Vision becomes blurry and indistinct when opening your eyes underwater without goggles. This happens because the human eye is adapted for clear vision in air, and submersion in water significantly alters the way light interacts with the eye. The blurriness is not due to water clarity, but rather an optical phenomenon.
The Science of Blurry Vision
Light travels at different speeds through different mediums, causing it to bend or refract when passing from one to another. In the human eye, most of the light bending, or refraction, occurs at the curved surface of the cornea, which is the transparent outer layer at the front of the eye. This is because there is a substantial difference between the refractive index of air (approximately 1.000) and the cornea (around 1.376 to 1.4). This large difference allows the cornea to perform about two-thirds of the eye’s total focusing power.
When the eye is submerged in water, light travels from water into the cornea. Water has a refractive index (about 1.33) very similar to that of the cornea. Because these indices are so close, light does not bend significantly when passing from water into the cornea, effectively eliminating the cornea’s primary focusing ability.
Without the cornea’s substantial focusing power, the eye’s internal lens alone cannot sufficiently bend light rays to focus them precisely onto the retina. Instead, light comes to a focus behind the retina, leading to extreme farsightedness, or hyperopia.
How Goggles Restore Clarity
Goggles or a diving mask effectively solve the problem of blurry underwater vision by re-establishing the air-cornea interface. These devices create a sealed air pocket directly in front of the eyes. This means that when light travels from the water, it first passes through the goggle lens and then enters this air-filled space before reaching the cornea.
By introducing this layer of air, the significant difference in refractive index between air and the cornea is restored. This allows the cornea to regain its natural ability to refract and focus incoming light rays. This restoration of proper light refraction enables clear vision underwater.
Is It Harmful to Your Eyes?
Opening your eyes underwater without goggles does not cause permanent physical harm or damage to the eyes’ structure or vision. The blurriness experienced is a temporary optical effect, a natural consequence of how light behaves when passing from water to the eye. Your eyes are not being damaged by the lack of clear focus.
However, direct exposure to water, particularly in swimming pools or natural bodies of water, can lead to temporary discomfort and irritation. Chlorine in pools can strip away the protective tear film on the eye’s surface, causing redness, stinging, and dryness. Saltwater can also dehydrate the eyes, leading to similar irritation and a gritty sensation. While these irritations are generally mild and short-lived, resolving within a few hours, water can also contain bacteria or other microorganisms that may cause infections like conjunctivitis. Therefore, while the blurriness itself is not damaging, protective eyewear is recommended to prevent irritation and potential infections.