Lord Rayleigh (John Strutt, 1842-1919) determined that the nitrogen and oxygen molecules in the atmosphere scattered sunlight, allowing us to see the sky. The scattering is strongest at the lowest wavelengths. Why then, don’t we see a violet sky? Our eyes are most sensitive to colors in the mid-section of the spectrum, about 550 nanometers. Because blue is closer to this wavelength, our eyes are more sensitive to it than indigo and violet. So, even though all three colors are scattered by the molecules in the air, humans see a predominantly blue sky. Because the shorter wavelength of sunlight are scattered by the atmosphere, the light transmitted has a yellow cast; the sun looks yellow rather than white.

There are two major reasons why the ocean and most bodies of water appear blue. The first can be observed by looking at the water on a cloudy day and then on a sunny day. There is a rather large difference in how blue the water appears to be on the two different days, because the water acts as a mirror for the sky. So on a sunny day with a blue sky, the water will have a richer blue color than on a cloudy day.

The second reason why bodies of water have a blue appearance is that water scatters short wavelength light more than the longer. In fact, water absorbs some orange, red, and the very long-wavelength infrared. As a result it absorbs more energy in the sunlight, increasing its temperature. The much larger amounts of reflected, short-wavelength light results in a crisp blue-colored body of water.

Some bodies of water may take on a more greenish or at times a brownish or black color. Usually this is due to other elements in the water such as algae, silt, and sand. Runoff water from glaciers is very white due to the tiny grains of silt in the water. Still, in the majority of cases, water looks blue.