Why goggles fog
Fog is a lot of tiny water droplets scattering light
Your face and eyes are warm, and pool or open water is cool. Warm, moist air inside the goggles meets the cooler lens, and the moisture condenses. Water molecules are more attracted to each other than to the plastic, so on an untreated lens they pull together into thousands of tiny beads. Each bead bends and scatters light, and the lens looks milky.
You can't stop water from condensing on a lens. What you can change is the shape it takes.
The fix: make water spread out instead of bead up
If the lens surface attracts water more strongly than the water attracts itself, the droplets flatten and join into one thin, continuous layer. Light passes through a smooth film almost undisturbed, so you see clearly. This is what anti-fog technology is designed to do, and surfaces that behave this way are called hydrophilic, meaning “water-loving”.
Why most fixes don't last
- Sprays and wipes leave a temporary film that rubs off and washes away.
- Soap, baby shampoo and saliva lower surface tension briefly, then need repeating.
- Factory coatings are often hydrophilic layers that absorb moisture. They work well for a while, but they can wear away or saturate, much like a sponge that has soaked up all it can hold.
The next page explains a different approach.