Fresnel's Biprism Experiment: What Are Virtual Sources?

[gracy]

What about my 29th post,is it correct?

 

[Bystander]

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.

 

[gracy]

a prism with two acute angles

In my post 28 ,where are these two acute angles ?

 

[Bystander]

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.

 

[gracy]

If I take two acute angled prism forming one obtuse prism like this

The black line separates two acute prism,I don't think it is correct because two prism appears to be right angled prism.

 

[Bystander]

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.

 

[gracy]

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.

 

[gracy]

Is this the edge you are talking about.Note that line is going into the page.

 

[Bystander]

Yes, Ma'am.

 

[gracy]

. Actually, there are no refracting edges -

Where does refracting edges of both prism(which combine to form biprism) go?

 

[Bystander]

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."

 

[sophiecentaur]

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. :)