Effect on visibility of thin films due to interference

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SUMMARY

The discussion centers on the visibility of thin films due to light interference, specifically addressing the conditions under which light is reflected or refracted through films of varying thicknesses. Participants clarify that visibility is contingent upon light reaching the observer, with destructive interference potentially rendering films invisible. The conversation highlights the significance of phase changes upon reflection, particularly the π phase shift at higher index media, and references Newton's Rings as a classic example of thin film interference. The conclusion drawn is that both films can appear bright under certain conditions, contradicting the assumption that one film must appear dark.

PREREQUISITES
  • Understanding of thin film interference principles
  • Knowledge of phase shifts in wave optics
  • Familiarity with Newton's Rings and their implications
  • Basic concepts of constructive and destructive interference
NEXT STEPS
  • Study the principles of thin film interference in detail
  • Learn about phase shifts in optics, particularly at boundaries
  • Explore practical applications of Newton's Rings in optical experiments
  • Investigate the mathematical modeling of light interference in thin films
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Students of physics, optical engineers, and anyone interested in the behavior of light in thin films and interference patterns.

  • #31
brochesspro said:
Sorry, I didn't get you.
Suppose on hitting the film from the left only 5% is reflected. The other 95% reaches the far side of the film, where 75% is reflected. (Is this possible? I don't know.) So then 75% of that is reflected from the LHS, the other 25% emerging. The reflected portion returns to the RHS, 75% of that being reflected and 25% emerging as the third ray directed leftwards.
We have, in the first three rays going leftwards from the film:
5% in the first
95%*75%*25%=15.7% in the second
95%*75%*75%*75%25%=9% in the third
 
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  • #32
haruspex said:
Suppose on hitting the film from the left only 5% is reflected. The other 95% reaches the far side of the film, where 75% is reflected. (Is this possible? I don't know.) So then 75% of that is reflected from the LHS, the other 25% emerging. The reflected portion returns to the RHS, 75% of that being reflected and 25% emerging as the third ray directed leftwards.
We have, in the first three rays going leftwards from the film:
5% in the first
95%*75%*25%=15.7% in the second
95%*75%*75%*75%25%=9% in the third
So this supports my assumption, right?
 
  • #33
brochesspro said:
So this supports my assumption, right?
That was an explanation of how the third ray to the left could be more significant than the first one, so an analysis that only considers the first two is incomplete.
 
  • #34
haruspex said:
That was an explanation of how the third ray to the left could be more significant than the first one, so an analysis that only considers the first two is incomplete.
So in short, no, right?
If so, I think I will stop thinking about this problem.
 
  • #35
brochesspro said:
So in short, no, right?
If so, I think I will stop thinking about this problem.
Good.
 
  • #36
haruspex said:
Good.
Thank you for all your help. The others too.
 

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