Submerging your head underwater and opening your eyes often results in a sharp, stinging sensation that forces them shut. This common reaction is a predictable physiological response to an environment our eyes are not designed for. The discomfort and resulting blurry vision are caused by chemical imbalances and the fundamental physics of light.
The Reason for Immediate Discomfort
The immediate, burning discomfort that causes the eyes to close is primarily a chemical reaction between the water and the tear film. The tear film is a thin protective layer that maintains a specific salt concentration (osmolarity) similar to the cells of the cornea. When the eye is submerged, the external water disrupts this delicate balance.
In swimming pools, the water is usually hypotonic, meaning it has a lower salt concentration than your tears. This osmotic imbalance causes water to rush into the corneal cells, leading to temporary swelling and stinging irritation. Conversely, ocean water is hypertonic, or saltier than tears, which pulls water out of the corneal cells, causing dehydration and a similar painful sensation.
Beyond osmotic pressure, water often contains chemical irritants that wash away the protective tear film. Pool water contains chloramines, compounds formed when chlorine reacts with organic matter like sweat and skin cells. Chloramines are the primary source of eye redness and irritation. Without the tear film, the cornea is left exposed and hypersensitive, triggering the defensive reflex to shut the eyes.
Why Underwater Vision is Always Blurry
Even if you keep your eyes open past the initial stinging, underwater vision remains blurry due to a problem in light physics. To see clearly, the eye must refract light rays to focus them precisely onto the retina. The human eye relies on the cornea, the clear front dome, to perform roughly two-thirds of this total focusing power.
The cornea’s focusing power relies on the massive difference in refractive index between the air in front of it and the fluid inside the eye. When submerged, water replaces the air. Since water has a refractive index very similar to the fluid inside the eye, this change effectively neutralizes the cornea’s ability to bend light.
With the cornea’s primary function disabled, the eye is left with only the focusing power of the internal lens, which is insufficient to compensate. As a result, the light focuses far behind the retina, a condition known as hyperopia (farsightedness). This optical error renders all objects extremely out of focus, appearing as an indistinct blur.
How Protection Alters the Experience
Solutions for comfortable and clear underwater vision demonstrate that discomfort and blurriness are environmental, not biological, limitations. To restore vision, simple equipment must overcome the issues of chemical irritation and physics.
Goggles and diving masks solve both problems simultaneously by creating a watertight seal around the eyes. They prevent water, chemical irritants, and pathogens from touching the sensitive corneal surface, eliminating osmotic and chemical discomfort.
The flat lens of the mask reintroduces a layer of air between the water and the cornea. This air gap restores the necessary difference in the refractive index between the air and the cornea, allowing the cornea to perform its light-bending function. The result is a return to normal, clear vision, provided the eye is looking through air.