The solar radiation is the main source of energy for the physical, chemical and biological processes in the aquatic system. Solar radiation reaches the surface of the water, partly as direct electromagnetic radiation and partly as diffuse sky radiation. How much of the light penetrates the surface is determined by the angle of the light and the properties of the surface. The reflection increases significantly at lower angles. Strong winds which lead to  formation of whitecaps can reduce the reflected light by half.

Objects under the surface appear to be larger than they actually are, when observed from above the surface. The intensity of light decreases rapidly under the surface. Approximately 65 percent of the energy in the visible light is absorbed in the first metre of water. The light that reaches the surface from the sun and atmosphere has a wide range of wavelengths. More than 50 percent of the energy in the light lies in the visible spectra.

Light of different wavelengths is absorbed unequally fast by the water and thus reaches different depths. In clean water, red light is absorbed first, then orange, violet and yellow. Blue and green light reach lower depths. Already at 20 metres depth everything is coloured blue. There is total darkness below 200 metres. Some of the light is reflected back to the surface by water molecules and particles. This light gives the ocean it's colour. If the water has a low particle content, then the blue light is least absorbed and thus spreads widely. Blue is therefore referred to as the ocean's "desert colour". Animals that are red in broad daylight appear to be black in greater depths, where there is no red light. This makes it easier to hide from enemies. Why then are not these animals red to begin with? One reason could be that the red colour requires less energy to produce.

In freshwater and coastal areas, high concentrations of humic substances can turn the colour of the water from blue to green, yellow-green or brown. Also, particles in the water, such as chlorophyll, can change the colour of the water by changing the spread and reflection of the light. The particle's own colour can sometimes “dye” the water, e.g. red by substantial plankton blooms of red dinoflagellates. On a cloudy day the reflection of the clouds will cause the surface to appear grey. Optical measurements can be used to trace and identify different masses of water. The optical properties of the water can also be used to trace pollution, surface chlorophyll or the topography of the bottom. The different wavelength's ability to penetrate the water can be measured by holding a coloured glass filter in front of the eye when measuring the secchi depth or with a photocell fitted with a glass filter.