*19.  ssm Orange light (λvacuum = 611 nm) shines on a soap film (n = 1.33) that has air on either side of it. The light strikes the film perpendicularly. What is the minimum thickness of the film for which constructive interference causes it to look bright in reflected light?

Now we need to consider constructive interference, so our phase equation is now

 

 

 

This is the condition for constructive interference.  We do not use (m-1/2), because we do not want negative signs showing up.  Now solve for thickness

 

 

For minimum thickness we can make m = 0.  Minimum thickness is

 

 

 

 

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This page last updated on July 13, 2018