Why do things turn dark when wet?
In: Chemistry
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 😭.
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