The critical angle is the angle of incident in the optically dense media that produces the angle of refraction as 90°.
sin c =1/n, where n is the refractive index and c is the critical angle.
Also, n1sinФ1=n2sinФ2
In this case we have,
1.33×sinc = 1.309×sin90° Where c is the critical angle.
sin c =1.309/1.33
sin c = 0.981995498
c = 79.11°