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Sunlight reaches the surface of the ocean and refracts. So it is still there. And its speed is about $225000$ km/s in water which is still incredibly fast. Light is a massless electromagnetic wave. So why does it fade away to a complete darkness at very deep parts of the ocean?

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    $\begingroup$ Not very deep. Typ. 200m, 1000m at most: oceanservice.noaa.gov/facts/light_travel.html $\endgroup$
    – Karl
    Jul 25, 2023 at 19:42
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    $\begingroup$ Why can't sunlight reach the other side of an aluminium sheet? Transparency isn't a "on or off" proposition. Some things are more transparent than others, but other than perfect vacuum, nothing is perfectly transparent. $\endgroup$
    – Luaan
    Jul 27, 2023 at 9:51
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    $\begingroup$ Just to prompt some consideration of what might be happening. If you hold a sheet of paper up to the sun you can see light through it, if you hold a book up you can't. Why do you think this is? $\endgroup$ Jul 27, 2023 at 12:21
  • $\begingroup$ Scattering and absorption perhaps? $\endgroup$
    – my2cts
    Feb 21 at 13:09

4 Answers 4

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Light is an electromagnetic wave and when it passes through (cold) matter it can be absorbed via reaction with the electrons in the atoms that make up the matter. Typically, the absorption is an all-or-nothing process: an individual photon will interact with an electron (either one in an atom or molecule or, in the case of a metal, a 'free' conduction electron) and will be completely absorbed. To explain the process in detail would require a venture into solid state and quantum physics.

For most materials, this happens within a distance in the range of nanometres to millimetres, but even for materials that we typically think of as transparent -- such as air, glass, and water -- the absorption still occurs. Water is relatively transparent in the visible spectrum, particularly for blue light - see the graph below. But even in the purest water, a photon only has about a 50% chance of passing through 50 m of water without being absorbed.

So, for example, less than 1/4 of the light that enters water at sea level will reach 100 m depth, less than 1 millionth of it will reach a depth of 1 km, and by the time you reach the deep ocean at several kilometres, the chance of even a single photon arriving from the surface is vanishingly small.

Absorption spectrum of liquid water (source):

enter image description here

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    $\begingroup$ Just out of curiosity: Do you know what would be the least transparent material, or are there materials that absorp light so quickly that it is impossible to create a sheet thin enough that it lets light through? $\endgroup$
    – gnasher729
    Jul 25, 2023 at 12:36
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    $\begingroup$ @gnasher729 That probably depends a lot on the wavelength and can't be answered generally, at least not in a comment. Sounds like a good question on its own though $\endgroup$
    – Bergi
    Jul 25, 2023 at 13:38
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    $\begingroup$ @gnasher729 Single layers of graphene (one-atom thick) absorb 2.3% of incident light in the visible range ( ncbi.nlm.nih.gov/pmc/articles/PMC7917831 ). While 2.3% doesn't sound like much, it's a lot for a single atomic layer! $\endgroup$ Jul 25, 2023 at 13:52
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    $\begingroup$ A single layer of graphene is 5 orders of magnitude thinner than a human hair. Even 100 layers would block 94% of incoming light, never mind 100,000 layers. $\endgroup$
    – chepner
    Jul 25, 2023 at 19:01
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    $\begingroup$ My approximation was for the 'best-case' photon. The euphotic zone extends to about200 m. That is the maximum depth at which photosynthesis can occur - so some light can penetrate to that depth. The 'rule-of-thumb' of 50% at 50 m, is for blue wavelengths around 500 nm - red light fades much faster (as shown by the graph). In perfect conditions about 1/16th of those blue frequencies can get to 200 m but red light is totally extinguished within the first 50 m or so. Of course, other factors such as turbidity and sea-surface conditions can (and do) further reduce solar flux at any chosen depth. $\endgroup$
    – Penguino
    Jul 25, 2023 at 21:44
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Consider that a cloud is made up of transparent water droplets. When light interacts with a water droplet, it changes direction without being absorbed. However, the interaction with very many such droplets makes it so that eventually the light's direction is completely randomized. If the cloudbank is thick, it's much more likely that the light will be scattered back out of the top of the cloud versus being diffused to the bottom. This is why

  • in the inside of a cloud (including ground-level clouds, also known as "fog"), light appears to be equally bright in all directions, and
  • the tops of thick clouds are brighter than their bottoms.

So light can't penetrate a cloud, even though clouds are made of transparent scatterers. The same is true for a pile of snow on the ground.

In seawater, you have the same sorts of considerations. The direction-randomizing density fluctuations are still there, but their size and separation length scales are larger than for water droplets in air. But seawater is also full of stuff: suspended debris like sand or fish poop, and even microscopic organisms whose entire purpose in life is to absorb some of the sun's energy and turn it into food. Sea foam, where there are lots of very small bubbles, is white from above and dark(er) from below, for the same reason as clouds. But in sea water, the absorption distance is generally shorter than the scattering distance, so light is more likely to be absorbed than it is to scatter back up to the surface. This is why shallow seawater can be transparent — especially if the suspended matter has been cleared out by a healthy population of filter-feeding animals — while light doesn't leak back up from deep seawater in the way it does from clouds or snowbanks.

Any non-uniform material is going to become opaque to light when it's thick enough. Consider that the Sun's photosphere is a "thin" layer on the Sun's outer surface, "only" about 400km thick; on Earth the density of that layer would be a pretty good vacuum.

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    $\begingroup$ The top answer suggests it's dark in the deep sea because of absorption by water. This answer suggests it's due to absorption by suspended matter, combined with a long optical path length due to scattering. Which effect is dominant? How much light would reach through a theoretical column of 1 km of distilled water, vs. how much through 1 km of Vienna Ocean Water? $\endgroup$
    – gerrit
    Jul 25, 2023 at 7:14
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    $\begingroup$ (Correction: Vienna Ocean Water is actually pure water with no salt. I meant a comparison with real ocean water.) $\endgroup$
    – gerrit
    Jul 25, 2023 at 7:30
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    $\begingroup$ If you watch enough nature documentaries, you'll see the one about shallow-water dolphins who hunt by changing the dominant effect, stirring up turbid circles around clear-water fish, who panic and jump out of the turbid ring, into the mouths of the dolphins. My experience with swimming pools is that I can always stand chest-deep and see my feet on the bottom, while my experience with beaches is that I can almost never see my feet. This suggests it's a mistake to ignore the suspended matter, even though the water itself does have an absorption length which is small compared to a "deep" ocean. $\endgroup$
    – rob
    Jul 25, 2023 at 12:35
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    $\begingroup$ The discussion about scattering in clouds is correct but is not a significant contributor to bulk light attenuation in sea water. And while suspended particles will cause the light to attenuate faster, pure water will still fully attenuate all the light at sufficient depth $\endgroup$ Jul 25, 2023 at 17:17
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    $\begingroup$ @gerrit As a (rather inexperienced) diver, the answer is somewhat "a bit of both", but scattering is normally the dominant effect. There's a dive site called Silfra in Iceland, on the border of the two tectonic plates. The water there is so clear that you actually take special training for diving there, because you can see so much further than you expect to, and that has safety implications for buddies staying close together. $\endgroup$
    – Graham
    Jul 25, 2023 at 17:34
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Photons get absorbed on their way down, due to the finite interaction cross section $\sigma\;[cm^2/g]$ in a material of density $n\; [g/cm^3]$. The photon mean-free path is then $l=1/(n\sigma)$.
This has nothing to do with the speed of light, as the absorption of a bundle of photons is statistical, but steady-state.

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Photons that make up light collide with the water particles and are absorbed.

The attenuation $\mu$ per distance of water is approximately constant for a given wavelength.

The equation for the loss is exponential, $I = I_0 \exp(- \mu x)$, where

  • $I$ is the intensity of light, equal to energy per unit of area and time, and
  • $I_0$ is the initial intensity
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    $\begingroup$ If a photon loses energy, it becomes a photon with a different frequency. Your view predicts that white sunlight would become redder in the deep sea, as blue photons lose energy and are converted into red photons. But the opposite happens: seawater is much more efficient at absorbing red light, and the deep sea is blue. $\endgroup$
    – rob
    Jul 24, 2023 at 20:22
  • $\begingroup$ So this losing of energy should happen below 1 second. Because the light can travel 225000 km/s in water. $\endgroup$ Jul 24, 2023 at 20:56
  • $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Jul 25, 2023 at 7:33
  • $\begingroup$ @rob, I now have a new understanding of the term "deep blue sea" $\endgroup$
    – Jasen
    Jul 25, 2023 at 23:50
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    $\begingroup$ @Jasen Twenty years ago, I took a touristy submersible ride on a Caribbean reef. My eyes adjusted to the color difference, so I didn't notice it too much. My camera did not have any color-balance adjustment. The pictures came back impossibly blue. $\endgroup$
    – rob
    Jul 26, 2023 at 4:33

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