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Deviation and dispersion in a prism

  1. Sep 15, 2011 #1
    [PLAIN]http://img718.imageshack.us/img718/1433/unledkzg.jpg [Broken]
    Could you explain the reason for this?This would help me so much
    Thanks in advance
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 15, 2011 #2


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    Staff: Mentor

    The reason for what, specifically?
  4. Sep 16, 2011 #3
    it seems that no body can see the picture
    Sorry I will reupload it again
  5. Sep 16, 2011 #4

    [PLAIN]http://img718.imageshack.us/img718/1433/unledkzg.jpg [Broken]
    Last edited by a moderator: May 5, 2017
  6. Sep 16, 2011 #5


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    Staff Emeritus
    Science Advisor

    No, it was just me at work. I can see the picture fine here at home. I don't think we understand what you are asking. You're going to have to type out a question, otherwise we have no idea what you are confused with.
  7. Sep 16, 2011 #6
    Oh ok
    Small deviation means large dispersion and vice versa,so why?
    Is that the reason why the prism should be in the minimum deviation position to disperse light?
  8. Sep 17, 2011 #7
    Angle of deviation + Angle of prism = Angle of incidence + Angle of emergence

    Let Angle of prism be constant ie k

    Angle of deviation =[itex]\partial[/itex]
    Angle of incidence=i
    Angle of emergence=e


    [itex]\partial[/itex] = k - i+e

    Now realize that

    [itex]\partial[/itex] [itex]\propto[/itex] i

    Forget about e now because each colour has different e

    In your first image of large deviation , and small i the e will be very small tally the relation.
    In your second image of small deviation , and large i the e will be very large tally the relation.

    If e will be large so more dispersion and vice versa.

    If you want to know the proof of relation then feel free to ask.

    In case of minimum deviation i=e , so ∂ = k - 2i ,
    there is maximum dispersion also.

    Claculate dispersion like this :
    ev+ei+eb+eg+ey+eo+er where v,i,b,g,y,o,r are different colours : 7
    Last edited by a moderator: May 5, 2017
  9. Sep 17, 2011 #8
    I don't understand those strange equations you are writing Could you use simpler equations?What kind of equations are they?
    I just understand that Angle of deviation + Angle of prism = Angle of incidence + Angle of emergence
    Thanks very much!
  10. Sep 17, 2011 #9
    I think this script doesn't work on my computer
    Could you use images or words please?
  11. Sep 18, 2011 #10
    ok I can see the equations now but I have some questions:
    We studied that alpha=i+e-k not k-i+e so how do u explain that?
    how do u consider that alpha is directly proportional to i?This is not a direct relation
    I haven't known that
    I can't imagine how I can forget about e , doesn't e depend on i not on the color?
    which relation?Anyways I certainly want to know the proof
    Thanks very much for your help
  12. Sep 18, 2011 #11
    Sorry , my fault , its ∂ = -k + i+e
    I mistyped it !

    I think the following sites may crystal clear your concepts better than I can :

    The proof of relation ∂ = -k + i+e : http://www.askiitians.com/iit_jee-Ray_Optics/Prism
    The factors affecting deviation in prism :http://en.wikipedia.org/wiki/Minimum_deviation
    Relation between dispersion and deviation in a prism :http://www.physicsclassroom.com/class/refrn/u14l4a.cfm

    Minimum deviation : http://www.mtholyoke.edu/~mpeterso/classes/phys103/geomopti/MinDev.html

    Hope this helps :)
  13. Sep 18, 2011 #12
    Ok,I'll check them and complete this discussion
    Thanks very much
  14. Sep 18, 2011 #13
    Still confused Can't anyone in the world solve my problem :(
  15. Sep 18, 2011 #14
    Can A prism disperse light when it is not the in the positsion of minimum deviation?
  16. Sep 18, 2011 #15
    I don't have a formal proof or a conceptual explanation, but sometimes I understand things while working on a numerical problem. Here's what I would do in this case:

    1) Using Snell's law, I compute the angle of deviation when it is minimum (when the ray of light is horizontal inside the prism). I use a typical refractive index: 1.50 .

    2) For a different color, I repeat step 1 with a slightly different refractive index (let's say 1.45).

    3) Subtracting the values obtained in steps 1 and 2, I find the angle of dispersion.

    4) I repeat step 1, 2 and 3 for a random case where the deviation is not minimum (angle of incidence = 45 degrees, for example). As a result, I should find a smaller angle of dispersion.
  17. Sep 18, 2011 #16
    Why ? Have you gone through those sites which I gave in post #14? Your question is not refined and hence it is quite hard for me to explain it to you.

    Yes , why not? I recommend that you should go through those sites again and read them carefully one by one !

    [Post by YPelletier]
    This is what I typed in my earlier post. I am not sure what OP is searching for ? I gave answer and even gave reference sites?

    What is the question Misr ? Please type your question again so that I can be clear what answer do you want exactly ?
  18. Sep 18, 2011 #17
    You mean I should use numbers?I don't really understand what you are trying to say
    could you use an example?

    I want to prove that small deviation means large dispersion and vice versa
    I already read the physicsclassroom tutorial in your link and I even printed it few weeks ago but no use..I can't find the answer to my question
  19. Sep 19, 2011 #18
    Ok ,how toi do that?
    I tried this on another problem
    I wanted to show that the thin prism is always in the position of minimum deviation whatever the angle of incidence
    but I failed , so could you help with this idea ?
  20. Sep 20, 2011 #19
    When white light falls on a prism do all the wavelength of white light have the same angle of incidence?
  21. Sep 23, 2011 #20
    When white light falls on prism , it is dispersed after the refraction not at that flick of second. At the line of separation between two media it is after all a monochromatic beams of light. It will have same angle of incidence though.

    Prove it experimentally , your question :small deviation means large dispersion and vice versa

    Take angle of incidence = 40o

    Compute by Snell's law :

    sin 40o/sin r = 1.52

    Make use of trigonometric tables.
    Find for r.
    Now find angle of deviation using this formula :

    d=i-r or d=r-i

    Whatever comes greater.

    Now use this formula :
    e= d+A-i

    Where A is angle of prism. Make it constant ie 60o.
    Here consider e as angle of dispersion.
    Find e or angle of dispersion.

    Repeat it again and again using different angle of incidence. 50o , 60o , 70o etc.

    Note your observations.


    (My exams are going on and that is why busy.)
    Last edited: Sep 23, 2011
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