Refraction is the bending of light as it passes from a medium of one optical density into a medium of a different optical density, as from air to water or water into air. The amount of bending is dependent upon the incident angle of the light. In the diagram below, a light ray, “A” strikes the water at right angles and passes through the surface without bending. But as the incident angle decreases (becomes less than 90 degrees) the light bends more and more–rays “B” and “C.” Light striking the surface parallel to the surface, bends downward 41 1/2 degrees (dotted lines). NOTE: light passing from the water into air will bend exactly the opposite of light passing from air into water.
Since light is coming into the water from all directions, refraction creates a cone of light with its base on the surface and its apex at the fish’s eye. The apical angle of the cone is 97 degrees. The base of the cone is a circular opening at the surface through which the fish sees the entire outside world. This opening is called the “Fish’s Window,” and was first described in 1621 by Willebrord Snell van Royen. Only the light passing through the window enters the fish’s eye. Notice line “D,” It’s a ray entering the water beyond the window; refraction bends it such that it cannot reach the fish’s eye. There are a number of somewhat startling ramifications to the window relative to the fish’s vision, and to the angler’s perspective of the fish. We will take these up in future posts. Watch for them.