Why do our eyes not fog up like glasses or camera lens’
In: Biology
Fogging happens when moisture condenses onto the surface forming many tiny water droplets. The water itself isn’t the issue, it’s the large number of individual droplets which act as surfaces for light to scatter in all directions instead of passing straight through the glass. The droplets make the surface “rough” if you will.
To solve the issue you can go both ways: either remove the droplets as they condense, or have them spread out forming a continuous film of water on the surface. In both cases the “roughness” is eliminated and the surface becomes clear again. Our eyes work using the latter, there’s always a film of moisture on the surface which keeps them optically clear. The former is the mechanism used by most anti-fogging glasses instead (super-hydrophobic coatings on rearview mirrors for example).
Glasses fog up when they are too cold compared to whatever hot steamy air blows over them causing condensation to form.
Sometimes that “too cold” happens by just being “room temperature” when our warm breath blows over them. Other times it might happen when we walk from a cold air conditioned room outdoors into the warm summer air.
Our eyeballs don’t fog up in those situations because of the fact that our body heat keeps them warm, so they are rarely “too cold” enough for condensation to form. (Also, even if a teeny tiny bit of condensation did start to form, we reflexively blink our eyes, which basically works like a car’s windshield wipers!)
Our eyes are already full of water, even if moisture was to condense on them, it wouldn’t make a fog, it would just add more water, but that doesn’t happen because eyes are not cold, they are at body temperature
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