Physics trivia

Color of water

Water is a partially transparent substance and capable of absorbing longer wavelengths of light, such as red, orange and yellow.

The bluish color of water arises from the absorption of visible light. Water can absorb wavelengths close to red and orange.
What is the color of water?

Water is not capable of emitting visible light . Like many substances and bodies around us, such as air, walls, clouds, trees, among others, it only reflects the light that falls on it, therefore, we say that water is a secondary source of light.

Also called illuminated bodies , secondary sources of light have their color defined by the light that illuminates them , and water is no exception. The bluish hue of water arises from its ability to absorb the portion of light corresponding to the longest wavelengths of the visible spectrum: the “warm” colors, such as red, yellow and orange. We call all the colors that we can see the visible spectrum .

Sunlight is polychromatic , that is, in it we can find all the wavelengths corresponding to visible light , between red and violet . When visible light travels through great depths of water, absorption by the aqueous medium becomes significant, and other wavelengths, such as blue and violet, are reflected by water molecules in a process called scattering . A natural consequence of this is that when we look at water in a glass, it appears almost transparent to us.The deeper the water, the more light it will absorb. Shallow water therefore tends to be more transparent.

The absorption of light by water depends on the wavelength of light. In pure water, for example, red light can reach depths of up to 15 m; yellow light , up to 30 m; orange light , in turn, travels up to 50 m before being absorbed. Blue light is the one that can penetrate the furthest, illuminating up to 100 m depth. From that distance, however crystal clear the water, we wo n’t see any visible light .

Some impurities, such as microorganisms and mineral salts, diluted in water can further decrease light transmission . These impurities can also absorb light differently, changing the hue of the water. In coastal regions, for example, it is common for sea water to have greenish or even brown tones .

Another interesting factor is that when stirred , water tends to reflect more light . This occurs because the agitation of the water favors the dilution of atmospheric gases in its interior, forming small bubbles. This dilution of gases decreases the absorption of light by the water, making the reflection of light more evident. That’s why the crests of the waves have light tones , close to white. Similarly, the white color of the ice is justified. When water starts its melting process, freezing starts from the outside in ., trapping gas molecules in the central regions of the ice.

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