Why do things turn dark when wet?
In: Chemistry
The smooth liquid coating causes the surface to go from rough (microscopically) to smooth (due to the surface tension of the liquid), which changes the way light reflects off the object.
With a rough matte object, incoming light diffuses, or bounces out equally in all directions. But with a shiny object, the reflected light all bounces out in a single, focused direction (assuming you have a single bright light source).
So if an object gets wet, and goes from matte to shiny, the distribution of the light leaving the object becomes less uniform; more of it is now heading in a particular direction (the reflection vector from the main light source), and less of it is now heading in all the other directions. If less light is heading in most directions, then from those directions, the object will appear darker.
Many people will say to you that it’s bacause light bounces a bit less from a wet rock, and a bit more from a dry one. That’s not true.
When the rock is wet, light does not bounce any “bit less” . It bounces just as much, only, it bounces a bit more in one “favourite” direction, and bit less in all other directions.
If that one favourite direction is right toward your eye, then a lot of bounced light hits your eye, and you see the rock brighter. Did you notice that wet stuff is not only bit darker, but also a lot more shiny? That’s why. When you see the shine, it’s because when you are in the way of all the light bouncing in its favourite direction.
But this means there’s less light bouncing in all other directions. So, whoever is not in the right spot to see the shine, sees the rock a bit darker
Water is like a light tube, and lets the light bend around fuzz, dust, which make things look brighter.
It lets the light go deeper into something, even a “flat surface”. More light is absorbed rather than reflected, so it’s darker.
All the dust, other small particles and roughness of the surface have a lot of facets that bounce light into your eyes. Weting the surface makes it more smooth. So the light bounces more uniformly in one direction. So it looks darker, but also shinier if you angle it right!
Basically, black means less colors being reflected. Light absorbs the lighter colors like yellows and greens first. So you see the stuff while that’s being absorbed
why it is tagged as `Chemistry`?
The best explanation can be done with a wet paper towel. If you wet a small spot on a paper towel and look at it normally, the wet spot looks darker. If you hold the towel up to a light, you’ll see that the wet spot is actually brighter than the rest. This mean that more photons are passing through the wet spot, meaning fewer photons are reflecting off of the wet spot. Since we perceive reflected photons as the “brightness” of something, the spot appears darker.
When things are dry light bounces off them in lots of directions so you tend to see light from all directions bounced back in all directions.
When things are wet the light tends to bounce off in straighter lines (because the surface of water is smoother). When you catch the angle opposite bright light sources you see loads of the total light hitting the surface (so it looks bright). Everywhere else you see much less light because most of it is bouncing off in the few bright directions.
The total amount of light coming off the surfaces is essentially the same. When things are wet some of the angles are very bright while most is dark. When they are dry all angles are evenly quite bright.
Interestingly the amount of perceived darkening is directly related to how rough the original surfaces are. For example polished surfaces don’t get darker at all.
There are more complex interactions at play in most surfaces to do with the surface of the water and the surface of the underlying material, the colour of light, the Fresnel effect and the rate at which light is actually absorbed by a material. But you could study for years and you still wouldn’t get to the bottom of it all.
Alrighty. Let’s stick to rocks as a reference since everyone who’s ever lived has a very firm grasp of rocks. With that, most of us have also washed a stone or two and witnessed the very phonmena you are asking about.
The big principal we are dealing with is how light scatters off of a surface as it is reflected, so now I’ll chew that into toddler food for you.
A stones surface, if we used a magnifying glass to look, will look very rough. That roughness, with it’s irregular scratches and pockmarks doesn’t reflect light all in the same direction. The surface acts like a pile of table salt, it scatters the light everywhere, because every angled surface of every imperfection on the stones surface will reflect light in a different direction. This also means that no matter where you are looking on the stone it looks “brighter” because light is reflecting back from every portion of the surface back to the eye.
Now if we took sand paper and sanded the surface, in only one direction, it would remove many of the irregularities. It would replace the random surface scratches and pockmarks with a series of uniform scratches. So now that the scratches are all in the same direction the light isn’t scattered as much, so the surface becomes more uniformly reflective. So, the total amount of surface area reflecting light is less, but it’s reflecting the light more like a mirror does now, so our imagine of the rock surface is also clearer.
Roughly the same thing happens when we get the rock wet. The water fills in all the voids, as if they were sanded off, and the light scatters less. So now the surface is reflecting less light, but the light it reflects is also more uniform. That’s why a rock with, let’s say rings, looks darker and you can see the rings better. The water “smoothes” the surface and improves the “image quality” if you will.
My mouth is so sore now from all that chewing 😭.
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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