<p>3c:
At the critical angle, the refracted angle all but disappears; it is 90 degrees, so sin(theta) = 1. So, n2 = n1sin(incident angle that results in a 90 degree refracted angle). Because you know n1 (index of refraction for air) and you can measure the incident angle, you can determine n2, the index of refraction for plastic.</p>