Tools · Snell's law

Why everything looks closer

Light bends when it crosses from one material into another of different optical density — that's Snell's law:

The number n is how much a material slows light; for air it's about 1.00, for water about 1.33. At your mask, light leaves the water and enters the little pocket of air in front of your eyes, and it kinks at that boundary.

Your brain never learned about that kink. It assumes light always travels in straight lines, so it traces the bent rays back along a straight path and places the object where those lines cross — which turns out to be bigger and nearer than the real thing. The magnification is just the ratio of the two n values, about 1.33: things look roughly a third larger and a quarter closer. Slide the real distance and size and compare.

Two consequences follow, and divers feel both. First, the fish you'll describe at the bar afterward is really about three-quarters its apparent size — the magnification works on your fish stories too. Second, and more useful: when you reach for something, it's farther than it looks. An object that appears one metre away is really about 1.3 metres off, so a new diver reaches and comes up short. The math is just the index ratio run the other way:

It's the same physics as a straw looking broken in a glass of water, scaled up to a whole sea. Experienced divers stop noticing because the brain quietly recalibrates — you learn the new rule for reaching and judging size without thinking about it. But it never goes away; the light is still bending at your mask on every single dive. And it is only one of the tricks the water plays on the senses — it also drains the colour out of what you see and scrambles where a sound comes from.