Fresnel's Biprism Experiment: What Are Virtual Sources?

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Discussion Overview

The discussion revolves around the concept of virtual sources in Fresnel's biprism experiment, exploring how these virtual sources are formed and their role in the context of interference patterns. Participants engage in clarifying the relationships between rays and virtual sources, as well as the conditions necessary for observing interference.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants inquire about the definition of virtual sources in the context of Fresnel's biprism experiment.
  • Others describe how rays from an actual point source create virtual sources S1 and S2 when refracted through the biprism.
  • Participants discuss the tracing of rays (R1, R2, R3, R4) and their relationships to the virtual sources, with some seeking clarification on their labeling.
  • There are questions regarding the conditions necessary for observing interference patterns, including the alignment of the biprism's edge with the slit.
  • Some participants express confusion about the geometry of the biprism and the definition of acute angled prisms.
  • Discussion includes the implications of the prism's orientation on the visibility of interference patterns, with some participants providing explanations based on ray behavior.

Areas of Agreement / Disagreement

Participants generally agree on the basic definitions and relationships between rays and virtual sources, but there are ongoing questions and some confusion regarding the geometry of the biprism and the conditions for interference. Multiple viewpoints exist regarding the necessity of the prism's alignment with the slit.

Contextual Notes

There are unresolved questions about the precise geometry of the biprism and the implications of its angles on the formation of virtual sources and interference patterns. Some assumptions about the behavior of rays and the definitions of prisms are not fully clarified.

Who May Find This Useful

Students and enthusiasts of optics, particularly those studying interference phenomena and the principles of light refraction in prisms, may find this discussion beneficial.

  • #31
What about my 29th post,is it correct?
 
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  • #32
Yes, those are all less than 90 degrees, therefore, the prism is "acute." The Fresnel prism is not an "acute" prism, but a prism with two acute angles. I doubt that it's necessary to be too picky about slightly sloppy geometry in the textbook.
 
  • #33
Bystander said:
a prism with two acute angles
In my post 28 ,where are these two acute angles ?
 
  • #34
Those are the two 30 minute of arc, or half degree, angles --- it's actually more like a pair of knife edges, or wedges, but technically still a triangular prism.
 
  • #35
If I take two acute angled prism forming one obtuse prism like this
upload_2015-2-7_14-23-7.png

The black line separates two acute prism,I don't think it is correct because two prism appears to be right angled prism.
 
  • #36
gracy said:
I don't think it is correct
You've got it correctly --- the text says "two acute angled prisms joined at the bases," and the "bases" count as your black line. It's a confusing explanation of the shape of the prism, and totally irrelevant to the actual discussion of what the prism does. What it's doing in the textbook is a mystery.
 
  • #37
Bystander said:
The edge that has to be parallel to the slit/source is the edge between the two surfaces that are 179 deg. from each other.
Is this the edge you are talking about.Note that line is going into the page.
 
  • #38
gracy said:
Is this the edge you are talking about.Note that line is going into the page.
 

Attachments

  • upload_2015-2-7_15-5-52.png
    upload_2015-2-7_15-5-52.png
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  • #39
Yes, Ma'am.
 
  • #40
Bystander said:
. Actually, there are no refracting edges -
Where does refracting edges of both prism(which combine to form biprism) go?
 
  • #41
Looking at the last attachment, it looks like the cross-section of a roof, yes. The refracting faces of the prism are the two sloped surfaces of the "roof."
 
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  • #42
gracy said:
This
Fresnel's biprism experiment is for interference not diffraction ,right?
Try to avoid classifying in that way. It's all diffraction. Interference is just a simplification in which you can identify isolated sources rather than continuous ones. The calculations are just easier for Interference situations.
BTW, there is confusion with the term "diffraction grating" which is usually explained in terms of interference (Summation) rather than diffraction (Integration). You can't win, if you get hung up on classification. :)
 
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