text: how do i work out the critical angle of something? if i have something like glass (n=1.50) to air? n = sin i / sin r.  when i = 90 degree r is called critical angle.   sin 90 = 1  therefore, n = 1/ sin r  sin r = 1/n =1/1.5= 0.6666.   r = sin inverse of 0.6666 = 41 degree 48 minute 37 second.