Refraction is generally accompanied by "partial" reflection. When waves are refracted from a medium of lower propagation speed to a medium of higher propagation speed (e.g., from water to air), the "angle of refraction" (between the refracted ray and the line perpendicular to the refracting surface) is greater than the "angle of incidence" (between the incident ray and the perpendicular). As the angle of incidence approaches a certain limit, called the "critical angle", the angle of refraction approaches 90°, at which the refracted ray becomes parallel to the surface. As the angle of incidence increases beyond the critical angle, the conditions of refraction can no longer be satisfied; so there is no refracted ray, and the partial reflection becomes total. For visible light, the critical angle is about 49° for incidence at the water-to-air boundary, and about 42° for incidence at the common glass-to-air boundary.