Why is the water in the 28 th Law blue?

The reason is simple and a little unexpected-because the water is really blue! If your bathtub is deep enough, you can see deeper blue-such as the blue of the sea. Attached is the UV-visible diagram of water to prove my conclusion [1]:

Ultraviolet-visible absorption spectrum of water in visible region. It can be seen that the absorption of water in the red area is obviously higher, so the water should appear blue.

It is generally believed that water is obviously colorless! For example, if I take a glass of water, it looks colorless. That's just because the water we see in our daily life is too shallow! So blue is not obvious enough. If you have a deep swimming pool with white tiles, you can see that the water is blue when you fill it with water.

Why is water blue? This is because water absorbs red light slightly-this absorption comes from the molecular vibration of water. Generally speaking, molecules vibrate in the infrared region and will not be absorbed in the visible region; However, if a plurality of molecular vibration modes overlap, the visible light region can be reached. For example, gaseous water molecules have three vibration modes at wave numbers 3650, 1595 and 3755 (

,

,

)。 For example, in liquid water, five vibration modes are superimposed (

), its corresponding absorption is at 660 nm[2], which is the red light region. Other combinations will also have different absorption wavelengths, but the more times of superposition, the lower the probability of occurrence. Because blue light has higher energy and needs to be superimposed more times, the absorption of blue light region is much lower than that of red light region [3]. After absorbing more light of other colors, it will appear blue; Because the absorption rate is still relatively low (because several vibration modes are superimposed together), the color is very light.

The overtone absorption will be in the visible region, and the higher the order, the lower the probability of occurrence. Therefore, the absorption probability of the blue light region with higher energy is much lower than that of the red light region, resulting in the ultraviolet-visible spectrum shown in the first picture.

In addition, can the water capacity of the bathtub at home meet the depth of color? According to the UV-VIS data I posted, the absorption coefficient above 700nm is about 3/m, so the corresponding attenuation length is about 33 cm. So as long as the water depth reaches more than 33 cm, you can see a very light blue; The absorption coefficient near 630 nm is about 0.3/m, and the corresponding attenuation length is 3.3 meters. Then if you want to have a more obvious blue color, the depth needs to be more than 3 meters.

Summary: Pure water itself is blue, and the water you usually see is just too small, and the blue is too light to see. And your bathtub is deep, and you can see light blue. If there is more water, such as the sea, you can see deeper blue (of course, the sea water needs to be purer).

In my other answer, many people are arguing whether the sea is blue or not because of what I call molecular vibration absorption or Rayleigh scattering. I want to quote from the popular science website of the University of California, Santa Barbara (UCSB) about why the ocean is blue [4]. They said:

Because of scattering, the ocean is not blue. On the contrary, due to selective absorption, the ocean is blue (as is pure water). In visible light, the longest visible wavelength is absorbed the most (red-orange, 600-700 nm), then the middle visible wavelength (yellow-green, 500-600 nm), and finally the short visible wavelength (blue-purple, 400-500 nm). This explains why very pure water, without many particles, looks dark blue.

My simple translation: the ocean is not blue because of (Rayleigh) scattering; It appears blue because of its absorption spectrum-the same reason that water appears blue. In the visible region, red light and orange light with long wavelength are absorbed the most, followed by yellow light and green light with medium wavelength, while blue light and violet light with short wavelength are absorbed the least. This also explains why very pure water without any particles appears dark blue.

Besides, some people think that the ocean is blue because it reflects the blue of the sky. A popular science article by a professor of oceanography mentioned [5]

A common misunderstanding is that the ocean is blue because the sky is reflected on its surface. This is not true, but people believed it decades ago. The real reason why the ocean is blue is because water, pure water, is blue.

A common misunderstanding about why the ocean is blue is that the surface of the ocean reflects the blue of the sky. Everyone thought so decades ago, but today we know it is wrong. The real reason is that the sea is blue, because pure water is blue.

So UCSB and the professor of oceanography have the same explanation as me-that is, the sea is blue, mainly because of the absorption of visible light caused by the overtone of molecular vibration mentioned in the article.

In addition, the blue spring in New Zealand is the purest stream in nature at present, and it is blue [6].