How Thin Must the Oil Film Be to Appear Red?

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To determine the minimum thickness of an oil film that appears red due to thin film interference, the film must meet specific optical conditions. The refractive indices of the oil and water are crucial, with the oil having a higher index (n = 1.49) than water (n = 1.33). For the reflection from the top surface to be in phase with that from the bottom surface, the film thickness must satisfy the equation related to the wavelength of light in the film. The minimum thickness can be calculated using the formula for constructive interference, which involves the wavelength of light in the oil and the refractive indices. Understanding these principles is essential for solving the problem effectively.
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A thin film of oil (n = 1.49 is spread over a puddle of water (n = 1.33). In a region where the film looks red from directly above (lambda = 629 nm), what is the minimum possible thickness of the film?

______nm

Could someone please help me with this? I have no clue on this one, it was the only of the assignment I couldn't solve...

Thanks!
 
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thin film interference

What conditions must be met so that the reflection from the top surface (air/oil interface) is exactly in phase with the reflection from the bottom surface (oil/water interface)?
 

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