What is the critical angle of glass?

When light in glass of index of refraction n hits the surface at angle A relative to the surface normal (perpendicular line to the surface) it generally exits into the air at larger angle B. These three variables are related by Snell's Law: n Sin(A) = (1)Sin(B) (air has index of refraction approx. = 1 ) The critical angle A for internal reflection occurs when the exit angle is 90 so the exit light skims the surface. Anything larger then critical A and the light gets reflected back into the glass. So in Snell's law let B = 90; nSin(A) =(1)Sin(90) = (1)(1) so Sin(A) = 1/n For glass n ~ 1.5 so Sin(A) ~ .67 This is the sine of the critical angle. Now figure out what angle has a sign of .67 and Whala you have it.