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Oil film thickness for total reflection 500nm

  • Thread starter messick11
  • Start date
  • #1
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Homework Statement


I was asked to determin the film thickness for an oil film (n=1.33) for a light beam (500 nm) to give total reflection. I could calculate refraction angle for air to oil, but have no idea as to how to approach this question.


Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
1,506
17
This is an interference question. Light reflected from the front surface of the film interferes with light reflected from the back surface. For total reflection the 2 reflected waves must be in phase.
Points to consider
1) at the air/film surface a phase change of π (λ/2) occurs (there is no phase change at the film/air surface)
2) In the film the light travels a distance = 2 x thickness of the film and the wavelength changes.

Hope this gets you started !!
 
  • #3
2
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Thank you VERY much. I never thought of it as an interference problem!
 
  • #4
1,506
17
Its a pleasure. Did you get 94nm (λ/4 for the wave in the oil)?
 

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