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Fresnel's biprism

  1. Feb 6, 2015 #1
    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?
     
  2. jcsd
  3. Feb 6, 2015 #2

    Quantum Defect

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    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.
     
  4. Feb 6, 2015 #3
    Can you please point out R1,R2,R3,R4 in my above picture.
     
  5. Feb 6, 2015 #4

    Quantum Defect

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    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.
     
  6. Feb 6, 2015 #5
    upload_2015-2-7_1-2-23.png
    Have I labeled it correctly?
     
  7. Feb 6, 2015 #6
    How?
     
  8. Feb 6, 2015 #7
    upload_2015-2-7_1-20-7.png
    Have I labeled it correctly?
     
  9. Feb 6, 2015 #8

    Quantum Defect

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  10. Feb 6, 2015 #9

    Quantum Defect

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    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).
     
  11. Feb 6, 2015 #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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  12. Feb 6, 2015 #11

    Quantum Defect

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    That s it.
     
  13. Feb 6, 2015 #12
    I didn't understand.Is any typo here?
     
  14. Feb 6, 2015 #13
    So,S1 is virtual source of R1 and R2. And S2 is virtual source of R3 and R4 ,right?
     
  15. Feb 6, 2015 #14

    Quantum Defect

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    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.
     
  16. Feb 6, 2015 #15

    Quantum Defect

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    Exactly right! You understand the whole thing.
     
  17. Feb 6, 2015 #16
    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.
     
  18. Feb 6, 2015 #17
    This
    Fresnel's biprism experiment is for interference not diffraction ,right?
     
  19. Feb 6, 2015 #18

    Quantum Defect

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  20. Feb 6, 2015 #19
    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
     
  21. Feb 6, 2015 #20

    Quantum Defect

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    The dashed bit is showing the region where you will see interference. i.e. the image plane at R.
     
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