Open water reflects the changing patterns of blue and white and grey above, and this alone can impart to it a wide variety of colour. But in the case of the sea there are other reasons why there is a preponderance of blue. Water slowly absorbs any light that passes through it, and sea water absorbs the longer wavelengths of red and orange light more efficiently than the short blue wavelengths - in effect filtering the sunlight to leave it with a distinctly bluish tinge.
In addition, as the sunlight passes down into the depths, the tiny particles of liquid sea-water obstruct some of the surviving blue light-waves and "scatter" them back in the direction of the surface, where they emerge again to be seen by an observer.
So the sunlight entering the sea is first filtered until it is blue, and then some of this blue light is scattered back in the direction of someone watching from above.
Voila! - the sea is blue.
But if you have ever stood by the lakeside like Sir Bedivere, and listened to ". . . .the ripple washing in the reeds, and the wild water lapping on the crag", it may have occurred to you to wonder why a lake is usually so much darker in colour than the sea.
As in the case of the sea, the reflection of the sky may sometimes give to a lake the appearance of being whitish-blue in colour. More often, however, reflections of mountains and greenery around the shoreline will impart a much darker colour to the water. But like the sea, the colour of a lake is also largely determined by light that has first penetrated the water and then been diverted upwards again in the direction of an observer.
The colour of this scattered light varies from lake to lake, depending on the range of impurities suspended in the water, and the extent to which they affect the different wavelengths making up the sunlight.
Very pure lakes often deliver a distinctly bluish tinge, just like the sea. Increasing proportions of iron salts or humic acids, however, result in scattered light that varies from light yellow to a darkish shade of brown. And sometimes, if the water is rich in large particles of peat washed down from surrounding bogs, the sunlight may be completely absorbed as it tries to penetrate the depths: no light remains to be scattered upwards and the lake takes on a sinister - or romantic - black appearance.