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So, I was reading about the Casimir effect. Two mirrors facing each other attract to each other in a vacuum. The reason is due to pressure exerted on those mirrors from the multitude of EM waves (like light) outside of them, while in between those mirrors there are less waves present since long waves can't fit. The closer you bring those mirrors, the less waves are present and the stronger is the external pressure on them.

I always imagined waves as being made out of segments, like in the classic Nokia game Snake. I thought that the first segment of a wave to hit a reflective surface would be reflected first, while the last would be reflected last, and thus in between two mirrors the entire wave would simply fold. The first part hitting the first mirror would be reflected to the second mirror and hit it while the 5th part is only about to reach the first mirror.

Now I see I was wrong. Why?

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When you have two parallel mirrors you get standing waves between them, and the standing waves can only form if the spacing between the mirrors is a half integral number of wavelengths. Exactly this principle is used in the Fabry-Perot interferometer. The restriction of the wavelength happens because if the mirror spacing is not a half integral number of light wavelengths the reflected light interferes destructively with the incident light.

In the context of the Casimir effect the key thing to note is that wavelengths bigger than twice the mirror spacing are excluded because they cannot satisfy the criterion that the mirror spacing be a half integral number of wavelengths.

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Thank you for the answer, I think I understand you but just to make sure- – the rains of casimir Apr 17 '13 at 21:33
Thank you for the answer, I think I understand you but just to make sure- You are saying that the reason a wave cannot be broken between two mirrors is because when one part of the same wave meets the other, they cancel each one out. Like in those anti-noise headsets (a microphone recieves noise and a speaker nearby sends the same sound waves in an opposing direction). Does my understanding of you is correct? Thx :) – the rains of casimir Apr 17 '13 at 21:49
Yes, sort of. A noise cancelling system uses destructive interference, which also happens between the mirrors. However the standing wave between the mirror reflects back and fro many times so the interference is especially effective. – John Rennie Apr 18 '13 at 5:44
Another small (:P) question. I have read today as much as I could about standing waves, perhaps (probably) I'm missing something, but if this reversal of the long wave is detrimental to it, why aren't the standing waves affected. I appologise if I'm asking something fundamentaly stupid. XD – the rains of casimir Apr 18 '13 at 20:53

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