January 1, 2020

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Conclusion

Liquid water molecules are continuously interchanging: sometimes unconnected then other times interconnected molecule states.

This creates spaces between the water molecules through which light photons can pass.

Light moves at such high speeds and there are many photons in a beam of light.
Enough of those light photons are able to cross in a straight line to the other side of the water (and back) without hitting any atoms from any of the water molecules.

Water clarity is a function of homogeneity of the liquid (coffee is not transparent), the size of the individual elements making the liquid, and its depth (very deep bodies of water are not clear).

Why is water opaque in some phases and not in others?

Clear transparent water

One can see things easily through 1-2 feet of water but the same is not true of snow or fog.

How does that happen? Why is liquid water clear and transparent?

How do light photons get through and come back to us almost unchanged?

Hydrogen bonds

H2O with bonding between the hydrogen and oxygen atoms

1. First step to understanding water 

The first step to understanding the different phases of water is to appreciate the bonds that hold the oxygen and hydrogen atoms in water.

Those bonds can be as wispy as cobwebs or as strong as any structural molecular bond.

Water molecule

2. Unconnected water molecules

For liquid water, at any one split second in time, some of its molecules are not connected to any others.

In an unconnected water molecule, each hydrogen atom is turning its back to neighboring oxygen atoms.  Water molecules with two socially-distancing hydrogen atoms are – for that femtosecond (one-millionth of a nanosecond!) in time – unconnected.

Hydrogen bond network

3. Interconnected water molecules

Suppose that the hydrogen atom is connected through two arms. One arm is when the "parent" oxygen atom reaches out to its "baby" hydrogen atom.

The other arm is the "baby" hydrogen atom reaching out to a neighboring oxygen atom  (creating a hydrogen bond). The net effect is that the hydrogen atom is wedged between two oxygen atoms. All three are connected (see the two green arms).

hydrogen and covalent bonds
Keep in mind that - in liquid water - the hydrogen atoms are constantly transforming from a socially distancing atom to an atom that wants to connect. The two states interchange at an incredibly fast speed. Each femto second - or one billionth of a nanosecond - the connections take place and then disappear; back and forth.

Gibbon
Suppose the wires of the chainlink fence are made of extremely fine mesh, such that each fiber has a width under 0.1 mm (the limit of one’s ability to see).

The light will go through; however, the light photons that come back will seem to join together. With our normal eyesight, we would not be able to see the very fine wires. All the light reflecting back to our eyes from the gibbon would merge into one picture: the chain link fence would be invisible!

4. How is some material clear and transparent?

Basically, three things must take place for things to look clear and transparent:

  1. There is no refraction of light: The material is homogenous. When the material is heterogeneous, there is haphazard scattering. 
  2. There is no absorption of light: The light photons that hit electrons on their way through are not absorbed; they are reflected back to the viewer and/or find spaces between the molecules to go all the way through and come back undisturbed.
  3. The material is made of ultra-fine particles: The material is so fine and the particles are of such a small size, that the light that comes back seems to be a uniform beam (our eyes can’t distinguish 2 light rays separated by less than 0.1mm: they will look as one!)

Any material made up of homogenous nanoscale particles, even if they are closely knit, will seem transparent because the material is made up of such minute particles. The light coming back will seem to come back as one beam, rather than interrupted rays.

water emotions

5. Why is water clear and transparent?

Assuming the depth of the water is not too much, the light photons can find spaces between the water molecules to get through to the other side and then back to our eyes. 

Water molecules are intermittently unconnected at the femtosecond level! it is those spaces that light can find a path through and back!

Suppose one were a nanoscopic ant walking in between those water molecules.

To pass through to the other side of a water droplet would mean that the ant would have to be lucky enough for the water molecule to change to an unconected one as soon as the ant reached that molecule.

This would be the equivalent of a car crossing town and the traffic lights changing to green as soon as the car approached the light.

For light photons, this is exactly what happens:

water allowing light to go through
Light moves at such high speeds and there are many photons in a beam of light. 


Enough of those light photons are able to cross in a straight line to the other side of the water (and back)  without hitting any atoms from any of the water molecules.

Furthermore, water molecules are very small (each about 0.0000003 mm); thus, light coming back does not seem to have anything preventing it from going through and coming back undisturbed.

Picture Credits


  1. Horia Varlan. “See through raindrops on hibiscus petals”. Flickr – Photosharing! Taken on Dec 10, 2008.
  2. Horia Varlan. “Large rocks inside the crystal clear waters of the sea”. Flickr – Photosharing! Taken on Aug 5, 2009.
  3. Thomas Brueckner. Mundy2. hydrogen bonds. Flickr photo-sharing, taken on Oct 7, 2005.
  4. By bobyramone. Shutterstock ID: 63406936. water molecule. 
  5. Masakazu Matsumoto. Water, hydrogen bond network structure of water. Flickr – photosharing. Uploaded July 10, 2008. 
  6. Keith Roper. “Gibbon, San Diego Zoo, October 2013”. Flickr – Photosharing! Taken on Oct 9, 2013.
  7. Juman Hijab. Water in glass on the counter. Taken in December 2016.

References

  1. Martin Chaplin. Water Structure and Science: Hydrogen bonding in water (1). Updated Dec 18, 2019.
  2. Wang SF, Zhang J, Luo DW, Gu F, Tang DY, Dong ZL, Tan GEB, Que WX, Zhang TS, Li S,  Kong LB , Transparent ceramics: processing, materials, and applications. Progress in Solid State Chemistry Volume 41, Issues 1–2, May 2013, Pgs 20–54. 

Updated July 29, 2020.

About the author 

Juman Hijab

Juman has been in clinical practice as a physician for more than three decades. Her lifelong interest has been in the chemistry of life.



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