Understanding the Optical Mystery: Why the Sea Sometimes Looks Turquoise Instead of Blue on a Clear Sunny Day
The color of the sea can vary significantly depending on several factors including the presence of particles, the depth of the water, and the angle of sunlight. Here’s why the sea sometimes appears turquoise instead of blue:
1. Water Depth
In shallower areas, sunlight can penetrate the water more effectively. The water might reflect more of the green wavelengths of light, especially if the bottom is sandy or contains light-colored corals, leading to a turquoise appearance. This phenomenon is particularly noticeable in tropical and shallow coastal waters where the light colors of the seabed contribute to a more vivid turquoise tint.
2. Sediment and Algae
The presence of certain sediments and microscopic algae can also affect water color. For example, phytoplankton can give the water a greenish tint, contributing to a turquoise hue. In estuaries and coastal waters, sediments from land runoff can mix with phytoplankton to create a more distinct turquoise color.
3. Angle of Sunlight
The angle at which sunlight hits the surface of the water can change the way colors are perceived. When the sun is high in the sky, the reflection and scattering of light can enhance the turquoise color, especially on clear days. This is due to the way light is reflected and refracted at different angles, creating a more vivid turquoise hue in the surface water.
4. Refraction
The way light is refracted as it passes through the water can also influence color. Turquoise waters often occur in areas where the water is clear enough to allow this refraction to enhance green wavelengths. In shallow waters, this refraction is more pronounced, leading to the turquoise coloration observed.
5. Surface Reflection
The interaction of light with the surface of the water can create different visual effects depending on the wind and wave conditions, which can influence how colors are reflected back to our eyes. In calm conditions, light reflects more directly, while in choppy seas, light is scattered and diffused, leading to a more pronounced turquoise hue.
The Human Eye and Color Perception
The color we see on the surface of the ocean is the result of the inherent optical properties of seawater, which vary widely depending on its optical type, environmental variables, and the position of the sun in the sky, cloud cover, and the roughness of the sea surface.
Ambient Luminosity and Color Perception: Another key factor determining the color that our eyes and brain detect is the ambient luminosity, not to be confused with radiance or irradiance. Under bright light conditions, such as on a clear sunny day, the types of cells that are most active are the cones, which are more responsive to longer, greener wavelengths. Conversely, in a dim light environment, such as on a heavily overcast day or at night, the rods are more active and are more responsive to smaller, bluer wavelengths.
Biological property of our eye-brain system: It is likely one of the most important or even the dominant reason why the same patch of ocean surface may look deep blue on a cloudy day and then suddenly appear turquoise if the sky clears. This is because the cones, which are most active in bright light, are better at detecting blue-green colors, whereas the rods, which are more active in dim light, contribute to the perception of blue colors.
Water Changes and Optical Properties
While the apparent and inherent optical properties of seawater can significantly change in a matter of hours or days, they are influenced by a wide range of environmental factors. For example, the ocean may look blueish in a sunny day and turquoiseish on the following day if the currents bring a plume of sediments or waters with otherwise different optical properties.
Overall, the color of the sea is a fascinating interplay of physics, biology, and environmental variables that create the vibrant and ever-changing colors of our oceans.