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Snell's Law of Refraction:
The refraction of light towards or away from the normal follows a very predictable mathematical relationship known as Snell's law.
n1 • sin Θ1 = n2 • sin Θ2
where n1 and n2 are the indices of refraction of the two individual media and Θ1 and Θ2 are the angles of incidence and refraction within those media. Knowing any three of the four quantities in the equation allows one to predict the fourth variable.
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The most sure approach to this problem involves performing calculations of the critical angle for each of the four boundaries. Get organized and record the results in a legible manner. Then rank the four boundaries from the largest value to the smallest value.
The only alternative involves doing the same as above, but simply finding the ratio of indices as opposed to the actual critical angle. The ranking in terms of critical angle will follow the same order as ranking the boundaries in terms of the n2/n1 ratio.
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On most calculators, the sin-1 function can be used by pushing two buttons. If you've never used it, then look above the button labeled sin (for sine). You will likely see the sin-1label. Usually, the angle can be determined by pressing the 2nd-Sin button in consecutive fashion. On simpler, 1-line calculators, it is usually necessary that you first determine the ratio of n2/n1 and then click the sin-1 label. On more complex, multiline calculators, the 2nd-Sin buttons are first pressed and then the n2/n1 ratio is entered.
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