Note:  Do not rely on this information. It is very old.

Critical Angle

Critical Angle, in Optics, is the greatest angle to the perpendicular, at which rays of light can be refracted from one medium to another: Critical angles only exist when light passes from one medium to another in which its speed is greater, as, for example, when it passes from water to air. The critical angle in this case is about 48-1/2°; for crown glass to air it is 40-1/2°.