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Refraction of Light question assistance needed

  1. May 19, 2015 #1
    As revision for my upcoming physics exam, I'm doing an old exam paper from a previous year of the course, for which the answers aren't published. However, I'm stuck on a basic angle of refraction question:
    "A light ray makes an angle of 35degrees to the surface of an oil layer. [A diagram is attached, which shows the light ray travelling through air until it hits the oil surface]. The air's n=1.00, and the oil''s n=1.47. What is the angle of refraction of the light ray in the oil?"

    It's a multichoice answer, and the answers are A) 35degrees, B) 56.1degrees, C) 55.9degrees, D) 33.9degrees, and E) 67.7.
    However when I use the n1*sin(theta1) = n2*sin(theta2) method, my answer is always equal to 22.97degrees? Which is equal to none of the multichoice answer values? And because there are no published answers, I can't check the correct answer.
    Have I done something wrong? Or is there a mistake in the exam paper? Thank you for any and all help!
    [P.S. my calculator is definitely set in degrees.]

    Relevant equations:
    n1*sin(theta1) = n2*sin(theta2)

    The attempt at a solution:
    1 * sin(35) = 1.47 * sin(theta2), therefore sin-1 (sin(35)/1.47) = theta2 = 22.97degrees.
  2. jcsd
  3. May 19, 2015 #2


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    Homework Helper

    Try 90o - 35o as the incident angle.
    The incident angle for Snell's law is measured with respect to the normal to the surface.
  4. May 21, 2015 #3
    Thank you very much! A dumb mistake on my part; you've saved me from embarrassing myself in front of my lecturer when I went to ask him for help, and the extra info at the bottom was also very helpful, thank you again!
  5. May 21, 2015 #4


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    Homework Helper

    Don't be afraid to make mistakes or ask questions (and hopefully do not break anything in the process). That is (mostly) how we learn.
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