Why does the colour of the sea change from one area to another?
How far can light travel through water?
How deep before it gets dark?
What happens to light at the sea surface?
Optical oceanographers look for answers to questions like these. Here are some reasons why we want to know:
Sunlight provides energy for photosynthesis. Without it marine plants cannot grow, and there would be little food for fish and other animals.
Heat from the sun warms surface water, and drives the ocean currents that modify our climate.
Water evaporating from the sea falls as rain and fuels tropical storms. This evaporation depends on energy from sunlight.
What happens to light as it travels through water can give us information about the content of the water.
Around the world a lot of effort is spent measuring water colour and working out exactly what it tells you. But you don't always need expensive instruments. You can learn a lot by looking carefully at the water or photos such as those on the right.
Sky reflection increases with viewing angle.
Refraction makes this fence look as if it bends at the water surface.
On calm days the water surface acts almost like a mirror. But look straight down, and the mirror disappears. Instead you see what is under the surface.
If you look further away, your viewing angle increases, and the surface reflection becomes stronger. At the same time more of the light from under the surface is reflected back down.
Surface reflection is a problem for ocean colour measurements. That's why satellites look fairly straight down. They also avoid looking in a direction where they can see the sun's reflection.
Have you ever put a stick into the water and watched it bend at the surface? You know it's straight, so the bend is an optical illusion.
The reason is refraction. Light from under the water is bent as it passes the surface into the air. As a result the underwater part of the stick seems to be in a different place from where it really is.
Refraction can make things in the water appear larger than they are. It is also the reason why waves often focus sunlight into patterns of light and shadow on the bottom.
Inside a breaking wave. The blue-green colour of the deeper water is a result of red and yellow light
being absorbed more strongly than green and blue light. Photo:
Divers on the English south coast. Tiny particles in the water scatter light and makes everything look blurred. The green colour is due to chlorophyll and dissolved organic matter, which both absorb blue light. Photo: Ken Collins.
Once in the water the light continues to travel downwards, gradually growing weaker. How deep it goes depends on the type of water and on the angle of the sun.
There are two reasons for the loss of light as you go deeper:
Absorption - photons disappear and the energy they contain is turned into heat or used for photosynthesis.
Scattering - photons change direction, but do not disappear. Often the new direction is upwards.
Absorption is what gives water it's colour. Open ocean water is usually blue because the blue photons travel furthest before they are absorbed. Water with lots of phytoplankton (microscopic plants) is often bright green. Chlorophyll in the plants absorbs blue light, so now the green photons travel furthest.
Pure water scatters very little light. When the water is clear and clean most of the photons disappears into the deep. That's why clear water seems quite dark when you look straight down.
Small particles in the water (plant cells or mud) scatter much more light. In this type of water many of the photons change direction and travel back up. Seen from above this type of water has a much lighter colour.
In the water particle scattering has another effect that divers around the UK know well. Just as in fog, the scattering blurs details, and you can only see a short distance ahead. The light also fades faster. In water with lots of small particles, it can be dark at just a few metres depth.