Fresnel's biprism

  • Thread starter gracy
  • Start date
  • #1
2,486
83
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?
 

Answers and Replies

  • #2
Quantum Defect
Homework Helper
Gold Member
495
116
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
2,486
83
Can you please point out R1,R2,R3,R4 in my above picture.
 
  • #4
Quantum Defect
Homework Helper
Gold Member
495
116
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.
 
  • Like
Likes gracy
  • #5
2,486
83
upload_2015-2-7_1-2-23.png

Have I labeled it correctly?
 
  • #6
2,486
83
R1 and R2, can be traced backwards to form S1, while R3 and R4 can be traced backwards to form S2.
How?
 
  • #7
2,486
83
upload_2015-2-7_1-20-7.png

Have I labeled it correctly?
 
  • #9
Quantum Defect
Homework Helper
Gold Member
495
116
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
2,486
83
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?
 

Attachments

  • #11
Quantum Defect
Homework Helper
Gold Member
495
116
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.
 
  • #13
2,486
83
So,S1 is virtual source of R1 and R2. And S2 is virtual source of R3 and R4 ,right?
 
  • #14
Quantum Defect
Homework Helper
Gold Member
495
116
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
Quantum Defect
Homework Helper
Gold Member
495
116
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
2,486
83
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
2,486
83
This
Fresnel's biprism experiment is for interference not diffraction ,right?
 
  • #19
2,486
83
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
Quantum Defect
Homework Helper
Gold Member
495
116
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
2,486
83
Why should refracting edge of biprism be exactly parallel to slit to see interference pattern consisting of alternate bright and dark bands?
 
  • #22
2,486
83
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
2,486
83
Please answer. Tomorrow is my test.
 
  • #24
Bystander
Science Advisor
Homework Helper
Gold Member
5,203
1,225
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.
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
2,486
83
If the edge of the prism is not parallel to the slit/source, the wave fronts won't be parallel.
Why?
 

Related Threads on Fresnel's biprism

  • Last Post
Replies
1
Views
6K
Replies
2
Views
13K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
7
Views
3K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
9
Views
5K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
1
Views
1K
Replies
1
Views
1K
  • Last Post
Replies
7
Views
5K
Top