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Minimum thickness of thin film constructive interference 
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#1
Aug1008, 12:59 PM

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1. The problem statement, all variables and given/known data
What is the minimum (nonzero) thickness of a benzene (n = 1.501) thin film that will result in constructive interference when viewed at normal incidence and illuminated with orange light (lamba = 615 nm)? A glass slide (ng = 1.620) supports the thin film answer= 204 nm 2. Relevant equations I rearranged an equation to get t= (m=.5)(lambda) / 2*n but i'm thrown off by how they give me two n values? 3. The attempt at a solution I tried using my equation plugging in each n, but did not get the answer. please help, thanks in advance :) 


#2
Aug1008, 01:11 PM

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Hi crazyog,



#3
Aug1008, 01:41 PM

P: 50

well they are referring to the benzene so would it be the 1.501 for n.
would the equation be t= (lambda)/4*n hm...but im still not getting the answer? 


#4
Aug1008, 01:52 PM

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Minimum thickness of thin film constructive interference
So what equation give constructive interference for this case? (I think there should only be two alternatives.) 


#5
Aug1008, 02:00 PM

P: 50

oh ok so (615*10^9)/(2*1.501) = 2.04*10^7
Got it! thanks! so the key was that it is constructive interference, which they stated in the problem... is there a way to tell if they don't state it? like if they say they would the find the maximum or minimum, does that refer to constructive and deconstructive? 


#6
Aug1008, 02:13 PM

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P: 2,249

If instead, for example, the benzene was on top of water (with an index n=1.333), then the equation in your original post would have been the correct one for constructive interference, because that case would only have one phase reversal. I'm sure you've noticed by now in your studies that there are many nonstandard way of wording physics problems, so no matter how many keywords you keep in mind, you'll still run across problems that are worded in a new way, and you just have to think about it to figure out what they want. 


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