We call Earth the Blue Planet because it’s covered in water—but what makes water blue? And why is it not blue in your glass or your ice cubes?

Water has many rare properties, some of which we’ve covered on previous episodes.

But this property, its blueness, is unique to water and goes right to its core. It’s the only molecule in the universe that vibrates a color.

How is that?

The water molecule is made up of two atoms of hydrogen and one of oxygen.

The hydrogen atoms are very lightweight, but their bonds are very strong. Imagine ping pong balls connected by metal springs. It doesn’t take much to start water molecules vibrating.

In basic terms, light itself is vibrating electromagnetic energy. Different colors have different vibrational frequencies.

Water molecules vibrate fast enough to reach the lower frequencies of visible light, where they absorb red light, so we see the blue light that remains.

This effect is too faint to notice in your glass of water. But it’s already visible in a bathtub or swimming pool, and gets more pronounced when the effect is amplified, as light passes through more and deeper water.

Scuba divers know this well. Past a certain depth, blue becomes the only color—till they turn on a flashlight, which reintroduces full-spectrum white light, and all the colors appear again.

There are other molecules that vibrate, but none that can reach into the visible light spectrum. Only water is true blue.