what is the general trend of the critical angle as the refractive index increases?

Answer 1

http://www.physicsclassroom.com/class/refrn/Lesson-3/The-Critical-Angle
nr must be smaller than ni because there is no such thing as a sine with a value greater than one. As nr increases closer to n incident the critical angle gets closer to 90 degrees (sin 90 = 1.00) In other words as nr increases to close to ni, the incoming ray can be just about parallel to the interface and still pass through, just about parallel to the interface on the other side.