Light reflected from film, find maximum thickness of film, draw phase diagram.

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Homework Statement


The light reflected from a soap bubble (n=1.4) appears bright red (wave length= 640 nm) at its center. What is the minimum thickness (in nm) there? Dray Phase Diagram.


Homework Equations


lambda(n)=lambda/n n=index of refraction
2*n*t=m*lambda
OR
2*n*t =(m+.5)*lambda
OR
2*n*t=m*lambda
I dont know which one to use.

The Attempt at a Solution


I tried skipping the first equation, because we know the light is INSIDE already, we should have the lambda(n) needed for the second three equations. I plugged numbers into both and got answers. 230 nm is what I believe the answer is.
I understand that each of these are for different situations of constructive vs. destructive interference, but I do not understand how to tell which situation is happening in this question! I do not know when to change m either!
Thanks so much for the help.
 

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