Why is the sea blue or green? Hold up a glass of seawater and it appears colorless. But stand on the deck of a ship and the sea will appear to have color. Why? To understand it, let me briefly outline what water does to light.
Visible white light is made up of a spectrum of all the colors-red, orange, yellow, green, blue, indigo, and violet. When we look at an object and see it as blue, we are seeing the blue light of the spectrum reflected from the object. All other colors are absorbed and cannot be seen. In the case of the sea, red light is absorbed as soon as it breaks through the water's surface. And by a depth of about 25 feet virtually all the red light discernible to the human eye is gone; a bright red air tank on a diver, for example, would seem a dull dark brown. At a depth of 75 feet a yellow air tank looks more greenish blue, because the discernable yellow light has been absorbed by the water. The still shorter rays of light are almost all absorbed by 100 feet. All that remains are the shortest rays: blue, indigo, and violet. Below 100 feet or so, all light appears a monochromatic blue. So, when the sea is pure and clear, as often is the case in the open ocean, the least-absorbed shade of the spectrum blue, is reflected to our eyes.
The sea isn't always blue, however. Some seas appear bluish-green or green or brown or even red. These colorations are partly due to the reflection of clouds, but are caused mainly by various particles, mineral or organic, that are in suspension in the water. In some areas, especially along coastlines and in shallower seas, organic matter decomposes and produces a yellow pigment that when mixed with blue light makes the sea appear bluish-green or green. Brown coloration may be caused by sediments, stirred up from the bottom, hanging in suspension in the water and reflecting its brownish coloration. In many coastal areas, at widespread intervals, blooms of a species of dinoflagellate that is red in color become so numerous they cause the waters to appear red. This is the famous "red tide."
When light passes from air to water-that is, from a relatively thin medium to another medium 800 times denser-its speed is reduced from about 186,000 to 140,000 miles per second. Crossing the surface, for the same basic reason, light is bent. This is called refraction. Each color of the spectrum has a different wavelength; red has the longest and bends the most. The shortest, blue and violet, are less refracted.
Penetrating the sea, light is not only refracted and absorbed. It is also scattered, slightly by the water molecules (some scattering occurs even in distilled water) but mainly by particles of sand, salt, and minerals in suspension. The light rays bounce from one to another until their energy is spent. The scattering process limits the distance one can see; even in the middle of the ocean visibility at 300 feet is exceptional-and reduces and subdues direct penetration of sunlight. In water 100 feet deep there are no shadows.
Dive into the sea, open your naked eyes, look around. What do you see? Fuzzy shapes, completely out of focus. No detail. Washed-out colors, hardly anything recognizable. Out of the water, light passes through the air and into your eye-an eye containing a fluid similar in density to seawater. The difference in the density of air and this fluid bends or refracts light rays as they enter your eye. The refracted light focuses on the retina. Underwater, however, light passes from seawater into your eye bending very slightly because of the similar density of the two fluids, so everything appears out of focus. You become extremely farsighted.
If you interpose a pocket of air between water and your eye, your eye lens functions properly. To form this air pocket, you don a diver's face mask. Nevertheless, a mask is not a perfect instrument. The refraction of light through a flat surface of separation between water and air, like the plate glass window of an aquarium corrects the refractive distortion. The additional of artificial light further corrects and/or compensates for the loss of color at depth.