Have you ever wondered why the sea appears blue? When we look at the vast ocean, we see this beautiful blue color stretching to the horizon. Many people think it's simply because the ocean reflects the blue sky above, but this is actually a misconception. The truth involves fascinating scientific phenomena working together to create this stunning blue appearance.
To understand why the sea is blue, we first need to understand light itself. Sunlight appears white to our eyes, but it's actually made up of all the colors of the rainbow. When white light passes through a prism, it separates into its component colors: red, orange, yellow, green, blue, and violet. Each color corresponds to a different wavelength of light. Red light has the longest wavelength at about 700 nanometers, while violet light has the shortest at around 420 nanometers. Importantly, shorter wavelengths carry more energy than longer ones. This difference in wavelength and energy is crucial for understanding how light interacts with water molecules in the ocean.
The primary reason the sea appears blue is due to a phenomenon called Rayleigh scattering. This occurs when light encounters particles that are much smaller than the wavelength of light itself, such as water molecules. Here's the key: shorter wavelengths scatter much more strongly than longer wavelengths. This relationship follows the inverse fourth power law, meaning scattering is proportional to one over lambda to the fourth power. Since blue light has a wavelength of about 470 nanometers and red light has a wavelength of 700 nanometers, blue light scatters approximately five times more than red light. When sunlight enters the ocean, the blue wavelengths get scattered in all directions much more than the red wavelengths, which is why we predominantly see blue light coming from the water.
In addition to scattering, water molecules also selectively absorb different wavelengths of light. This absorption spectrum shows that water absorbs red and infrared light much more readily than blue light. When light enters the ocean, the longer wavelengths like red and orange are absorbed quickly, while shorter wavelengths like blue can penetrate much deeper. This selective absorption works together with Rayleigh scattering to enhance the blue appearance of water. As we go deeper into the ocean, more and more of the red light gets absorbed, leaving predominantly blue light to scatter back to our eyes. This is why even in very clear water, we see that characteristic blue color rather than the full spectrum of white light.
Now let's see how ocean color changes with depth. As sunlight penetrates the ocean, different colors are progressively filtered out due to absorption. Red light is absorbed within the first 10 meters, which is why everything looks bluish-green to divers even at shallow depths. Orange light disappears by 30 meters, and yellow light is mostly gone by 100 meters. At depths greater than 500 meters, only blue light remains, creating the deep blue color we associate with the open ocean. This filtering effect has important implications for marine life and underwater photography. Many deep-sea creatures have evolved to be blue or transparent to blend with their environment, while underwater photographers must use artificial lighting to reveal the true colors of marine life at depth.