Why some things are darker when wet (1988) | Hacker News Reader

Why some things are darker when wet (1988)

fermatslibrary.com

116 pointsluisb14 comments

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.

That is an excellent summary. It was like an answer that failed to produce any questions from me and I feel like I understand it now.

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?

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.

Oil also makes paper transparent. Here is why:

http://www.businessinsider.fr/us/why-does-grease-make-paper-...

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

The scene: https://www.youtube.com/watch?v=S3MEj9f8PXY&t=1m28s Episode (Season 7, 1995): 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.

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

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

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.

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

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

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.

Probably the best approximation of this right now is the simplest: make the clothing of a low-reflective (black) material.

You can try a version of this at home: the discoloration of a black fabric is much less visible than that of a white fabric. An engineered low-reflective material should work better something merely dyed black, though.

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 :)

Reply on news.ycombinator.com