# Fresnel's biprism

In Fresnel's biprism experiment what is meant by virtual sources?

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?

Quantum Defect
Homework Helper
Gold Member
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.

Can you please point out R1,R2,R3,R4 in my above picture.

Quantum Defect
Homework Helper
Gold Member
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.

gracy

Have I labeled it correctly?

R1 and R2, can be traced backwards to form S1, while R3 and R4 can be traced backwards to form S2.
How?

Have I labeled it correctly?

Quantum Defect
Homework Helper
Gold Member
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).

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).

Is it right?

#### Attachments

30.1 KB · Views: 655
Quantum Defect
Homework Helper
Gold Member
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.

That s it.
I didn't understand.Is any typo here?

So,S1 is virtual source of R1 and R2. And S2 is virtual source of R3 and R4 ,right?

Quantum Defect
Homework Helper
Gold Member
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.

Quantum Defect
Homework Helper
Gold Member
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.

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.

This
Fresnel's biprism experiment is for interference not diffraction ,right?

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?

Quantum Defect
Homework Helper
Gold Member
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.

Why should refracting edge of biprism be exactly parallel to slit to see interference pattern consisting of alternate bright and dark bands?

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?

Bystander
Homework Helper
Gold Member
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.

If the edge of the prism is not parallel to the slit/source, the wave fronts won't be parallel.
Why?

Bystander
Homework Helper
Gold Member
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.

gracy
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.

Here is what written in my textboook about biprism

I am not getting .What is acute angled prism?

Last edited:
.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.

Bystander