Question: If water is transparent, why does the ocean look blue? But sometimes it looks other colors too like green or red, why?

Hayley Evers-King answered on 13 Mar 2013:

Water looks blue because of the way it absorbs, scatters and reflects the different colours of light. Remember that light is made up of colours (think of a rainbow). Pure water absorbs the red and some of the green parts of light, which leaves the blue bits to be reflected back towards your eyes when light shines on the ocean.

The ocean looks green and red when there are things in it other than pure water. Usually when the ocean is green, it is because there are tiny plant like creatures in it (called algae or phytoplankton). These are very similar to plants on land, they photosynthesise and contain a pigment called chlorophyll. To power photosynthesis, chlorophyll absorbs blue and red light. It doesn’t absorb the green light, which leaves the green to be reflected back at you. Red colour can come from algae/phytoplankton that have red pigments or from the way their cells scatter the light.