Light

As light passes through water, it is absorbed and scattered by water molecules, ions, and suspended particles, including silt and micro-organisms.  It is also absorbed by organisms for photosynthesis, to be used in their life process.  This decrease in the intensity of light over distance is known as attenuation.  Seawater transmits only a small portion of the electromagnetic spectrum, primarily in the visible range.  Light energy is attenuated very rapidly with depth, particularly the longer infrared wavelengths.  The intensity of light at any depth can be calculated using Beer's Law: I_z = I₀e ^–kz therefore by rearranging terms  z = -ln(I_z/I₀)/k

The attenuation coefficient k varies with the clarity of the water.  The clearer the water, the smaller the attenuation coefficient and the greater the light penetration.  In typical open ocean water, about 50% of the entering light energy is attenuated in the first 10 m (33 ft), and almost all the of the light is attenuated 100 m (330 ft) beneath the surface.

This graph shows how much visible light (%) broken down into the familiar spectrum of red, orange, yellow, green, blue, and violet and to what depth it can reach.