The first thing you will notice underwater if looking through a diving mask or swimming goggles is that everything looks bigger. In fact, about 33% bigger. So a 3 feet long stick would look like 4 feet long. Now, as you can very well imagine, this will create some confusion in eye-hand coordination for the uninitiated. But generally our brains adjust to this pretty soon, with bit of patience and practice.
So why does it happen?
Simple answer – Refraction of light. As described by Snell’s law, light changes direction (bends) when it passes from one transparent substance to another with a different refractive index (optical density). Since our eyes are used to seeing in air and cannot focus properly in water, we wear a diving mask/goggles. This creates a pocket of air in which our eyes can function effectively, but this also forces light traveling from an object in water to ‘bend’ as it passes this pocket of air before reaching our eyes. The refractive index of air is about 1, but water has an index of 1.33 (refractive indices). This change of angle is not anticipated by our brain and we perceive the size of the object 33% larger than it actually is.
This is one of the reasons that some scuba diving agencies mandate skills practice in shallow, safe environment like a swimming pool before an actual open water dive. The skills practice also help in strengthening the eye-hand coordination. Some diving instructors also employ underwater games, like swim through hoops, underwater toypedos and frisbees. Even experienced divers who have not dived for a while should keep this mind when diving after a significant gap. Most scuba diving training agencies also have refresher programs for such scenarios to encourage this.
Something to keep in mind next time you’re swimming over a reef, it may be farther away than it seems!