Why is Rainwater Clear?

Clean water looks clear because it doesn't have any dirt or particles that make it cloudy or block light. Light can easily pass through it without getting absorbed or scattered, making it transparent and allowing us to see through it. However, the clarity of water is also influenced by the inherent properties of the water itself.

Clean Water and Its Clarity

Clear water is clear because of an intricate combination of physics, chemistry, and biology. At some level, this is just a coincidence. Every material absorbs different wavelengths of electromagnetic radiation to different degrees. The absorption spectrum of water is quite specific. For a given wavelength, the y-axis gives approximately the fraction of light absorbed in traveling through unit thickness of water. For a more intuitive way to look at it, the reciprocal of the number on the y-axis gives you the distance in meters that light of a given wavelength can travel without undergoing significant absorption.

Water's Absorption Spectrum

Here is how to read the absorption spectrum graph of water:

Visible Light: The visible spectrum falls between the electronic and vibrational absorption bands of water. This means that water is transparent to visible light, which is why we can see through clean water. Infrared Light: Moving to the near infrared, as little as a few centimeters of water is enough to absorb light. This is why the deep ocean floors are dark. Ultraviolet Light: In the ultraviolet region, the thickness that light can travel through is as low as a few nanometers, making it highly absorbed by water.

So, what is going on? There are different underlying atomic and molecular processes that give rise to absorption. These processes include:

Electronic Transitions: Molecules absorb a photon and move to a higher excited electronic state. Rotational and Vibrational Transitions: The electronic state remains the same but the rotational or vibrational state is changed. These transitions occur at lower energies than electronic transitions.

In the case of water, the visible spectrum lies between these absorption bands. This is not a coincidence. Our eyes evolved while we were still in water, so they are photosensitive to water-transparent regions of the spectrum. When we evolved out to land and became human, the spectral range did not change significantly. Our eye “optics” still consist of a lot of liquid, which is why we can see through water.

Advances in Research

For those interested in more scientific details of water and light absorption, you can find more of my answers related to topics of my research here.

Understanding the clarity and transparency of rainwater not only helps us appreciate the beauty of clean water but also underscores the importance of protecting our natural resources. By understanding how water behaves, we can develop better strategies for water conservation and environmental protection.