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Light refraction problem

  1. Jul 14, 2007 #1
    1. The problem statement, all variables and given/known data

    A light ray enters a rectangular block of plastic at an angle 1 = 44.0° and emerges at an angle 2 = 79.0°, as shown in Figure P22.61.
    [​IMG]

    Figure P22.61
    (a) Determine the index of refraction of the plastic.
    wrong check mark
    (b) If the light ray enters the plastic at a point L = 50.0 cm from the bottom edge, how long does it take the light ray to travel through the plastic?
    ns


    2. Relevant equations
    n1sin(1)=n2sin(2) or sin(critical)=n2/n1

    3. The attempt at a solution
    i attempted arcsin(11/44) and arcsine(79/44) but both were wrong. Can someone please help me get on the right path?
     
  2. jcsd
  3. Jul 14, 2007 #2

    Doc Al

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    Staff: Mentor

    Treat this as two refractions and apply Snell's law at each interface. Hint: How does the second angle (the angle of refraction) at the first interface relate to the first angle (angle of incidence) at the second interface?
     
  4. Jul 14, 2007 #3
    I have sin(44)=nsin(theta2) and sin(79)=nsin(theta2) but that cannot be correct. I know that the angle of incidence and angle of refraction are equal. Please help Im mising something here.
    Thanks
     
  5. Jul 15, 2007 #4

    Doc Al

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    Staff: Mentor

    True, that cannot be correct, since [itex]\sin 44 \neq \sin 79[/itex].
    They are related, but not equal.

    For clarity, I'll rewrite the equations as:
    [tex]\sin 44 = n \sin \theta_a[/tex]
    [tex]\sin 79 = n \sin \theta_b[/tex]

    Find the simple trignometric relationship between [itex]\theta_a[/itex] & [itex]\theta_b[/itex] by examining the diagram. Look for triangles.
     
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