The critical angle is defined as the incident angle producing an angle of refraction of:

1. The critical angle is defined by what the refracted ray does. 2. As the incident angle grows, the refracted ray bends further from the normal. 3. The critical angle is defined as the incident angle that produces a particular angle of refraction. 4. At that point the refracted ray grazes along the surface. _Source: OpenStax College Physics for AP Courses 2e (CC BY 4.0), Ch 25 'Geometric Optics', sections 25.1-25.7_