Water Is Clear to Our Eyes but Not to Most Light

Water lets visible light pass much more easily than many other electromagnetic wavelengths, so it looks clear to us while absorbing much of the radiation outside visible light.

Physics

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Water is transparent only to a limited part of the electromagnetic spectrum. Visible light falls inside a relatively low absorption window, which is why we can see through clear water, although different visible wavelengths are absorbed by different amounts. Outside that window, absorption becomes much stronger in several regions. Infrared is strongly absorbed through molecular motions and vibrations, while ultraviolet absorption also becomes stronger toward shorter wavelengths. The exact amount of transmission depends on wavelength, water purity, and how far the radiation travels through the water. This makes the everyday appearance of water a surprisingly narrow slice of what water does to electromagnetic radiation. The idea is also useful for understanding underwater optics: the light that travels farthest through natural water is not simply every color equally, but a limited range within the visible region.

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