) the 'random direction(s)' Madidus was discussing would have no 'say' anymore?
But otherwise it would?
---------
Can't you , if you like, see all interactions (valence jumps) as 'reflections'?
Or is it just my brain overheating:)
Ahhh, steeaaam...
-
Would that mean that in this case (reflection) the 'random direction(s)' Madidus was discussing would have no 'say' anymore?
This it what happens when people want photons to be bullets. As you say, they would have to 'go in random directions' if that's all they were they would behave differently (like a hand grenade). If you allow them to take the form of waves - spreading out from each atom and interfering with waves from other atoms then it is possible to believe they are going to do exactly what they actually DO do - form a beam.
This 'bullet' idea still survives since long before Newton and it really gets in the way of understanding of even the simplest situation. There can never be any 'proof' that a photon is a particle whilst it is on its way between A and B because you would need to introduce a 'C' at which to detect it (and interact with it). Why not just let it be energy which is 'guided' by the way a wave will behave? Don't keep trying to say what it 'really is' 'cos it isn't 'really' anything.
-
Well Sophie, I'm kind of 'old fashioned' I'm afraid.
As you probably noticed:)
Not that I don't see the merit of your and others ideas, especcially mathematically.
Just that I find invariant mass so frustrating when trying to see it as waves.
You say "Don't keep trying to say what it 'really is' 'cos it isn't 'really' anything."
Now you got me confused, if that now was directed at me?
The only thing I've said is that I really would like to know what 'invariant mass' is.
Mostly because what I see all around me, 'clashes' with how we, successfully, describe those properties mathematically.
I see matter, not waves and the mug I'm holding is of 'invariant mass', not light.
So?
-
"Don't keep trying to say what it 'really is' 'cos it isn't 'really' anything."
That wasn't aimed at anyone in particular but it applies to posts in almost every thread involving photons. And it keeps leading people up gum trees.
Yes- you 'see' matter when you hold your mug because it is interacting with photons. If you allow the waves to predict how things behave between interactions and then, sometimes, choose to regard the interactions as involving particles then you don't need to have a problem.
(Except the problem of accepting that idea, of course). :)
-
Sophie, what can one say?
You shine :)
---
better:)
-
You shine girl:)
A common misconception......
-
I often look at people's profiles. It can be revealing.
It's too late to change my monicker.
-
Hey, it fooled me:)
What does it stand for?
That monicker
-
My Boat is called Sophie and she is an ancient Westerly Centaur.
The name seems to stir some loins, it seems!
-
And now that humor of mine will reach an even lower height:)
Sir loins?
Someday I will have to grow up, I know.
--------
Btw: motor or sails?
I used to have sails, quite fast too:)
----------------
Ok, a sloop right.
Looks nice.
-
Sir loins?
Well, there's a lot at stake.
26 foot sloop
scruffy but has 1970's charm and lots of room inside.
Lifted out today, as it happens. Back in in March.
-
Nice :)
And yes, the winter is here.
I used to sail a thirty feet, double rudders.
It was fast and could turn 'on a penny'
We christened it 'mine'
but it didn't have a lot of room though.
Sailing is a very relaxing occupation:)