
Why some things are darker when wet (1988) - luisb
https://fermatslibrary.com/s/why-some-things-are-darker-when-wet
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whatshisface
Summary: The absorbing surface gains its lightness or darkness from the
fraction of light rays that are absorbed when they hit it. When present the
liquid surface reflects a certain fraction of exiting light rays back down to
the material, giving them multiple chances of being absorbed (one chance for
every time they are reflected back down). Chances are involved because the
material's surface is rough, reflecting the light that is not absorbed back
out in random directions, some of which strike the back of the fluid-air
interface in a way that sends them back down to the material.

~~~
rcthompson
So this means that a damp fabric should appear darker than the same fabric
when dry, _but_ if you and the fabric are both completely submerged in water,
then the darkening effect should go away and the fabric will appear as light
as if it was dry, since there is no water surface on the fabric itself?

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onemoresoop
This is interesting as I deal with this quite often while painting and it
causes a few annoyances here and there as the true color is the dry color.
When experimenting with finishes, matte vs glossy I noticed how drastically
the color changes. When applying a glossy finish the color changes to its wet
version as the matte does the opposite. I intuitively concluded that it's got
to do with the last layer on the surface and how light is reflected by it.

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GorgeRonde
Oil also makes paper transparent. Here is why:

[http://www.businessinsider.fr/us/why-does-grease-make-
paper-...](http://www.businessinsider.fr/us/why-does-grease-make-paper-
translucent-2014-4)

~~~
croisillon
There is a Simpsons' episode where Bart eats such a fat donut that scratching
it against the kitchen wall releases daylight through it

~~~
EForEndeavour
The scene:
[https://www.youtube.com/watch?v=S3MEj9f8PXY&t=1m28s](https://www.youtube.com/watch?v=S3MEj9f8PXY&t=1m28s)
Episode (Season 7, 1995): [https://en.wikipedia.org/wiki/King-
Size_Homer](https://en.wikipedia.org/wiki/King-Size_Homer)

Homer declines a Krusty Burger, whereupon Bart rubs it on the restaurant wall,
turning the wall transparent with the sheer amount of oil in the burger. This
convinces Homer to eat it (as he's on a mission to gain weight to get on
disability and work from home), and causes a bird to fly straight into the
newly transparent patch of wall.

~~~
croisillon
Thank you, my memory did not betray that much ;)

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theoh
This paper is referenced by the following more recent paper about actually
simulating the effect:
[http://graphics.cs.yale.edu/site/sites/files/Wet.pdf](http://graphics.cs.yale.edu/site/sites/files/Wet.pdf)

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novia
This is a question I asked my parents as a child, when I noticed that the
driveway would change colors after pouring water onto it. I really wish my
parents had known such things. It could have easily stoked my interest in
physics.

~~~
jolmg
Lately, because of a car situation, I've been watching car mechanic videos
online. There's some old videos, like 1937 old, made by Chevrolet, that
explain really well how different mechanics of a car work. One explains a
transmission starting from levers[1], how they work to give mechanical
advantage, and how gears are basically spinning levers. I ended up feeling
like I could make a basic transmission if I only knew how to make metal pieces
in the shapes I want. From there I've been looking at people disassembling
automatic transmission and explaining the different parts. I gotta say,
learning how these machines work mechanical advantage to something so
convenient has really raised my interest in physics and mechanical
engineering.

I don't remember simple machines[2] being something so interesting back in
physics class. I don't blame the teacher, 'cause she was always enthusiastic
about physics. I wonder why this is more interesting now than then. It's
probably just that, now that I'm older, I can understand complex applications
of those fundamentals being taught at school.

[1]
[https://www.youtube.com/watch?v=JOLtS4VUcvQ&t=26s](https://www.youtube.com/watch?v=JOLtS4VUcvQ&t=26s)

[2]
[https://en.wikipedia.org/wiki/Simple_machine](https://en.wikipedia.org/wiki/Simple_machine)

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tlb
Can a fabric be engineered that doesn't get darker when wet?

One example is retroreflective fabric, which reflects light back where it came
from and therefore shouldn't be affected by internal reflection of a water
layer. But retroreflective fabric looks weird.

One idea is a dark material with lots of little pits, each with a white
bottom. Any light reflected by the white bottom and not hitting the walls of
the pit should be perpendicular enough to the surface that there won't be
internal reflection. The overall albedo should be insensitive to wetness.

It could revolutionize clothing for people who sweat too much.

~~~
luisb
I know there's research being done on fibres deliberately engineered to change
colour depending on pH levels to counterbalance the darkness.

For the sweat problem you could have a retroreflective layer on the inside of
the fabric :)

