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Published by Karnuvap 09-29-2008
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#1
By
modest
on
09-30-2008
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| Re: Anti-Reflection Coating I imagine (but don't know) that this is very much like interference by way of a classic double slit experiment. The interference is not so much a real physical thing that happens between 2 photons, as wikipedia's Double Slit page says: Quote:
I find it incredibly difficult to imagine "probability waves" interfering with themselves or considering every possible path through space before deciding on the best possible path - but this is only one of several things quantum that I find very counterintuitive. ~modest | |
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#2
By
Qfwfq
on
10-09-2008
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| Re: Anti-Reflection Coating Actually Modest, this phenomenon is much less counterintuitive than others. Karnuvap, what I think you aren't considering is that the photon (if we imagine it makes sense to follow its corpuscular trajectory) doesn't really have to "decide" or "know" at all when it reaches the inner boundary. If we imagine it being reflected "the first time" it will return to the outer boundary and, at that point, there isn't the same issue of local causality in the fact that it must be reflected. You could imagine that it may go back and forth an arbitrary number of times, with the "maybe" being only at each time it reaches the inner boundary. There would be a decreasing probability for increasing number of reflections. Strictly, though, if things are analysed properly, what modest says is correct except that the non-locality issue is in time rather than in space; indeed a non-constant amplitude bungles things up a little bit, according to how much the amplitude varies over the time to traverse the coating. However, at least one can imagine it in a less disturbing way by considering that, as a "traditional" corpuscle, it needn't go forward the very first time. |
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#3
By
Pyrotex 
on
10-10-2008
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| Re: Anti-Reflection Coating Quote:
We see the light coming in from the top. Some portion reflects off each surface it hits. Point 1: The smaller the difference between the two Indexes of Refraction (Ir) at each surface, the smaller the amount of light will be reflected at that surface. Since the AR-coating is in-between the Ir of air and the Ir of glass, both surfaces now reflect less light than an air/glass surface. Point 2: The thickness of the AR-coating is 1/4 wave. This effectively means (A) destructive interference at the top surface and (B) constructive interference at the AR-coating/glass surface. (A) effectively means that no light comes OUT of the air/AR-coating surface. Some is absorbed. Most is reflected back down. (B) effectively means means that at the AR-coating/glass surface, a higher % of light gets through. Point 3: Internal reflections in the AR-coating means that most of the light that did NOT get into the glass on its first pass, has a another chance to get into the glass. The overall effect of this point is much (if not most) of the light that originally reflected off the glass, now gets into the glass, even if it takes 1, 2, 3, 4, 5, or more internal reflections. Of the original amount of light that would have reflected off an unprotected glass surface, we now have the case that WITH the AR-coating, almost NONE of that light is reflected (thus the name "anti-reflection"); a small portion (<50%) is aborbed by the AR-coating; and most of that light now makes it into the glass. So, AR-coatings not only reduce reflections, they actually increase the amount of light that gets through. At no time did I have a double-slit up either sleeve.  | |
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#4
By
Karnuvap
on
10-11-2008
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| Re: Anti-Reflection Coating Quote:
But I was still concerned about the "1/4-wave meaning destructive interference at the top surface". The light that is attempting to exit destructively interferes with some of the incident light - OK with this but where does the energy go? Ans: It goes through the coating and into the lens instead. But how does it get from the top surface to the bottom and thus penetrate the lens? Ans: It "knows that it will be destructively interfered and so doesn't bother heading up towards the outer surface" I thought. But Pyrotex has helped me understand that the point of "pre-cognition" is the outer surface of the coating. It doesn't need to communicate with light at the bottom surface of the coating. OK, all it needs to do now is somehow "know" that it will be destructively interfered with should it venture "outside" and so it decides to be internally reflected instead. I am happier with this explanation but, you can see, it is still a little mysterious. Thanks guys for the explanations. I am much happier now. The Vap. | |
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