Michael Phelps has more going for him than immense talent. In Quest for Speed, Olympic Swimmers Use Physics over at Physics Buzz. There’s a brief discussion of the fluid mechanics topics involved in improving swim times, from analysis of swimmers’ motions to the pool design that reduces turbulent water to the new swimsuits.

But what goes unmentioned is the underwater photo, showing the phenomenon known as Snell’s Window. Snell’s law tells us that light moving from one index to another will refract; light entering the pool on its way to the camera must bend toward the normal, meaning that the light entering the lens is compressed from a hemisphere into a cone, and the index of water (1.33) dictating the cone’s apex of 97.5º. Outside of that angle we have total internal reflection from the pool’s bottom; this light cannot escape the pool as it can’t refract and enter the air.

Here’s another picture showing Snell’s Window.