Ask a child to draw water and, armed with crayons, they will make it blue. However, many adults would consider water to be colorless. The blue hue that large bodies of water have is not simply a reflection of the sky or light scattering on the surface – the kids are right, water is ever so subtly blue.

The color of an object or a substance depends on the light shone on the object and how that light interacts with it – the surface of a strawberry is red because the molecules there absorb visible light of all colors but red, which is reflected back to our eyes.

Before we dive into the matter of water, there are subtleties to discuss. How we interpret colors depends on cultural and subjective contexts, and the color blue is in general an excellent example of this – for example, my blue might not be the same as your blue.

Don’t Go Blaming Electrons, When The Atoms Are At Fault

Color usually depends on the interactions between electrons, the negatively charged particles surrounding the nuclei of atoms; and photons, particles of light. There is a varied selection of interactions. Light can be absorbed or emitted, or only partially reflected, but it can also be refracted and diffracted. 

Light can also experience Rayleigh scattering – the most glaring example of it is above our heads. The sky is blue because the white sunlight is scattered by different angles depending on the color (wavelength) of the photon. Blue more, and red less. So blue light can travel less in the atmosphere, giving the sky its hue for most of the day – but at sunset or sunrise, where light travel more in the atmosphere, it’s the reds and yellows that win.

Water is different. Water molecules selectively absorb light in the red portion of the visible spectrum. Certain specific wavelengths of light make the water molecule vibrate once they are absorbed – this is the only known natural color caused by vibrational transition.

The Reason For The Blueness Of The Sea

Assuming clarity and calmness, with just a few meters in depth, water will show its true color. The same is true for ice as well. Dig a hole in ice and you will see the blueness of the substance coming through.

So water is ever so slightly blue because it absorbs red photons. So a glass of water is clear, but a lake will show its blueness. So is the sea really blue, and it is just because of this absorption? Mostly, but not exclusively. The sea (as well as other water sources) has impurities and turbidity, which in some case might make it more or less blue. Water can also reflect and scatter light at its surface which can add to the blueness. That reflection might include the sky, making the blue of the sea brilliant on sunny days and gray when overcast.

The blue of water is subtle and unique – but by looking at it in a glass, you couldn’t tell.