text: n = sin i / sin r.\n\nwhen i = 90 degree r is called critical angle. \n\nsin 90 = 1\n\nTherefore, n = 1/ sin r\n\nsin r = 1/n =1/1.5= 0.6666.\n\n r = sin inverse of 0.6666 = 41 degree 48 minute 37 second.