Why does water appear shallower than it really is?
Every angler knows that when he sees a fish in clear water the fish appears closer to the surface than it actually is. The angler will need to allow for this difference when presenting his fly to the fish.
The reason for this apparent difference is the speed of light in the air compared with its speed in the water. Light travels at 300 000 km per second in the air but only ¾ of this in water. Both these speeds are so large that under most everyday circumstances we are not aware of their effects.
Imagine you are walking along a path with a fence on your left. If you put out your left hand then it will make contact with the fence and your left side will be slowed. This will cause you to turn towards the fence.
The same thing happens to light at the surface between light and water. The part of the light wave moving into the water is slowed down and the light changes direction and turns into the water. The reverse occurs when light moves out of the water into the air. So light coming to you from a submerged fish changes direction when it passes through the water surface.
However, our brain has learned through all its experiences that “Light travels in straight lines”. The only way our brain can cope with this is to imagine the fish is in the position it would be had the light not changed direction. The result; we “see” the fish closer to the surface than it really is. In practice, you need to add an extra third to the observed depth. A fish that appears 3ft deep is really 4ft down so you need to allow for this extra depth.