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Prism : what if incident angle is greater than apex angle?

  1. Oct 6, 2014 #1
    • Warning: Problem statement should include a description of the problem; Solution attempt must be included.
    1. The problem statement, all variables and given/known data
    incident angle : 60 degree
    apex angle : 30 degree
    prism refraction index : 1,6
    deviation angle : ?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 6, 2014 #2


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    You mean the angle of the prism? Why do you think it makes a problem if the angle of incidence is greater than that? What is the question?

  4. Oct 6, 2014 #3
    if i solve this equation to find first refraction angle (r1) :
    sin(I1) = np sin (r1), which (I1) is incident angle =60 degree, and np is prism refraction index = 1,6
    i find r1 = 32,76 degree.
    if beta = r1 + I2 , which beta is apex angle = 30 degree, and I2 is second incident angle,
    i find I2 = -2,76 degree
    i cant quite understand whats the meaning of (-) in my answer :/
  5. Oct 6, 2014 #4


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    It means that the second angle of incidence falls to the opposite side of the normal. See picture. You still can use ## |I_2|=|\beta-r_1|##. The second refracted ray will travel upward, in the direction of the apex.
    I suggest to draw the rays always before applying a formula. prismneg.JPG

  6. Oct 8, 2014 #5
    so i can still use
    Deviation angle = i1+r2-beta to find second refraction angle? (r2)
    thank you so much that enlightens me so much about prism.
    many thanks man
  7. Oct 8, 2014 #6


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    You find r2 by applying Snell's Law for I2. And I1 and r2 and beta determine the deviation angle.

  8. Oct 8, 2014 #7
    thank you so much.
    this really helpful
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