In seawater, the colors of the visible light spectrum do not penetrate equally. The penetration depth of light in seawater depends on the wavelength (color) of the light and the constituents present in the water. Generally, blue light penetrates deepest into seawater, followed by green light, while red light is absorbed more rapidly and does not penetrate as deeply. This phenomenon is often referred to as "color attenuation" or "selective absorption."
The reason for this color-dependent penetration is due to the interactions of light with water molecules and the various dissolved and suspended particles in seawater. Water molecules, particularly the water molecule itself (H₂O), absorb light strongly in the red part of the spectrum. Additionally, other substances like dissolved organic matter, chlorophyll (from marine plants and algae), and suspended particles (such as sediment and plankton) also contribute to the absorption and scattering of light in seawater.
The shorter-wavelength blue light is less absorbed by water molecules and other substances in seawater compared to longer-wavelength red light. As a result, blue light can penetrate deeper into the water column, potentially reaching greater depths. Green light falls in between blue and red light in terms of penetration depth.
In very clear and optically pure seawater, blue light can penetrate several hundred meters, while green light can reach depths of tens to a few hundred meters. Red light, however, may only penetrate a few meters or even less before being absorbed.
It's important to note that the exact penetration depths of different colors of light can vary depending on factors such as water clarity, concentrations of dissolved and suspended particles, and the specific absorption characteristics of the water body.