Oil film thickness for total reflection 500nm

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Homework Help Overview

The discussion revolves around determining the film thickness of an oil film (n=1.33) for a light beam of wavelength 500 nm to achieve total reflection. The problem is situated within the context of optics, specifically focusing on interference phenomena.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to understand how to approach the problem after calculating the refraction angle. Some participants suggest considering the interference of light waves reflected from the film's surfaces and the implications of phase changes at the boundaries.

Discussion Status

Participants have engaged in exploring the nature of the problem as one of interference, with some guidance provided regarding phase changes and the relationship between film thickness and wavelength. There is an acknowledgment of the original poster's realization of the interference aspect, indicating a productive direction in the discussion.

Contextual Notes

There is a mention of specific conditions for total reflection and the need to consider phase changes at the boundaries of the oil film, which may influence the approach to the problem.

messick11
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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

 
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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 !
 
Thank you VERY much. I never thought of it as an interference problem!
 
Its a pleasure. Did you get 94nm (λ/4 for the wave in the oil)?
 

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