Why are some things darker when wet? • Hacker News Board

Re: Why are some things darker when wet?

#51

Post by CrazyStat » Thu, Jan 09, 2020, 4:54 PM UTC

maskedinvader wrote: post #45

CrazyStat wrote: post #44

Earlier quoted context omitted.

How does absorption and reemission preserve phase information? What about total internal reflection? Why does the light travel through the entire body of a glass prism without issue and then suddenly get absorbed and reemitted at the surface? Glass (and water) are transparent specifically because they don't absorb photons in the visible spectrum, so how is it that they absorb and reemit?

IANAP but from my limited understanding (mostly reading Richard Feynman) light beam = stream of photons, they don't travel through entire body of glass prism without issue, they get absorbed and re emmitted along the way too (Which is what explains the different speed of light in different mediums)

I'm not convinced but even if we accept that explanation, why do all the atoms along the way emit the light in the direction it was travelling and then the ones at the surface suddenly emit it in a different direction?

Re: Why are some things darker when wet?

#52

Post by danans » Thu, Jan 09, 2020, 5:03 PM UTC

SamBam wrote: post #40

Asterate wrote: post #13

Earlier quoted context omitted.

Refractive index of a material is the ratio between speed of light in vacuum and speed of light in that material. Light tends to bounce back when encountered with a sharp change in refractive index. Being wet means that there's a water film covering the material, mediating the change in refractive index, resulting in reduced reflection. Apart from index mediation, the water film does something else. For rough/fibrous…

> When a water film is present, the surface becomes smooth, and the reflection will be specular, and only visible in one direction. So in most directions, the material will appear darker. Yes, that's precisely the part I was addressing in my last paragraph. If it's specular reflection, then in "most directions" it will appear darker, as you say, but in one direction it should appear brighter, even shiny. But I've nev…

> But I've never seen a damp rag be shiny in any direction.

The surface of a cloth is still much less smooth than a polished car, cd, etc, so is it possible that the specular reflection happens, but at a contrast that is too low for our eyes to detect, or in enough disjoint sections that we can't perceive it as a single effect?

There are plenty of phenomena that fall out of the range of our unaided perception.

We would, however, probably observe the specular reflection of instead of water we used a thicker transparent liquid, like clear glue.

Re: Why are some things darker when wet?

#53

Post by whatshisface » Thu, Jan 09, 2020, 5:04 PM UTC

melon_madness wrote: post #42

tigershark wrote: post #30

Earlier quoted context omitted.

That doesn’t look right at all to me. Why the photon is then re-emitted in exactly the same direction that it would have if it was reflected rather than a random direction?

Conservation of momentum, I think

If the emitted photon always had the same momentum as the absorbed photon, the light would go right through the mirror, passing on its way without changing direction.

Re: Why are some things darker when wet?

#54

Post by hn3333 » Thu, Jan 09, 2020, 5:06 PM UTC

CrazyStat wrote: post #51

maskedinvader wrote: post #45

Earlier quoted context omitted.

IANAP but from my limited understanding (mostly reading Richard Feynman) light beam = stream of photons, they don't travel through entire body of glass prism without issue, they get absorbed and re emmitted along the way too (Which is what explains the different speed of light in different mediums)

I'm not convinced but even if we accept that explanation, why do all the atoms along the way emit the light in the direction it was travelling and then the ones at the surface suddenly emit it in a different direction?

Probably it's just how the math works out. The atoms emit in all directions, however the waves traveling in all the other non-correct (according to reflections and refraction laws) directions cancel each other out, and only the ones with the new correct direction are still there / visible.

Re: Why are some things darker when wet?

#55

Post by elcomet » Thu, Jan 09, 2020, 5:07 PM UTC

aphextron wrote: post #18

This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually "bounce" off of a surface like little balls. They are always absorbed and re-emitted. The actual photons that hit an object are not the same ones that eventually enter your eyes. The atoms in an object are stimulated by photons hitting them to then emit a new…

I don't think it's hand wavey. It's just a different (simpler) model. To describe this phenomena (wet things are darker), you don't need to go into details about photons, or quantum mechanics or whatever.

Geometrical optics is really sufficient here, it explains it perfectly, and is much more intuitive than other more complex models.

Re: Why are some things darker when wet?

#56

Post by hn3333 » Thu, Jan 09, 2020, 5:12 PM UTC

Perhaps a related thought, but I find it fascinating that everything would reflect like a perfect mirror if there weren't these small imperfections in / the roughness of surfaces.

Re: Why are some things darker when wet?

#57

Post by jimmyseoul » Thu, Jan 09, 2020, 5:13 PM UTC

I thought this was going to be kinda dumb but this was actually really neat. You never really take the time to understand the everyday things such as objects getting darker when wet.

Re: Why are some things darker when wet?

#58

Post by Ceezy » Thu, Jan 09, 2020, 5:15 PM UTC

Water is not transparent, not 100%. So it just absorb light enought that you would noticed it whent something is wet

Re: Why are some things darker when wet?

#59

Post by chemeng » Thu, Jan 09, 2020, 5:20 PM UTC

whatshisface wrote: post #49

aphextron wrote: post #18

This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually "bounce" off of a surface like little balls. They are always absorbed and re-emitted. The actual photons that hit an object are not the same ones that eventually enter your eyes. The atoms in an object are stimulated by photons hitting them to then emit a new…

Having thought about it, I'm not sure if I agree. Light can reflect off of transparent crystals, which do not have the right electron energy levels for absorption (that's why they're transparent.) It's possible that the Feynman diagram of a reflection might involve the photon "going away and another one replacing it" (I don't know what the diagram looks like), but Feynman diagrams cannot be interpreted as sequences o…

They are not the same photons. They are emitted by the electron changing energy states after excitation of the incoming photon. But your intuition that there is some quantum weirdness is correct. Quantum Electrodynamics is the theory you're looking for. It unifies quantum mechanics and special relatively in the context of light/matter interaction. Given you seem to be familiar with Feynman. You're in for a treat! His book on QED is meant to be an accessible explanation.[1]

[1]https://en.m.wikipedia.org/wiki/QED:_The_Strange_Theory_of_L...

Re: Why are some things darker when wet?

#60

Post by ryanmercer » Thu, Jan 09, 2020, 5:23 PM UTC

I've always just assumed the increased surface area from expansion results in more light getting absorbed, not unlike how more particulate in the air (dust, smoke, sand) reduces the amount of light hitting something.