Fish Eye View


i have a question. What will a fish see when it looks up?


Neglecting the difference between fish eyes and people eyes, all any animal sees is the light that enters the eyes. When a fish looks up the sources of the light entering its eye are objects above the surface that are reflecting light from the Sun or the water surface reflecting back light which has illuminated the bottom and been reflected upward. On its way from an object above the surface to the eye of a submerged fish, light may be refracted(bent) at the water surface, reflected from the water surface, or absorbed by the water or the impurities suspended in it.

In looking through the surface at an object in the air, if the fish is not too deep, light from the object will not be too much absorbed and may penetrate to the depth of the fish. Whether that light actually reaches the fish depends on the angle from the vertical that the fish must look to see the object. The index of refraction of air is about 1.00 and for water it is about 1.33. This causes the refraction of light rays entering the water such that the fish sees objects farther from the vertical than they actually are.

At a particular angle called the critical angle, elevated objects appear to be all the way down at the water surface. Beyond this angle the surfaceappears to the fish like a mirror, reflecting light from objects on thebottom of the pool. The critical angle is the angle whose sine is 1.00/1.33 or about 49 degrees. At angles less than this the fish can see objects above the surface but it will judge any object other than one directly overhead to be closer to the surface than it actually is. Of course any movement of the surface will change the angle between the rays of light and the surface, distorting the image.