# Refraction and Lenses - Mission RL6 Detailed Help

 Indices of refraction for various materials are shown in the table below. Use these values to rank the critical angles of the following boundaries in descending order. List the boundary with the greatest critical angle first; the smallest critical angle last.
 The critical angle is the angle of incidence in the more dense medium at which the angle of refraction is 90 degrees. The critical angle (Θcrit) can be calculated using the Snell's law equation. The equation simplifies to:   Θcrit = sin-1 (n2/n1) where n2 is the index of refraction of the least dense medium (into which light is heading) and n1 is the index of refraction of the most dense medium (through which light is traveling).
 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.
 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.