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 = n_air / n_water ,

where n_air and n_water are the indexes of refraction of the air and water, respectively.