Light propagation in water

[Itoero]

How does light propagation in water works?

[Strange]

In what sense? It works as in any other medium: the light occasionally interacts with the molecules on the way through. 

Is there some specific aspect that is not clear?

[Moontanman]

The water filters out colors, red first blue or violet goes deepest.. I have cut myself while diving and the blood was fluorescent green, really odd looking.. 

[Strange]

Also, are you looking for a classical (EM wave) or a photon-based description?

[swansont]

14 hours ago, Itoero said:

How does light propagation in water works?

This is increibly vague. Please be more specific.

[Itoero]

21 hours ago, Strange said:

In what sense? It works as in any other medium: the light occasionally interacts with the molecules on the way through. 

How is that possible? Water exists mainly our of H20 molecules. Photons can't travel through H2O-molecules without interacting with them. Light is propagated in water via absorbing and reemitting of photons. 'The fact' that absorbing/reemitting of photons doesn't go instantaneously is why the speed of light in water is slower then c. A medium like water is really a collection of a huge number of tiny electrons and protons/neutrons.

I asked this because I wonder how the absorbing/reemitting works.(which causes light propagation)

And I'm wondering if the speed of light slows down, if you go deeper in an ocean.

Edited October 20, 2018 by Itoero

[Strange]

5 minutes ago, Itoero said:

How is that possible? Water exists mainly our of H20 molecules. Photons can't travel through H2O-molecules without interacting with it. Light is propagated in water via absorbing and reemitting of photons. 'The fact' that absorbing/reemitting of photons doesn't go instantaneously is why the speed of light in water is slower then c.

So you ask how it is possible and then give the usual (simplified) explanation of how it is possible!

5 minutes ago, Itoero said:

I asked this because I wonder how the absorbing/reemitting works.(which causes light propagation)

It is complicated because quantum theory is probabilistic. This means you can't say what specific path a particular photon follows or which atoms it interacts with. (Which is why it easier to think about this classically.) So to work out what happens you have to consider every possible path a photon could take - every atom it could interact with, and every angle it could be re-emitted - you then find that all these different paths interfere either constructively or destructively, giving you the most probable outcomes. It turns out that the most probable outcome corresponds to the classical wave description. (Feynman did some excellent lectures to a lay audience on QED, the theory that explains all this.)

Your "causes light propagation" is the wrong way round, I think. The light propagates normally, in between the interactions with the atoms. There is a lot of space between the water molecules where the light/photons propagate normally, with only occasional interactions.

Quote

And I'm wondering if the speed of light slows down, if you go deeper in an ocean.

Yes. Classically because the refractive increases with pressure (because the density increases). From the photon point of view because the molecules are closer together so the interactions are more frequent. 

[swansont]

Light is only going to be lost from the propagation if the medium has a resonance at the appropriate frequency, and the light can be removed, or emitted in another direction.

[Itoero]

On ‎20‎/‎10‎/‎2018 at 8:56 PM, Strange said:

It is complicated because quantum theory is probabilistic. This means you can't say what specific path a particular photon follows or which atoms it interacts with. (Which is why it easier to think about this classically.) So to work out what happens you have to consider every possible path a photon could take - every atom it could interact with, and every angle it could be re-emitted - you then find that all these different paths interfere either constructively or destructively, giving you the most probable outcomes. It turns out that the most probable outcome corresponds to the classical wave description. (Feynman did some excellent lectures to a lay audience on QED, the theory that explains all this.)

Thanks for that explanation.

On ‎20‎/‎10‎/‎2018 at 8:56 PM, Strange said:

Your "causes light propagation" is the wrong way round, I think. The light propagates normally, in between the interactions with the atoms. There is a lot of space between the water molecules where the light/photons propagate normally, with only occasional interactions.

According to what I read hydrogen bonds in water (which cause the cohesion) are about 0,197 nm to 0,3 nm. And a watermolecule's diameter is about 0,275 nm. Isn't this a small 'intermoleular' distance compared to the size of the molecule? IIs the size of the nuclei related to the diameter?

 

On ‎20‎/‎10‎/‎2018 at 8:56 PM, Strange said:

Yes. Classically because the refractive increases with pressure (because the density increases). From the photon point of view because the molecules are closer together so the interactions are more frequent. 

The reason light can't reach the ocean bottom is often said to be due the interaction of photons with opaque particles. But since intermolecular distance (hydrogen bonds)keeps growing smaller, shouldn't light in theory reach the oceaan bottom yet there is not enough to observe/measure it?

Since the distance between watermolecules grows smaller you need more and more power to shine light through water. This causes a 'limit' related to the depth of the oceans where Bioluminescence is still possible/doable/lucrative.

Edited October 23, 2018 by Itoero

[swansont]

1 hour ago, Itoero said:

 

According to what I read hydrogen bonds in water (which cause the cohesion) are about 0,197 nm to 0,3 nm. And a watermolecule's diameter is about 0,275 nm. Isn't this a small 'intermoleular' distance compared to the size of the molecule? IIs the size of the nuclei related to the diameter?

Physical size isn't the issue. Light propagates in a bulk material if there are no resonances available to allow for the absorption of the light. That's an issue of energy states, not sizes.

1 hour ago, Itoero said:

The reason light can't reach the ocean bottom is often said to be due the interaction of photons with opaque particles. But since intermolecular distance (hydrogen bonds)keeps growing smaller, shouldn't light in theory reach the oceaan bottom yet there is not enough to observe/measure it?

Light attenuation follows the Beer-Lambert law, which is exponential.  if you have attenuation by 10 or 20 orders of magnitude, the distinction of whether you have 10 photons per unit area, or 1, or if you're to the point where you expect an even smaller signal, is unimportant.

1 hour ago, Itoero said:

Since the distance between watermolecules grows smaller you need more and more power to shine light through water. This causes a 'limit' related to the depth of the oceans where Bioluminescence is still possible/doable/lucrative.

The distance between the molecules doesn't shrink all that much and, as I said, isn't the primary issue. 

"The bulk modulus of elasticity of water is approximately 2.2 GN/m2, which corresponds to a 0.05% change in volume for a change of 1 MN/m2 in pressure."

https://www.okan.edu.tr/uploads/pages/ce204-fluid-mechanics/Class2_25_02_2010.pdf