When submerging ourselves in water, a common experience is the sudden blurriness of our vision. This phenomenon stems from fundamental principles of light behavior and the unique design of the human eye. Understanding these scientific reasons helps clarify why our sight is compromised in an aquatic environment.
How Light Changes in Water
Light travels at different speeds through different mediums, such as air or water. When light passes from one medium to another at an angle, its speed changes, causing it to bend. This bending of light is known as refraction. The extent to which light bends depends on the “optical density” of the materials, quantified by their refractive index. A higher refractive index indicates greater optical density, meaning light slows down and bends more significantly. For instance, air has a refractive index of approximately 1.0003, while water has a higher refractive index of about 1.333. This difference causes light rays to refract as they cross the air-water boundary, which is why objects viewed partially submerged in water can appear bent or distorted.
Why Our Eyes Struggle Underwater
The human eye is adapted for clear vision in air, with its focusing power largely attributed to the cornea. The cornea, the transparent front part of the eye, accounts for approximately two-thirds of the eye’s total optical power. This focusing ability arises from the substantial difference in refractive index between air (around 1.0003) and the cornea itself (about 1.376). As light enters the cornea from the air, it undergoes considerable bending, allowing the eye to converge light rays onto the retina. When the eye is submerged in water, the interface changes from air-cornea to water-cornea. Since water’s refractive index (approximately 1.333) is very close to the cornea’s (around 1.376), the difference needed for effective refraction is drastically reduced. Consequently, the cornea loses almost all focusing power. Light rays entering the eye underwater are not sufficiently focused, causing them to converge behind the retina, resulting in a blurred image.
How Goggles Solve the Problem
Swim goggles and diving masks provide a solution to this optical challenge. Their design creates an air pocket directly in front of the eyes. This re-establishes the air-cornea interface that our eyes are designed to utilize for focusing. With air separating the water from the cornea, light travels from water, through the mask’s clear lens, then through the air inside the mask, and finally enters the eye. The light rays now encounter the necessary difference in refractive index between the air pocket and the cornea. This allows the cornea to refract light properly and focus images sharply onto the retina, restoring clear vision underwater.
Beyond Refraction: Other Causes of Murkiness
While the refractive mismatch is the primary reason for blurry vision underwater, other factors can reduce clarity. Suspended particles in the water, such as sand, silt, algae, and pollutants, scatter and absorb light. This scattering deflects light rays, reducing the amount of direct light reaching the eye and lowering contrast. The presence of these particles, often measured as turbidity, diminishes overall visibility. Water itself also absorbs light, and this absorption varies with wavelength. Longer wavelengths of light, like red and orange, are absorbed more quickly in water, disappearing within the first few meters of depth. Shorter wavelengths, such as blue light, penetrate much deeper. This selective absorption affects color perception, making the underwater world appear predominantly blue or green at depth and contributing to a general dimness.