Do Perfectly Collimated and Monochromatic Beams Show Interference Fringes?

  • Context: Graduate 
  • Thread starter Thread starter Gobil
  • Start date Start date
  • Tags Tags
    Beam Fringes
Click For Summary

Discussion Overview

The discussion revolves around the conditions necessary for observing interference fringes when using perfectly collimated and monochromatic beams in interferometry. Participants explore the implications of beam characteristics on the visibility of interference patterns, questioning the feasibility of achieving such ideal conditions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant suggests that perfectly monochromatic and collimated beams would not produce visible fringes due to the absence of divergence, proposing that only a spot of light indicating brightness proportional to phase difference would be observed.
  • Another participant acknowledges the need for some bias or fringes to enhance accuracy in interferometry but expresses uncertainty about the original question.
  • A third participant argues that Heisenberg's principle implies that perfectly monochromatic and collimated beams cannot exist, framing the discussion around the physical impossibility of such ideal conditions.
  • A later reply supports the idea that transverse modes in any beam lead to interference patterns, asserting that without these modes, spatial variance of interference would not occur.

Areas of Agreement / Disagreement

Participants do not reach a consensus. There are competing views regarding the existence and implications of perfectly collimated and monochromatic beams, with some asserting their impossibility and others discussing their theoretical implications.

Contextual Notes

Limitations include the assumptions about the physical realizability of perfect beams and the dependence on definitions of collimation and monochromaticity. The discussion does not resolve the mathematical or conceptual implications of these assumptions.

Gobil
Messages
59
Reaction score
0
Hi All,

Just got to thinking about interference, if we have two sources (say a laser beam split in two), and introduce a phase shift between the two by making them travel different path lengths, then recombine them, we see fringes in the wavefront right. (interferometry)

My problem is this, if we have two perfectly monochromatic and perfectly collimated (i.e. no divergence as they propagate out to infinity) beams, the fringes would not be visible right? as the transverse pattern occurs due to the fact that the beams are indeed slightly divergent. You would however see a spot of light whose brightness in proportional to the phase difference between the two beams.

so the only reason we have interferometry is because we can't make perfect beams.
Does this make sense?

Thoughts please.
 
Science news on Phys.org
If I understand you correctly, it is true that in order to use interfereometry as a tool, there needs to be some bias present (some fringes) to increase the accuracy.

But I'm not sure I understood your question.
 
I am not saying I agree, and I am not saying I disagree. What I am sure of is that Heisenberg's principle prevents both a perfect monochromatic or a perfectly collimated beam. Both concepts may be useful for our understanding, but neither is physically possible.

In this sense, it's not really a matter of "our ability" to make them, it's just that close analysis reveals that the idea of making such a perfect beam is as absurd as building an infinitely long rod or something.
 
yes, this is what I was getting at.

The fact that there are transverse modes in any beam means that interfering two identical beams will cause an interference pattern.

If there are no transverse modes, then there will be no spatial variance of interference in the plane of propagation
 

Similar threads

Graduate Light-beam propagation across impassable barrier?
  • · Replies 23 ·
Replies
23
Views
3K
High School Explanation for bright fringes in Single Slit Diffraction
  • · Replies 4 ·
Replies
4
Views
2K
High School What happens to the energy in destructive interference?
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Double slit displaying anti-fringes?
  • · Replies 8 ·
Replies
8
Views
21K
Graduate Do Multiple Diffraction Gratings Affect Interference Patterns?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Laser interference fringes fading in and out of view
  • · Replies 30 ·
2
Replies
30
Views
3K
Graduate The coherence length is much longer than the manufacturer claims
  • · Replies 10 ·
Replies
10
Views
3K
Graduate Can cooling enhance the monochromaticity of an electron beam?
  • · Replies 13 ·
Replies
13
Views
4K
Undergrad Distance between fringes in an interference pattern?
  • · Replies 4 ·
Replies
4
Views
5K
Graduate Question about dark fringe in diffraction
  • · Replies 1 ·
Replies
1
Views
3K