Question

What is the critical angle of glass-air interface?

Answer 1

critical angle is given by 
n=1/sinC
sinC= 1/n                      n=4/3
sin C= 3/4
C = sin inverse(3/4)
C= 48.5 degresss

Answer 2

Critical angle  is possible when light rays pass from  glass into air.  Hence according to the law of refraction we have:

$\mu_{glass}\ Sin\ i = \mu_{air}\ Sin\ r\\\\r= \pi /2,\mu_{air}=1\ \ ,\ i_{C}=Critical\ angle\ in\ glass.\\\\ i_{C} =Sin^{-1}\ (1/\mu_{glass})$

$\mu_{glass}=1.3\ to\ 1.6\ \ in\ general.\\ \\For\ \mu_{glass}=1.3,\ \ i_C=sin^{-1}(1/1.3)=50.3\deg\\\\For\ \mu_{glass}=1.6,\ \ i_C=sin^{-1}(1/1.6)=38.7\deg$