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Critical Angle Lab

  1. May 20, 2012 #1
    I am trying to design a refraction lab involving the critical angle and I have an idea but i'm not sure if it is good or, for lack of a better word, stupid. Here it is:
    Using a laser and a rectangular container filled with water, I will shine the laser at the container and rotate the angle at which I shine it through. Then I will record the angle at which the laser no longer goes all the way through but reflects, or the critical angle. I will do this for 5 different substances with different indexes of refraction.
    Looking at the equation ni sin(i) = nr sin(r), I can plot ni vs sin(r), where r is the measured critical angles, because ni is air which is about 1. Then, the slope of this graph will be equal to sin(i). The slope is equal to 1 making the angle i 90 and proving the equation for the critical angle.

    I'm pretty sure the math is correct but overall, is this a nice lab proving a know equation or is stated in the definition of the critical angle that that angle is 90 and therefore this is a, for lack of a better word, stupid lab.

    Thanks for the help!
    Last edited: May 20, 2012
  2. jcsd
  3. May 20, 2012 #2


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    There is a major problem with your setup. I'll give you two clues. First, critical angle is only observed going from higher optical density to lower. Second, if the walls of container are parallel, the light will exit the container under the same angle the light entered the container.

    You need to modify your setup if you wish to observe critical angle.
  4. May 21, 2012 #3
    If you want the refractive index of liquid with respect to air you should think about shining the light through the liquid to emerge from the liquid/air surface.
    This means shining the laser through the bottom of the container.
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