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Optics and thin film interference question. 
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#1
Feb2009, 02:19 PM

P: 27

1. The problem statement, all variables and given/known data
The thin film is of two glass with air in the middle. refractive index of... air: n=1.0003 glass: n=1.5 lamba=632.8nm Q1.) Calculate optimum thickness of the film (for 1st order interference) Q2.) Why does the distance between interference fringe beome longer (increased fringe spacing) when the film thickness is reduced? 2. Relevant equations We aren't given any, we have to find them ourselves. I found: 2t=m(lamba/n) and 2t=(m+1/2)(lambda/n) one is constructive one is destructive, not sure which is which for this case. not sure if they are even relevant here. 3. The attempt at a solution Q1.) I found: t=m(lambda) / 2n and used that to get 3.19 x10^7 m Is that correct? Q2.) I drew a diagram that explains it, and I think it has to do with missing fringes in between due to destructive interference, however I don't have any equation that shows that. I think this is relevant: 2t=m(lamba/n) I think that's for constructive interference. t is directly proportional to m (# of visible fringes). When t is decreased, the # of fringes decrease. Thus, providing empty spaces between fringes. But how do I prove that it's every other fringe that disappears? Is there any equation that shows thickness is inversely proportional to fringe spacing? 


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