In air, light is refracted as it passes through the
cornea at the front of the eye, and then again as it
travels through the lens. The lens is mainly there to
provide fine adjustment because the cornea is fixed;
in fact, two thirds of the focusing power of the eye is
supplied by the cornea. But underwater, the
refractive index of the cornea is virtually the same
as the water in front of it, and it loses almost all of
its focusing power. The lens has a higher refractive
index, but it cannot compensate enough, so you
become extremely long-sighted. ----