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Homework Help: Snells law - help please

  1. Apr 29, 2014 #1
    Snells law -- help please

    A light ray approaches a glass block at 30° to the normal. The refractive
    indices of air and water are 1 and 1.5 respectively At what angle will the light
    be refracted?

    This uses Snells law

    I did Sin 30 / Sin r = 1 / 1.5
    I then multiplied both sides in order to get rid of sin 30.
    After that, I got Sin r = 1/1.5(xSin30)
    This gave me an answer of -1.48
    When I do inverse sine it gives me an error
    What did I do wrong???
  2. jcsd
  3. Apr 29, 2014 #2


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    Staff Emeritus
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    Check your algebra !
  4. Apr 29, 2014 #3
    I have
  5. Apr 29, 2014 #4
    Check it again and again, just to be sure, a minus answer should be rather unlikely in elementary examples.
  6. Apr 29, 2014 #5
    You did your fractions wrongly.

    You should have got this:

  7. Apr 29, 2014 #6
    Make sure your calculator is set for degrees instead of radians. You're getting a negative sign because you're using radians. That's what happens when you blindly follow the calculator without a clue of what to expect. You end up eating calculator garbage.
  8. Apr 30, 2014 #7


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    Gold Member

    What dauto said. and:

    Since you have done that,(Did it using radians), be sure to reset your calculator especially before exams.
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