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 
= 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 total internal reflection.
