There is a source of light at the bottom of the lake. The rays from the source reach
the surface of the water and continue through the air at a refracted angle, some
rays are also reflected back into the water. As you increase the incident angle,
more and more rays are reflected into the water. There comes a time when all of
the rays are reflected, this happens when the angle of incidence is equal to or
greater than the critical angle, which is defined by the ratio of the two
indexes of refraction:
sin θc = nair/nwater
where nair and nwater are the indexes of refraction
of the air and water, respectively.
Thus, increasing the angle of incidence above the critical angle will result in
the light being totally internally reflected.
