At sufficiently large angle of incidence, a ray in a medium with higher refractive index is totally internally reflected at the interface with a medium of lower refractive index. The film clip at right shows an experiment in which the angle of incidence in glass was successively increased. Initially, we see both refraction into air and reflection in the glass. As the angle of incidence increases, the refacted beam becomes weaker and, at the critical angle, it disappears. At larger angles, there is only reflection: the internal reflection is total.
Swimmers notice that, from below, the air water interface looks 'silver', meaning that it reflects. For a swimmer, the line of sight is usually not very far from horizontal, so the angle of incidence is large: well above the critical angle for water, which is 49°. So the swimmer sees reflections of other underwater objects, and very little light that is diffracted directly from the surface.
Ray and wavefront animation
Animation of wavetronts in reflection
In this animation, incident and reflected rays are shown in grey. The incident wavefronts are the red lines with positive slope, those with negative slope are the reflected wavefronts.