
Why are some things darker when wet? - aryankashyap
https://aryankashyap.com/why-are-some-things-darker-when-wet
======
aphextron
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 photon via the photo-electric effect,
which we perceive as light "bouncing".

~~~
tigershark
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?

~~~
mac01021
I think the two slit experiments indicate that a photon does exactly have a
direction of travel?

~~~
lonelappde
Only after it arrives, so the situation's a lot more nuanced than that.

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SamBam
> Once you spill water on the shirt, that part of the shirt is now covered
> with a thin film of water. So, any light which has to reflect off that part
> of the shirt has to go through water.

> Before water is spilt, 100% of the light travelling towards that part of the
> shirt will hit the surface. But now only a fraction of the light moving
> towards it will hit its surface. This is because the light now has a layer
> of water to go through. And due to the reflectance of water, not all light
> at the air-liquid-interface (border between air and water) goes through the
> water. Some of it is reflected.

This is not clear, or at least needs another step. It is saying that part of
the reason the shirt looks darker is because some of the light never had a
chance to reflect off the shirt, because it reflected off the water first. But
the observer only cares whether light is reflected at all, not whether it
reflected off the shirt or the water.

I assume the unwritten part is that this specular reflection is only
reflecting light in one direction, instead of diffusely, so if the observer is
not in that line of reflection, they won't see the light. But this explanation
seems wrong to me, as it implies that a wet shirt _will_ have some brighter
highlights at some angles, and yet I have never seen this.

~~~
iforgotpassword
Yes, wouldn't that only apply to soaking wet cloth? If it's just a little wet
it still looks darker but not shiny, and there is no specific angle you can
look at it and see the light source reflected. So it appears to still be
diffuse in that state. Where does the light go?

~~~
arh68
It goes through, doesn't it? I can't find hard numbers, but basically Wet
Clothes Don't Stop Sunburn. The UV rays don't bounce at all, unless it's off
the skin. It mostly all turns to heat eventually (AFAIK).

~~~
frandroid
Wet clothes just blur light!

------
mjd
If you find this kind of thing interesting, I strongly recommend the book [The
Nature of Light and Colour in the Open
Air]([https://store.doverpublications.com/0486201961.html](https://store.doverpublications.com/0486201961.html))
by Marcel Minnaert. He discusses why wet things are darker, why shadows of
leaves have little circular gaps, how to see the "green flash" that sometimes
precedes a sunset, and dozens of others, some you might have noticed and
wondered about, and some you won't realize you noticed until he points it out.

~~~
mturmon
Seconded. It’s a great physics based explanation of everyday light properties.

------
phaemon
This is the theme of Bon Jovi's ill-fated concept album "Dark When Wet", the
followup to their smash hit "Slippery When Wet". Such commercial success must
have gone to their heads to attempt such an audacious second project as a
sequel based on Quantum Electrodynamics. It was doomed to failure.

Re-interpretations of their classics, such as "Wanted, Dead or Alive -
Schrödinger Redux" failed to capture the live animal energy of the original,
and "Livin' on a Fermion", with lyrics such as "Pauli used to work on the
docks/That's not even wrong/He'd have fallen right through/It's tough" left
not only their core audience baffled, but indeed all sentient beings.

Hardly surprising that Mercury Record buried all mention of the album (and
will deny it to this day). They returned to more familiar ground with "Jersey
Shore", but lament the death of the Concept Album with this last, brave
attempt.

~~~
kupopuffs
I'm sorry what

~~~
phaemon
Google it

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Robotbeat
Index of refraction matching (or, effectively _impedence matching_ ). Snow is
white because you have a bunch of air/ice/air/ice/air/ice interfaces. The
amount reflected at each interface is determined by the Fresnel Equations:
[https://en.wikipedia.org/wiki/Fresnel_equations](https://en.wikipedia.org/wiki/Fresnel_equations)

Replace the air gap with liquid water, and the slush will be much darker
because the difference in index of refraction at each intereface (now
liquid/ice/liquid/ice etc) will be much less.

Basically, in electrical terms, less impedance mismatch so less reflection.

Same thing works for other things besides snow.

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matt-attack
On a related note, I find it fascinating that modern CGI cannot quite simulate
wet fabric. I watched Frozen 2 and the rendering is absolutely breathtaking on
most materials. Water, fabric, hair, etc. All impeccable.

But the minute fabric gets wet, it looks so unrealistic. It looks plastic-y or
greasy. I can only assume this is an active area of research, but we’re
clearly not there yet.

~~~
kyberias
Well, we may well be able to simulate it perfectly, but it might not fit into
Frozen 2's limited budget for rendering.

~~~
matt-attack
I thought I'd read up, and googled "cgi wet fabric". For me the number one
result was my own comment above!

~~~
kyberias
For me, when I google "cgi wet fabric" the top result is your comment here
referring to the top result that you found in Google when googling "cgi wet
fabric"!

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raphlinus
I am not an expert on this, but I suspect that the "diffuse reflection"
picture is not telling the whole story here, as it shows reflections purely
from the surface. From what I've seen, in most diffuse materials it's
subsurface scattering that dominates. So I think the _main_ contribution of
wetness is that it decreases the internal scattering because the indices of
refraction are more matched. The absorption remains similar, and I believe
reflectance is mostly a function of the ratio of scattering to absorption
(Kubelka-Munk theory).

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hammock
This article and all the comments seem like fancy and longwinded and sometimes
misleading theories of something that basically boils down to "a lot of light
gets trapped at diffuse angles in the short distance between the rough surface
of the fabric and the surface of the water on top of it."

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ggggtez
I think this is a good reference for anyone who touches computer graphics.
It's not really clear enough for Physics (see all the comments here), but as
tips for understanding how computer tend to model things, it's fine.

For example, a raytracer can simulate specular (mirror) or diffuse (matte)
reflections by simply playing with the incident angle when the ray hits an
object. The more jitter, the more matte the object appears. Similarly, you can
get very nice "milk" or "skin" coloration by following the incoming ray
slightly below the surface before reflecting back out, the same way this
discusses having a thin layer of water would prevent some of the light from
reflecting back out.

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rahuldottech
Related Q/A on physics.SE:
[https://physics.stackexchange.com/questions/30366/why-do-
wet...](https://physics.stackexchange.com/questions/30366/why-do-wet-objects-
become-darker)

~~~
Lxr
This seems like a better explanation, because the effect is stronger for
things that absorb water than for things where the water sits on top.

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Zenst
Short version: It changes the refraction index and less photons bounce of in
the direction of the observer, so it looks darker.

~~~
k__
How does it manage to direct the photons away from the observer?

I mean, it obviously doesn't know where they stand.

~~~
skykooler
It directs them into a more specific spot from which the material appears
bright. This is why wet things can look "shiny".

~~~
k__
Thanks, that makes sense!

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fbelzile
Wow, is this the same reason why you're more prone to sunburn if you have
water on you or are in a swimming pool? Does the light refracted under the
surface of the water bounce around and hit your skin multiple times?

~~~
tartoran
Yes, the water drops on the surface of the skin act as mini lens.

~~~
RealityVoid
I am unconvinced by this argument. After all, the same light flux is hitting
your body. Only thing a lens would do is make it focus in different spots than
it otherwise would, but then you'd get spots with burns and spots without
burns. This is not what I see happening at all.

~~~
tartoran
If the drops or any layer of water were staying still you'd probably get some
patters, but they don't.

~~~
tartoran
I don't know why I am being downvoted here. Here's a link to a Scientific
American article [0]

[0] [https://www.scientificamerican.com/article/the-magnifying-
ef...](https://www.scientificamerican.com/article/the-magnifying-effect-of-a-
water-drop/)

~~~
morsch
Nobody doubts that a drop of water can act as a kind of lens. The question is
then if a large number of very small water-drop-lenses contribute to sunburn.

The argument is that the light flux (the total amount of energy) hitting your
body is the same. A lens just focuses the energy of a wide area (the size of
the lens) onto a smaller spot, it doesn't add any energy to the equation. If
the lenses are kept totally still, you would get a higher flux (more of a
sunburn) in the focal point, and less energy (less of a sunburn) around the
focal point. If the lenses move around a lot, it all averages out into not
having any effect at all.

Here's some discussion on the topic:
[https://physics.stackexchange.com/questions/71263/why-
does-w...](https://physics.stackexchange.com/questions/71263/why-does-wet-
skin-sunburn-faster)

There's a counterargument to the above in that thread: the droplet-lenses have
a larger surface area than the area they're covering, which lends them the
ability to gather more light flux than the area they're covering. This seems
rather theoretical, though.

------
mannykannot
Start with a dry surface and consider the cone of light, scattered from a
point on it, that will pass through your iris. If, as shown in the final
diagram, the water, when added, forms a flat surface, and you are looking at
it fairly perpendicularly, then none of the rays that are totally internally
reflected would have been in that cone and ended up entering your eye anyway -
they are too oblique. They do not contribute to the brightness either before
or after the water is added.

On the other hand, when water is added, the above cone of light is broadened,
on account of refraction, so it is no longer the case that all the light
within it will enter your eye.

These considerations must be modified (I am not sure how) if the water forms a
thin, capilliary-adhering film that conforms to the roughness of the
substrate.

Finally, if the substrate is transparent or translucent (e.g. sand (quartz
crystals)), the change of refractive index at its surface is lessened in the
presence of water, causing more transmission / absorbtion and less reflection
/ scattering there.

------
c1ccccc1
This is not the full explanation, as can be checked with a simple experiment.
Dip a piece of cloth in a glass of water and hold it there submerged beneath
the surface. The submerged part will still look darker than the dry part,
despite the fact that there is no film of water covering it.

Other comments here have put forward the explanation that the water matches
the index of refraction of the material much more closely, meaning that light
is more likely to pass straight through the material instead of bouncing off.
This explanation seems much more likely to be correct to me.

Another experiment: Put some wet spots on a cloth and hold it up to a light
source. More light will pass through the wet spots than the dry cloth. This
certainly suggests that the reason wet cloth reflects less light is because
more is passing through. (I have tried both these experiments with paper towel
used as the cloth.)

------
kyberias
The original article: [https://fermatslibrary.com/s/why-some-things-are-
darker-when...](https://fermatslibrary.com/s/why-some-things-are-darker-when-
wet)

------
robbrown451
Here are my own (much shorter) attempts to explain the same thing:

[https://www.quora.com/Why-does-snow-appear-white-in-
colour-w...](https://www.quora.com/Why-does-snow-appear-white-in-colour-while-
water-is-transparent-colourless-1/answer/Rob-Brown-13)

[https://www.quora.com/Why-do-damp-items-such-as-fabrics-
appe...](https://www.quora.com/Why-do-damp-items-such-as-fabrics-appear-
darker/answer/Rob-Brown-13)

------
jimmyseoul
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.

------
throwaway_tech
>This is why you can’t see things in dark; no light means that there is no way
that you can channel the information about the surroundings into the person’s
eye.

This is not exactly accurate to my understanding. As carl Sagan once said "we
are star stuff", and just like stars wherever there is a human to see - even
in the "dark" \- there is light.

Take the image of the sun/sun rays, moon and eye from the article, its
important to remember all matter is emitting light (that includes the moon and
the eye and human attached to the eye) not just the Sun.

Even in the "dark" everything including the person will be emitting light
(electromagnetic radiation). Humans mostly emit electromagnetic radiation in
the infrared wave length, but humans also emit some electromagnetic light in
"visible" wave lengths. The issue is obviously humans have not evolved eyes
that see "infrared" light like other animals, and similarly the visible light
humans emit is below the intensity human eyes evolved to see. However, if
human eyes were sensitive enough all humans would appear as shining stars
emitting their own light.

Not sure its a thought experiment, but I always thought to understand human
sight look at your hand. Then pretend you saw infrared and imagine what you
hand looks like (or google a hand in infrared), then do the same as though you
saw x-ray wave length light. Now try to imagine if you could see all 3
spectrum at once...what would that look like?

~~~
thaumasiotes
>> This is why you can’t see things in dark; no light means that there is no
way that you can channel the information about the surroundings into the
person’s eye.

> The issue is obviously humans have not evolved eyes that see "infrared"
> light like other animals, and similarly the visible light humans emit is
> below the intensity human eyes evolved to see. However, if human eyes were
> sensitive enough all humans would appear as shining stars emitting their own
> light.

In other words, the quote you pulled is completely accurate. It didn't say
there was no way to channel the information about the surroundings into any
photosensitive device. It says quite clearly that you can't channel the
information into a human eye.

> Not sure its a thought experiment, but I always thought to understand human
> sight look at your hand. Then pretend you saw infrared and imagine what you
> hand looks like (or google a hand in infrared), then do the same as though
> you saw x-ray wave length light. Now try to imagine if you could see all 3
> spectrum at once...what would that look like?

Probably much like looking at a solid object embedded in colored but not
opaque glass. The ability to see things inside other visible things is not
foreign to the visible light spectrum.

~~~
throwaway_tech
> It didn't say there was no way to channel the information about the
> surroundings into any photosensitive device.

No it specifically said

>"This is why you can’t see things in dark; no light means...".

Not sure how to make the distinction any simpler, maybe you can follow:

If there is no light, then it is dark (true) - thats what you claim is being
said, but thats not what is said, what is said is

If it is dark, then there is no light (false) - any time you have been in the
dark, there has always been light.

>Probably much like looking at a solid object embedded in colored but not
opaque glass. The ability to see things inside other visible things is not
foreign to the visible light spectrum.

Sure the non-imaginative approach is to say just superimpose 3 images on top
of one another with each image having some transparency. And sure that may
make sense for objects like bone inside the persons outer skin...but emitted
heat is not a solid object inside another solid object, it neither embedded in
the object (its emitted) nor solid.

~~~
samatman
This is pedantry.

'light' means either electromagnetic radiation or visible light, depending on
context.

In this context, it means visible light. I'm not sure what you get out of
pretending otherwise.

~~~
throwaway_tech
>In this context, it means visible light.

When did "I pretend otherwise"? My comment(s) have nothing to do with the
meaning of light. I'm not sure what you get out of pretending otherwise.

Either way in the dark when you can't see, the author is wrong, that doesn't
mean there is no light...there is light, at minimum the person who can't see
is emitting it. Many people don't know that and find it interesting.

Its not pedantry its fundamental laws of physics.

~~~
samatman
You're doing it again.

Humans do not radiate visible light.

Also colloquially referred to as light.

~~~
throwaway_tech
Now I see the problem and confusion on your end...you don't think humans emit
visible light.

Like I said, most people don't know and are fascinated, that is why I shared
it...I don't find many who dispute it, but here are some articles about the
studies that confirmed it:

[https://www.sciencealert.com/you-can-t-see-it-but-humans-
act...](https://www.sciencealert.com/you-can-t-see-it-but-humans-actually-
glow-in-visible-light)

[https://www.nationalgeographic.com/science/phenomena/2009/07...](https://www.nationalgeographic.com/science/phenomena/2009/07/20/photographing-
the-glow-of-the-human-body/)

[https://journals.plos.org/plosone/article?id=10.1371/journal...](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0006256)

------
yogrish
ELIF5 explanation:
[https://www.reddit.com/r/explainlikeimfive/comments/e6czsd/e...](https://www.reddit.com/r/explainlikeimfive/comments/e6czsd/eli5_why_do_things_turn_dark_when_wet/?utm_source=share&utm_medium=ios_app)

------
etoxin
So in theory if we wet a surface painted in Vantablack (the darkest substances
known) it should become even darker.

------
20191224234044
Does this have anything to do with red look darker when there's less light?

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

edit: maybe not. Looks like one is physiological, the other physical

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sebastianconcpt
Differently from diffusive reflection: specular reflection.

 _In this type of reflection, the angle at which light hits the surface is the
same angle at which the light leaves the surface (reflected). This type of
reflection tends to happen on smooth surfaces like a mirror or glass._

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ryanmercer
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.

------
hn3333
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.

------
asimjalis
So why is a swimming pool not darker when there is water in it?

------
ngvrnd
Why are some things not darker when wet?

~~~
mattb314
I'd also like an answer to this one. It seems like most of these explanations
would also apply to plastic or metal getting wet, so why does cloth get so
much darker than other materials?

It feels like the fact that water is absorbed has to matter here--wood, dirt,
cloth and sand all darken when they absorb water but not when the water
remains in a bubble on the surface (as it does on cloth/sand/wood treated with
a hydrophobic coating).

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bacelyy
I read this article is very good ! I like that thanks for your sharing
@aryankashyap

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marban
[https://pictures.abebooks.com/RECYCLE2/22655932905.jpg](https://pictures.abebooks.com/RECYCLE2/22655932905.jpg)

~~~
aryankashyap
?

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

~~~
klodolph
If this were true, a glass of water sitting on top of concrete would look
darker than a puddle of water on the same concrete. That’s why we know that
this is not the explanation.

------
mlurp
I think it's a funny coincidence that he says "Light acts as a medium of
information transfer here.", because IMO this article is only a medium of info
transfer from the source he links at the bottom (to be fair, at least he links
it) without adding much.

I don't think there's anything wrong with making personal blog posts about
learning well established concepts, but promoting them is the reason we have a
million Medium/etc articles that are just regurgitating a paper without adding
anything of substance.

