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News and Features

October 2022

Optics

The Color of Water

Figure 1:  Crater Lake in Oregon is more blue than the sky. [USRA]

Water is Blue

The color of water is in the news. Jefferson Public Radio’s web site published an article about research at Crater Lake in Oregon, and the National Park Service near Seattle is warning hikers to avoid ice that is not blue.

The JPR article reminds us that water is blue:

Keeping Tabs On Crater Lake
Juliet Grable, JPR, 2022-Sept-11

Notice how blue the water is in the first photo of the article, and in all the photos actually. More information about that research can be found in the following report:

Limnological Monitoring Program
National Park Service, 2020

The water of Crater Lake has been so blue because it is so deep and so pure. It is the deepest lake in the United States, with no river runoff, formed in the cone of a volcano. Boating is only allowed on official research craft, and on a single tour boat that ferries tourists around the lake.

Figure 2:  The one and only tour boat plying the blue water of Crater Lake. [Wiki]

Figure 3:  Crater Lake, with patches of snow on the shore, viewed from hiking trail on Garfield Peak. [Wiki]

Cleaner water is naturally blue, because it absorbs other colors, while scattering blue (instead of absorbing the blue light). The further that light travels in cleaner water, the more blue it becomes. With Crater Lake being so deep, light has plenty of water volume to scatter in.

If water has more impurities, the water will appear to have colors that are introduced by the impurities. For example, dirt suspended in water can make the water brown, algae can make water look green, etc. In those examples, the introduced particles absorb blue light and reflect other colors (like they do on land), counteracting how water does the opposite of that.

It's not that those types of impurities add other colors. Rather, they take away blue light which would otherwise be scattered instead of absorbed. That reduces the total amount of light that penetrates the water. Light can travel further in cleaner water, and thereby become more blue.

Blue is the intrinsic color of water. The colors we observe in the world are the colors of light that objects reflect (or disperse) instead of absorb. For example, green leaves appear green to us because the green leaves are absorbing all the other colors of light, reflecting only the green light that strikes the “green” leaves. Likewise, cleaner water disperses blue while absorbing the other colors of light, if there is enough water to absorb the other colors and disperse the remaining blue light.


Absorption Coefficient

Absorption coefficient is a measure of the distance (how far) a color (wavelength) of visible light (or any electromagnetic radiation) can travel in a medium before being absorbed by the medium. In this case, water is the medium.

The lower the absorption coefficient, the further that color can transmit into the medium.

This graph shows absorption coefficients for different colors (wavelengths) of light in water:

Figure 4:  Absorption coefficients for different colors of light in water. Wavelengths/frequencies in the visible light range are displayed with their respective colors. Absorptance coefficients of wavelengths can be found in Table 3 of [Pope & Fry]. RGB colors of wavelengths can be generated with [λtoRGB]. Those two sources (or similar sources) can be used to generate this graph.

Most transmittance (least absorption) of visible light through water is in the violet and blue wavelengths. The area depicted gray on the right of the graph is the beginning of the infrared (IR) spectrum, showing higher absortance than the visible light wavelengths.

If the water is heavy water (with a neutron in each hydrogen atom instead of no neutrons), the absorptance coefficients are red-shifted into the IR spectrum:

Figure 5:  Absorption coefficients of regular water and heavy water.

That would make heavy water more transparent than regular water, in the visible light range.

Heavy water occurs naturally as a very small fraction of regular water. Extracting it from regular water is expensive, and usually only done in small quantities.

Figure 6:  Sample of heavy water. [Wiki]

For wider IR spectrum range transmittance of regular and heavy water, see Figure 1c, in Max & Chapados.


Snow and Ice

The solid phase of water is snow and ice. And just like liquid water, snow and ice are intrinsically blue, if there are not enough impurities to absorb the blue as explained above for regular water.

Figure 7:  Snow and ice are naturally blue, underneath any surface dirt and debris, for example in broken off pieces. [Wiki]

If you dig a hole in clean snow, it will look blue inside:

Figure 8:  A core hole in snow. [Wiki]

Ice caves are naturally blue, even when lit with standard artificial lighting (if the lighting is not supposed to be colored):

Figure 9:  Ice tunnels in Iceland. [IcelandUnlimited]

Holes in snow and ice can form naturally, for example with melt water creating streams:

Figure 10:  Raging stream (lower left) flowing from under the snow and ice of Morteratsch Glacier in Switzerland, August 2021. [Wiki]