Cause of interference pattern in the Michelson Interferometer.

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Discussion Overview

The discussion revolves around the cause of the interference pattern observed in the Michelson Interferometer, focusing on the formation of circular fringes and the role of path differences in the interference of light waves. Participants explore theoretical aspects and practical implications of the setup.

Discussion Character

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

Main Points Raised

  • Some participants question how the path difference between light waves remains constant while still producing circular fringes.
  • Others suggest that varying the length of one of the mirrors by micrometers is necessary to observe the interference effects.
  • One participant emphasizes that the purpose of the interferometer is to detect very small differences in length, implying that fringes should be expected.
  • A participant expresses confusion about why the fringes are circular, comparing it to Newton's rings where the thickness of air is constant over concentric circles.
  • Another participant proposes that the circular pattern arises because the wave-fronts of both beams expand in a cone, leading to different travel paths at various points on the detector screen.
  • It is noted that a simple analysis indicates the interference condition is met when light travels perpendicularly to the mirrors, but reflections at small angles result in longer path lengths that vary for the two mirrors.

Areas of Agreement / Disagreement

Participants express differing views on the reasons for the circular fringes, with no consensus reached on the underlying cause. The discussion remains unresolved regarding the specific mechanisms leading to the observed patterns.

Contextual Notes

Participants have not fully explored the assumptions regarding the geometry of the interferometer and the nature of the light waves involved, leaving some aspects of the discussion open to interpretation.

Zanychap
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The books say that interference happens basically between the light waves from the two mirrors. But it seems to me that the path difference between the pairs remains constant for all the waves (as long as both the mirrors are perpendicular to each other). So why do circular rings form? How does the path difference vary?
 
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You need to vary the length of one of the mirrors over very small differences. Typically micrometers will do it for the visible light spectra.
 
The point of the Interferometer is to see _very_ small differences in length and changes thereof. In general, I would challenge anyone to set one up such that it doesn't have fringes...
 
The point that I am not getting is why are the fringes circular.
Like in Newton's rings I know that the thickness of air is constant over concentric circles causing circular rings. I just can't understand what causes this in Michelson's.
 
Ah...

I think it's because the wave-front of both beams expands in a cone, so the travel path is different at each point on the detector screen.
 
The usual simple analysis gives you the interference condition when the light goes through exactly perpendicular to the mirrors. This is the center of the actual interference pattern. At other points on the screen, the light reflects off the mirrors at a small angle. This makes a longer path which is generally slightly different in length for the two mirrors.

http://www.phy.davidson.edu/stuhome/cabell_f/diffractionfinal/pages/Michelson.htm
 
Thanks.
 

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