Some people wonder "what clouds smell like," while others ponder why we see blurry underwater. Both questions are equally valid and could spark lengthy debates, but we’ll focus on answering the second one as clearly as possible so anyone can understand.
Surely, at some point while diving without recreational scuba gear or a snorkelling mask, you’ve opened your eyes underwater only to make out blurry shapes or the colour of the sea or pool—barely distinguishing anything else. There’s a reason for this, rooted in human biology.
In the human eye, the lens acts as a converging lens, focusing the images we perceive onto the retina. When a light ray passes from one transparent medium to another, its path bends—a phenomenon known as refraction. Light refracts through the lens and projects onto the retina, with the lens "focusing" by adjusting its curvature.
Above water, with the eye exposed to air, the lens has a refractive index that allows us to focus images. Underwater, however, the water’s refractive index is similar to that of the lens, so the lens can no longer function effectively as a converging lens. As a result, light rays and images reach the back of the eye almost parallel.
What’s the consequence? We experience a kind of extreme hyperopia that makes everything appear undefined and blurry.
The solution? Create an airspace in front of the eye so the lens can resume its role as a "focusing lens." A diving mask corrects this effect by preventing water from touching the eye.
As a fun fact, the Phoenicians—the earliest known seafarers—would fill their mouths with oil before diving to the seafloor. As they descended, they’d release it slowly, and the resulting "film" supposedly improved their underwater vision.
