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Homework Help: Index of Refraction

  1. Mar 28, 2010 #1
    1. The problem statement, all variables and given/known data
    While studying physics at the library late one night, you notice the image of the desk lamp reflected from the varnished tabletop. When you turn your Polaroid sunglasses sideways, the reflected image disappears.

    If this occurs when the angle between the incident and reflected rays is 110 degrees, what is the index of refraction of the varnish?

    2. Relevant equations

    tan theta = n2/n1

    3. The attempt at a solution
    I used n2 = air (1) and solved for n1 but was incorrect. Where am I going wrong?
  2. jcsd
  3. Mar 28, 2010 #2


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    Well because it is reflected, it sounds like you have a case of total internal reflection. You'll need to use sinC = 1/n.
  4. Mar 28, 2010 #3
    Ah so close. You are correct in using the Brewster's angle formula. However notice the trick in the problem. They tell you the angle between the rays NOT the angle the rays make with normal.
  5. Mar 29, 2010 #4
    so would i do 180-110 or something like that? the angle thing is messing me up
  6. Mar 29, 2010 #5


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    During reflection angle of incidence is equal to angle of reflection. In the problem, the sum of the angle of incidence and angle of reflection is given. from that find the angle of incidence which is the polarizing angle.
  7. Mar 29, 2010 #6
    Yes what rl.bhat said is correct. If you are still having trouble thinking about it, try drawing a diagram and setting the reflected and incident angles to the normal equal. Also set the angle between the two rays equal to 110.
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