Why should the sea be blue? And why should the sky be blue? For the matter of that, why should anything be blue? What do we mean by colour?—whether blue or green, red or yellow?
We mean that particular tint—or sensation—which the object in question causes us to see—or feel. The object receives sunlight, and reflects it to our eyes. And since sunlight is white or yellowish-white, one might expect that everything we look upon would appear to us white or yellowish-white.
But everything does not. And the reason is that a ray of sunlight is really a bundle of lesser rays, each of which has its own colour. If a ray of light is made to pass through a prism, these lesser sub-rays are spread out upon wall or floor, always in the same order, from violet at one end to red at the other end. Light is believed to be due to enormous numbers of most minute wavelets; and for each colour the wavelets have a definite but different size. They are smallest at the violet end, and largest at the red end.
Now when a ray of sunlight falls upon anything—leaf or flower, earth or water—some of the sub-rays are absorbed or taken in, and some are rejected or refused admission. A healthy leaf, for instance, absorbs the red, the yellow, the blue, the violet, and refuses the green, which is therefore thrown back from the leaf-surface to our eyes, making it green to us. A ripe tomato absorbs the yellow and green, the blue and violet, and reflects the red. A lump of blacklead absorbs greedily all the rays, reflecting none, and so to our eyes it is black or without colour. The petals of a white rose, on the contrary, absorb so little colour of any kind, that practically the entire ray is sent again to us as white light.
If this is the manner in which the ocean is blue, it means that sea-water, and indeed water generally, must in itself be actually blue, as a cake of ultramarine, a sapphire, a corn-flower, a forget-me-not, are blue.
Could that be the case? For a long while men decisively answered—No. Everybody knew that water was colourless, so the idea was flung aside as impossible. Another explanation came up instead.
The sea is full of fine dust; and it was suggested that, as vast multitudes of these floating dust-particles are exceedingly small, they might be able to reflect only the smaller blue light-waves, and not the larger yellow or the still larger red waves. Thus the sea would naturally seem to be blue.
After much discussion, careful experiments were made. Different objects were lowered to various depths, and the effect upon their colouring was noted. The outcome of these and other tests went far to prove that the old certainty was wrong, and that water is in itself blue.
Somebody may feel inclined to protest. “Water blue! But that is out of the question. I have good sight, and I assure you there isn’t the least sign of colour in water. Not the faintest. If you dip a bucket in the sea, you will see for yourself.”
Yet all the while the colour may be there, actually existent, though not to be detected by your eyes or mine in a pailful.