Fresnel's Biprism Experiment: What Are Virtual Sources?

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In summary, The Fresnel biprism experiment involves using a prism to create virtual sources of light, from which the refracted rays appear to come from. These virtual sources, S1 and S2, are used to create an interference pattern on the image plane at R. In order for this interference pattern to be visible, the refracting edge of the biprism must be parallel to the slit/source. If not, the wave fronts will not be parallel and the interference pattern will not be visible. This experiment is for interference, not diffraction, and the place where interference occurs is in the region marked by the dashed lines on the image.
  • #1
gracy
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In Fresnel's biprism experiment what is meant by virtual sources?
upload_2015-2-6_22-59-53.png

In this image s1 and s2 are called virtual sources.In my textbook it is written that refracted rays appears to com e from these virtual source.What is virtual source?
 
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  • #2
gracy said:
In Fresnel's biprism experiment what is meant by virtual sources?
View attachment 78697
In this image s1 and s2 are called virtual sources.In my textbook it is written that refracted rays appears to com e from these virtual source.What is virtual source?

If you follow the rays from the actual point source, S, you will see a virtual source S1 formed by rays entering the upper part of the prism from the actual source S, and a virtual source S2 formed by rays entering the bottom part of the prism.

Labeling the rays R1 through R4 (top to bottom) coming from S, R1 is bent to become parallel with the optical axis, and is the upper ray reaching the screen. R2 refracts downwards to the bottom of the screen. R3 refracts to the top of the screen, and R4 refracts to be parallel with the optical axis, at the very bottom of the screen. R1 and R2, can be traced backwards to form S1, while R3 and R4 can be traced backwards to form S2.
 
  • #3
Can you please point out R1,R2,R3,R4 in my above picture.
 
  • #4
gracy said:
Can you please point out R1,R2,R3,R4 in my above picture.

I don't have good software for this. I'll attempt to use words:

Four rays are emerging from S, moving left to right. The top one is R1, it grazes the top of the prism. R2 is directly below R1. The centerline (optical axis) is below R2, and bisects the prism. Below the centerline is R3, and at the very bottom is R4, which grazes the bottom of the prism. After refraction, R1 and R4 travel parallel to the centerline, R2 travels downwards and R3 travels upwards.
 
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  • #5
upload_2015-2-7_1-2-23.png

Have I labeled it correctly?
 
  • #6
Quantum Defect said:
R1 and R2, can be traced backwards to form S1, while R3 and R4 can be traced backwards to form S2.
How?
 
  • #7
upload_2015-2-7_1-20-7.png

Have I labeled it correctly?
 
  • #9
gracy said:
How?

Look at the rays on the right side of the prism. If you trace R1 back -- i.e. use a ruler to draw a straight line from right to left, it goes straight back to S1 (top line in S1). If you trace R2 back, it goes to S1 (bottom line to S1).
 
  • #10
Look at the rays on the right side of the prism. If you trace R1 back -- i.e. use a ruler to draw a straight line from right to left, it goes straight back to S1 (top line in S1). If you trace R2 back, it goes to S1 (bottom line to S1).
upload_2015-2-7_1-40-4.png

Is it right?
 

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  • #11
gracy said:
Look at the rays on the right side of the prism. If you trace R1 back -- i.e. use a ruler to draw a straight line from right to left, it goes straight back to S1 (top line in S1). If you trace R2 back, it goes to S1 (bottom line to S1).
View attachment 78706
Is it right?

That s it.
 
  • #12
Quantum Defect said:
That s it.
I didn't understand.Is any typo here?
 
  • #13
So,S1 is virtual source of R1 and R2. And S2 is virtual source of R3 and R4 ,right?
 
  • #14
gracy said:
I didn't understand.Is any typo here?

You have drawn in the lines correctly. R1 and R2 trace back to S1, and R3 and R4 trace back to S2.

I think that you understand.
 
  • #15
gracy said:
So,S1 is virtual source of R1 and R2. And S2 is virtual source of R3 and R4 ,right?

Exactly right! You understand the whole thing.
 
  • #16
Quantum Defect said:
Exactly right! You understand the whole thing.
All because of you.I will need further help of yours in this particular thread.I am posting my queries .I hope you are there.
 
  • #17
This
Fresnel's biprism experiment is for interference not diffraction ,right?
 
  • #19
gracy said:
Fresnel's biprism experiment is for interference not diffraction ,right?
If yes then at what point interference occurs?
I have pointed out in the image below ,is this a place where interference occurs?
upload_2015-2-7_2-17-38.png
 
  • #20
gracy said:
If yes then at what point interference occurs?
I have pointed out in the image below ,is this a place where interference occurs?
View attachment 78707

The dashed bit is showing the region where you will see interference. i.e. the image plane at R.
 
  • #21
Why should refracting edge of biprism be exactly parallel to slit to see interference pattern consisting of alternate bright and dark bands?
 
  • #22
Quantum Defect said:
The dashed bit is showing the region where you will see interference. i.e. the image plane at R.
This is the place where interference pattern can be seen,and the point which i have shown in the image is where interference will occur i.e two waves will overlap.Right?
 
  • #23
Please answer. Tomorrow is my test.
 
  • #24
gracy said:
be exactly parallel to slit to see interference pattern
The wave fronts that intersect to form the interference pattern need to be parallel to generate a visible pattern. If the edge of the prism is not parallel to the slit/source, the wave fronts won't be parallel.
gracy said:
i.e two waves will overlap.Right?
I think so. It's been a long time since I've looked at this type of problem.
 
  • #25
Bystander said:
If the edge of the prism is not parallel to the slit/source, the wave fronts won't be parallel.
Why?
 
  • #26
Referring to the drawing in the first post, we're looking at a cross-section of the prism and the slit: if the prism edge is not parallel to the slit, as we move "into or out of" the page looking at other cross-sections, the slit/source, "S," will be moving up or down (in the drawing). This will result in the two "virtual" sources moving in opposite directions, and twisting the two "virtual" slit images non-parallel from each other.
 
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  • #27
Bystander said:
if the prism edge is not parallel to the slit, as we move "into or out of" the page looking at other cross-sections, the slit/source, "S," will be moving up or down (in the drawing)
Which refracting edge?That's where I am getting confused. Biprism is made up of two prism,so there will be 2 refracting edges.
 
  • #28
Here is what written in my textboook about biprism
interference-17-1024.jpg

I am not getting .What is acute angled prism?
 
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  • #29
gracy said:
.What is acute angled prism?
Prism with all angles acute?Is the prism below acute angled prism as angle BAC,angle ABC and angle BCA all are acute angles.
upload_2015-2-7_13-31-48.png
 
  • #30
gracy said:
Which refracting edge?
Excellent question. Actually, there are no refracting edges --- the refractions occur at the surfaces between air and glass, the surfaces of the prism. 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.
 
  • #31
What about my 29th post,is it correct?
 
  • #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.
 
<h2>1. What is Fresnel's Biprism Experiment?</h2><p>Fresnel's Biprism Experiment is a classic optical experiment that demonstrates the phenomenon of interference of light waves. It was first performed by French physicist Augustin-Jean Fresnel in the early 19th century.</p><h2>2. How does the experiment work?</h2><p>In the experiment, a thin, transparent prism is placed in front of a light source, creating two virtual sources of light. These virtual sources interfere with each other, creating a pattern of bright and dark fringes on a screen placed behind the prism.</p><h2>3. What are virtual sources?</h2><p>Virtual sources are not actual light sources, but rather the result of the diffraction of light passing through the prism. They appear as if they are coming from two separate points, creating an interference pattern when they overlap.</p><h2>4. What is the purpose of the experiment?</h2><p>The purpose of Fresnel's Biprism Experiment is to demonstrate the wave nature of light and the phenomenon of interference. It also helped to support the wave theory of light proposed by Fresnel, which was in contrast to the particle theory of light proposed by Newton.</p><h2>5. What are the applications of this experiment?</h2><p>Fresnel's Biprism Experiment has been used in various applications, including studying the properties of light, calibrating optical instruments, and determining the wavelength of light. It has also been used to explore the wave-particle duality of light and to support the development of modern optics and diffraction theories.</p>

1. What is Fresnel's Biprism Experiment?

Fresnel's Biprism Experiment is a classic optical experiment that demonstrates the phenomenon of interference of light waves. It was first performed by French physicist Augustin-Jean Fresnel in the early 19th century.

2. How does the experiment work?

In the experiment, a thin, transparent prism is placed in front of a light source, creating two virtual sources of light. These virtual sources interfere with each other, creating a pattern of bright and dark fringes on a screen placed behind the prism.

3. What are virtual sources?

Virtual sources are not actual light sources, but rather the result of the diffraction of light passing through the prism. They appear as if they are coming from two separate points, creating an interference pattern when they overlap.

4. What is the purpose of the experiment?

The purpose of Fresnel's Biprism Experiment is to demonstrate the wave nature of light and the phenomenon of interference. It also helped to support the wave theory of light proposed by Fresnel, which was in contrast to the particle theory of light proposed by Newton.

5. What are the applications of this experiment?

Fresnel's Biprism Experiment has been used in various applications, including studying the properties of light, calibrating optical instruments, and determining the wavelength of light. It has also been used to explore the wave-particle duality of light and to support the development of modern optics and diffraction theories.

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