The critical angle is what is known as Snells-Descartes law, which describes the angle of incidence light takes as it passes through the boundaries of different media. The best example of this is seed when you take a glass half full of water and place a pencil in it. The pencil will appear broken as it enters the water, slightly below what the upper half of the pencil shows, and slightly enlarged. The reason for this optical illusion has to do with the critical angle light takes as it passes from air, which represents one medium, and passes through water, which represents a second medium. With respect to density, the speed of light slows down as it encounters media that are progressively more dense. Visible light from the sun travels the 8.3 light-minutes it takes to get to Earths atmosphere, then travels through Earths atmosphere. When it encounters the oceans, it only penetrates the first 100 meters or so, before it starts dissipating.