If water is transparent, why does the ocean appear dark?
Light that hits the surface of the water is scattered in directions other than where your eye is, so those parts of the ocean surface appear darker. And then there’s one part that directs the light *just right*: those are the shiny parts of the surface that gleam and sometimes ruin your beach photos.
For a similar reason, the layer of water on wet materials changes the amount of light reflected towards your eyes. Less light is reflected from those parts to your eyes, and so they appear darker.
Simplifying what [bibliophile785](https://old.reddit.com/r/explainlikeimfive/comments/e6czsd/eli5_why_do_things_turn_dark_when_wet/f9pb3v6/) [has already said](https://old.reddit.com/r/explainlikeimfive/comments/e6czsd/eli5_why_do_things_turn_dark_when_wet/f9pdjxl/).
When light encounters any boundary (air to thing, air to water, water to thing, etc.) some of it reflects and some of it gets absorbed. Now if you make the same object wet, light has 3 boundaries that it encounters (water / wet layer, thing, water / wet layer again once it gets reflected from thing), hence slightly more of it is absorbed and lesser of the reflected light meets our eye. So, the objects appears darker because of the lesser light it reflects.
I’ve seen two explanations:
1) Light can bounce around in the layer of water. More bounces -> more chances to be absorbed in the material. More light absorbed -> less light reflected to your eyes.
2) The rough surface of the material normally scatters light in all directions. The layer of water makes the surface smoother, more like a mirror. More of the light gets reflected in one direction, so the surface looks brighter from this direction, and darker from every other direction.
A wet cloth appears a couple darker because less light is reflected from a wet cloth. Any cloth is woven from a yarn or fibre. That fibre is in turn made of smaller micro-fibres. Light comes from the room lights, or from the Sun, and lands on the cloth. Some of the photons of light are absorbed, but some are reflected and land on your retina – and that gives you the sensation of seeing the cloth as having a certain level of brightness. But when the cloth gets wet, the water fills in the gaps between each individual strand of fibre, and also between each individual micro-fibre. When light falls on the wet cloth, some of it is now more likely to enter the water, and be bent away from your eyes. So some of the light that would have previously been reflected off the cloth back to your eyes, is now bent away.
Fewer photons of light get back to your eyeball, and so the wet cloth “appears” darker than the dry cloth. But as the water gradually evaporates, more and more light is reflected back to your eyeball, and you see the brighter colour of the fabric again…..
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