Seeing underwater in the ocean is very different from vision in air. The marine environment presents unique challenges to light, causing visibility to vary dramatically. Conditions can range from clear waters, where objects are discernible from a distance, to murky conditions with limited visibility. Understanding these variations involves exploring how light behaves in water and the elements suspended within it.
How Light Interacts with Ocean Water
Light changes significantly when it enters ocean water. Two processes, absorption and scattering, determine how much light penetrates and how clearly objects are seen. Water molecules and dissolved substances absorb light energy, converting it into heat. This absorption is selective: longer wavelengths like red, orange, and yellow light are absorbed quickly, often within the first few meters. Shorter wavelengths, such as blue and violet light, penetrate much deeper. This is why the ocean often appears blue, as blue light is scattered and reflected.
Light scattering happens when light rays are deflected by water molecules and suspended particles. This diffuses light, reducing visibility and contrast. While water molecules cause scattering in clear open ocean, suspended particles contribute significantly to blurring and reduced clarity elsewhere. The combined effects of absorption and scattering mean light intensity decreases rapidly with depth, and its color composition changes dramatically.
Factors Affecting Underwater Clarity
Several environmental and biological elements influence ocean water clarity. Suspended particles, like sediment, significantly reduce visibility. Sediment originates from rivers, coastal erosion, and seafloor disturbances. When suspended, these particles block and scatter light, making water cloudy and reducing how far one can see.
Plankton blooms are another major contributor to reduced clarity. These microscopic organisms multiply rapidly, creating dense concentrations that absorb and scatter light, sometimes making water appear green or brown. Organic detritus, decaying plant and animal matter, also adds to the particulate load, further diminishing visibility. Dissolved organic matter (DOM) from land runoff can absorb light, imparting a yellowish or brownish tint. Coastal areas and river mouths generally have lower clarity than the open ocean due to this influx of sediment and dissolved substances.
The Impact of Ocean Depth on Vision
Increasing ocean depth significantly alters underwater vision due to light interaction. Light intensity diminishes rapidly, leading to darker environments. In clear ocean water, visible light can decrease tenfold for every 75 meters of descent, leaving only about 1% of surface light at 150 meters. Beyond 200 meters, significant sunlight is rare, and below 1,000 meters, the ocean is in complete darkness.
Color perception also changes dramatically with depth due to selective light absorption. Red, orange, and yellow light are absorbed first, disappearing within the upper 10 to 40 meters. Objects appearing red at the surface will look gray or black at greater depths because no red light is available to reflect. As depth increases, only shorter wavelengths like blue and green light penetrate further, resulting in a monochromatic blue or greenish visual field. This explains why many deep-sea creatures appear red or black, effectively camouflaging them in the blue-dominated deep-sea.
How Human Vision Works Underwater
The human eye is adapted for vision in air, so seeing clearly underwater is significantly limited without assistance. When light passes from water directly into the eye, the minimal change in refractive index between water and the cornea reduces the cornea’s focusing power. This causes light to focus behind the retina, resulting in severely blurred vision.
A flat diving mask addresses this by creating an air pocket in front of the eyes. This air-filled space restores the necessary air-to-cornea interface, allowing the eye to refract light correctly and form a clear image. Even with a mask, objects underwater appear magnified by about 25% to 30% and closer than their actual distance. This distortion affects depth perception and the ability to accurately judge distances. While human eyes adapt to various light intensities, they are limited by the extreme darkness in deeper ocean zones.