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Homework Help: Prism - emergent angle anomaly

  1. Jun 19, 2009 #1
    1. The problem statement, all variables and given/known data

    A ray of monochromatic yellow light is incident in air on an
    equilateral triangular glass prism. This ray is in the same plane as
    the equilateral triangular cross section of the prism; the angle
    between the ray and the prism face is 60°, and the refractive index
    of the glass is 1.54 for this light.

    Find the angle the emergent ray makes with the prism face at
    which it emerges. Illustrate your answer with a diagram.

    The given answer is 60 degrees. The problem appears straightforward but it appears impossible to get the required result with the given data. Please check my solution and comment...

    2. Relevant equations
    Snells law: n1*sin theta1 = n2 sin theta 2

    3. The attempt at a solution

    For angle of refraction of the incident light - from air into the prism:

    sin r = sin 30/1.54, r = 18.94 degrees

    The angle of incidence upon striking the internal glass surface = 60-18.94

    Therefore the angle of refraction - from glass back into the air:

    = asin(1.54 * sin(60-18.94))

    The difficulty is that the inner sin function gives a value > 1

    Have I done something wrong?
  2. jcsd
  3. Jun 19, 2009 #2


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    Homework Helper

    60-18.94 = 41.06 is greater then the critical angle. Now draw the ray diagram and find the angle of incidence of the total internal reflected ray on the prism face.
  4. Jun 19, 2009 #3
    Quite so... many thanks for your help!!!
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