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Snell's law

  1. Nov 5, 2006 #1
    I have just conducted the experimental stage of a practical 'to find the refractive index of different objects using Snell’s law' (which states sine i/sine r =N). The objects chosen were Ice, Glass, and Perspex. As 3 trials were taken for all 3 objects, if these points were graphed, and a line of best fit was applied, I believe the gradient of this line would be the refractive index? Is this correct? Also does the I and R angles of the light leaving the prism hold any significance?

    attached are my results.


    I1 R1 N1 I2 R2 N2
    40 25 -5.63 25 38.5 -0.18
    10 8.5 -0.68 8.5 11.5 -0.91
    29 20.5 -0.67 20.5 30 -1.01

    I1 R1 N1 I2 R2 N2
    22.5 13.5 -0.61 12.5 22.5 0.14
    30.5 30 0.80 20 32.5 1.03
    20 14 0.92 14 25 -7.48

    I1 R1 N1 I2 R2 N2
    22 31 0.02 30 34 -1.87
    23 19 -5.65 19 24 -0.17
    2 1.5 0.91 1.5 2 1.10

    Note - The 1 results were those angles entering the prism, while the 2 results were those angles of the light leaving the prism.


    Attached Files:

  2. jcsd
  3. Nov 5, 2006 #2


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    Homework Helper

    You can obtain the index of refraction of the material of the prisma from the gradient of a graph of your data.

    To do this relabel your data: [tex]I_1,\ R_2\ =\ \theta _a[/tex] and [tex]R_1,\ I_2\ =\ \theta _p[/tex], where the a refers to angles measured in air and the p refers to angles measured inside of the prisma.

    The gradient of a graph of the sine of the angles in air against the sine of the angles in the prisma will then give the refractive index of the material. This follows from Snell's law.
    Last edited: Nov 5, 2006
  4. Nov 5, 2006 #3

    Thanks heaps for replying, when you tell me to relabel my data as [tex]I_1,\ R_2\ =\ \theta _a[/tex], what does the "," represent? is this some sort of function between the two?

  5. Nov 5, 2006 #4
    could anyone please help me with my last question? How should it be relabelled?
  6. Nov 5, 2006 #5
    no its just like that there is not funtction in that
  7. Nov 5, 2006 #6
    so could you please explain what hes saying?
  8. Nov 5, 2006 #7
    how ive been graphing so far

    this is what ive done, is this correct? so if i take the gradient this shall be the refactive index?

    Attached Files:

  9. Nov 5, 2006 #8
    help please?

    hi i was wondering if you could please help me with the graphing of snell's law, as ive been told you can graph somthing over somthing and the gradient of this line is the refactive index, are you able to assist as im quite confused as the current results i am getting are quite wrong!

    cheers matt
  10. Nov 5, 2006 #9


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    Homework Helper

    Sorry my explanations are clear only to me. Lets try again.

    Instead of labelling your data values as [tex]I_1\ and\ R_2[/tex] label them as [tex]\theta _a[/tex], that is these angles are the angles measured in air.

    Change your labelling for the [tex]R_1\ and\ I_2[/tex] angles to [tex]\theta _p[/tex], that is these are the angles measured inside the material of the prisma.

    Then plot a graph of [tex]\sin(\theta _a)[/tex] as a function of [tex]\sin(\theta _p)[/tex].

    The gradient of such graph will be the index of refraction of the prisma material. Hope this makes it clearer.
    Last edited: Nov 5, 2006
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