It’s not the water – it’s the interface

An intriguingly common misconception around our eyes underwater is the idea that we need to apply some compensating factor to a prescription for a patient to see underwater. While this is easily debunked, it is oddly pervasive, thanks to the internet. It is so common that some dive shops and even specialty websites still recommend modifying a patients power, although there is no consensus on the amount and whether we increase or decrease the power. So what’s the reality? We need to consider the way that our eyes achieve their power. Like all refractive surfaces it is the difference between refractive indices and the curvature that lends the surface its power.