Questions and Answers > Common Questions > General Q: Why does the sea sometimes look blue and sometimes green? A: As we know, the colour of a substance depends on the colour of light it reflects. Similarly, the sea has colour because of lights of different wavelengths is reflected and scattered differently in water. Red light is more readily absorbed by water while blue is more scattered. This phenomenon is only to apparent water of certain thickness. Therefore, the sea usually appears blue. However, the sea may also appear in other colors depending on its “ingredient”. Algae or sediment in coastal waters is one important factor governing the colour of the sea. The green/yellow pigments from the algae/sediment mix with the blue light would produce the commonly observed green sea colour. When the concentration of the algae is high, depending on the color of the algae, the sea may even turn red, brown, orange etc. This phenomenon is called “red tide”. Every year, there are around 20-30 red tide happening in Hong Kong according to the record of the Agricultural Fisheries and Conservation Department. GROUPS Common Questions Misunderstandings Technical Questions CATEGORIES General Beach Fishery Red-tide Enviornmental Protection Water Treatment Scientific