What does a "phase reversal" really do?

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Discussion Overview

The discussion revolves around the concept of phase reversal in the context of thin film interference, exploring its implications for light behavior and visibility. Participants examine how phase shifts affect interference patterns and the conditions under which light rays interact.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that a wave reflecting off a medium with a higher index of refraction undergoes a phase shift of 180 degrees, while a wave reflecting off a lower index does not.
  • Another participant suggests that a 180-degree phase shift is not visible, but it can lead to cancellation when two rays of light are superimposed with opposite phases.
  • Questions arise about the necessity of considering phase in thin film interference, with one participant wondering if it is solely for identifying destructive interference.
  • Further discussion indicates that while some rays may not interact, others converge at a point (like the retina), where their phase relationships become significant for the resulting light intensity.
  • Participants discuss the analogy of adding vectors to illustrate how phase can affect the resultant light when rays overlap.

Areas of Agreement / Disagreement

There is no consensus on the necessity of phase consideration in thin film interference, with some participants asserting its importance for interference effects while others question the interactions between rays.

Contextual Notes

Participants express uncertainty about the interactions between light rays and the implications of phase shifts, indicating a need for clarification on how these factors influence interference patterns.

Who May Find This Useful

This discussion may be of interest to those studying optics, wave behavior, or anyone seeking to understand the principles of interference in light.

cmlee1324
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While learning about thin film interference, I see diagrams quite often that display waves being inverted. For instance, a wave that bounces off outside the film is inverted because it is going from air to film (lower to higher index of refraction) while a wave that bounces INSIDE the film is not inverted (higher to lower index of refraction). This got me thinking.

Why do phase "reversals" (what my teacher calls a phase change of 180 I think) matter? If light is still in the same location at the same angle, why does the phase matter? Does light appear different when it has a different phase? If the period of light is so ridiculously fast, why would a nanosecond difference in phase appear different?

If this question makes no sense from the view of someone with more knowledge on the subject, may I request a very basic explanation of waves and how they interact with thin films and an eye?
 
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A phase shift of 180 degrees is not visible, but if we have two superimposed rays of light having opposite phase then we will find cancellation.
 
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So the only purpose of taking phase into account is to see if there is any destructive interference?

If so, why would that be necessary when talking about thin film interference? I would assume that there are no interactions between the rays; they glance off into infinity.

Do the rays interact? Is that the reason why 1 vs 2 phase reversals gives different equations for constructive and destructive interference? (m or m+1/2)
 
cmlee1324 said:
So the only purpose of taking phase into account is to see if there is any destructive interference?
Yes, pretty much. At least in a general interpretation. Although it's "interference" not just destructive interference.

cmlee1324 said:
If so, why would that be necessary when talking about thin film interference? I would assume that there are no interactions between the rays; they glance off into infinity.
Some may go different places. But some paths meet at a cell on your retina, or the photocell in your instrument. At that point the response they create is the result of addition, in which case phase can be really important. Like adding two vectors up, direction matters. Same with light.
 
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