8 ms·
Why are some things darker when wet?
- rahuldottech 7y agoRelated Q/A on physics.SE: https://physics.stackexchange.com/questions/30366/why-do-wet-objects-become-darker https://physics.stackexchange.com/questions/30366/why-do-wet...
- Lxr 7y agoThis seems like a better explanation, because the effect is stronger for things that absorb water than for things where the water sits on top.
- robobro 7y agoIs this a gimmick to boost your seo?
- deleted 7y ago[deleted]
- aryankashyap 7y agoNo, but the fact that you said it shows you've done something like that in the past.
- Zenst 7y agoShort version: It changes the refraction index and less photons bounce of in the direction of the observer, so it looks darker.
- k__ 7y agoHow does it manage to direct the photons away from the observer? I mean, it obviously doesn't know where they stand.
- skykooler 7y agoIt directs them into a more specific spot from which the material appears bright. This is why wet things can look "shiny".
- k__ 7y agoThanks, that makes sense!
- Khoth 7y agoThe actual short version is: diffuse reflection from the surface means that some of the light leaving the surface is at angles which makes it get totally internally reflected at the boundary between the water film and air, giving it another chance to be absorbed by the surface
- fbelzile 7y agoWow, 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 7y agoYes, the water drops on the surface of the skin act as mini lens.
- deleted 7y ago[deleted]
- RealityVoid 7y agoI 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 7y agoIf the drops or any layer of water were staying still you'd probably get some patters, but they don't.
- tartoran 7y agoI 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-effect-of-a-water-drop/ https://www.scientificamerican.com/article/the-magnifying-ef...
- morsch 7y agoNobody 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-wet-skin-sunburn-faster https://physics.stackexchange.com/questions/71263/why-does-w... 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.
- SamBam 7y ago> 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 7y agoYes, 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?
- thaumasiotes 7y agoDoes it become heat?
- arh68 7y agoIt 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).
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- mannykannot 7y agoStart 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.
- aphextron 7y agoThis 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".
- aryankashyap 7y agoCould you please elaborate on how it's hand-wavey. Happy to take constructive criticism and improve :)
- deleted 7y ago[deleted]
- ignoramous 7y agoThis might interest you: https://www.quora.com/What-are-the-physics-behind-colour/answer/Joshua-Engel https://www.quora.com/What-are-the-physics-behind-colour/ans...
- thaumasiotes 7y agoThe little balls model perfectly explains why light reflects off a mirror at the same angle at which it arrived. How does the absorb-and-reemit model handle this?
- silvestrov 7y ago"The Science Asylum" explains it: https://www.youtube.com/watch?v=cep6eECGtw4 https://www.youtube.com/watch?v=cep6eECGtw4
- lonelappde 7y agoThat's almost exactly the same as Feynman's QED lecture from the 1980s(?), but with MSPaint animation instead of a chalkboard.
- marban 7y agohttps://pictures.abebooks.com/RECYCLE2/22655932905.jpg https://pictures.abebooks.com/RECYCLE2/22655932905.jpg
- aryankashyap 7y ago?
- throwaway_tech 7y ago>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 7y ago>> 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.
- raphlinus 7y agoI 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).
- asimjalis 7y agoSo why is a swimming pool not darker when there is water in it?
- hammock 7y agoThis 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."
- Robotbeat 7y agoIndex 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.
- matt-attack 7y agoOn 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 7y agoWell, we may well be able to simulate it perfectly, but it might not fit into Frozen 2's limited budget for rendering.
- matt-attack 7y agoI thought I'd read up, and googled "cgi wet fabric". For me the number one result was my own comment above!
- kyberias 7y agoFor 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"!
- hn3333 7y agoPerhaps 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.
- jimmyseoul 7y agoI 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.
- Ceezy 7y agoWater is not transparent, not 100%. So it just absorb light enought that you would noticed it whent something is wet
- klodolph 7y agoIf 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.
- ryanmercer 7y agoI'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.
- mlurp 7y agoI 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.
- mrtweetyhack 7y agophotons get wet and don't travel as fast
- 20191224234044 7y agoDoes 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
- robbrown451 7y agoHere are my own (much shorter) attempts to explain the same thing: 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-does-snow-appear-white-in-colour-w... https://www.quora.com/Why-do-damp-items-such-as-fabrics-appear-darker/answer/Rob-Brown-13 https://www.quora.com/Why-do-damp-items-such-as-fabrics-appe...
- mjd 7y agoIf 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.
- kyberias 7y agoThe original article: https://fermatslibrary.com/s/why-some-things-are-darker-when-wet https://fermatslibrary.com/s/why-some-things-are-darker-when...
- c1ccccc1 7y agoThis 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.)
- phaemon 7y agoThis 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.
- sebastianconcpt 7y agoDifferently 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.
- ngvrnd 7y agoWhy are some things not darker when wet?
- mattb314 7y agoI'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).
- etoxin 7y agoSo in theory if we wet a surface painted in Vantablack (the darkest substances known) it should become even darker.
- bacelyy 7y agoI read this article is very good ! I like that thanks for your sharing @aryankashyap
- ggggtez 7y agoI 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.
- yogrish 7y agoELIF5 explanation: https://www.reddit.com/r/explainlikeimfive/comments/e6czsd/eli5_why_do_things_turn_dark_when_wet/?utm_source=share&utm_medium=ios_app https://www.reddit.com/r/explainlikeimfive/comments/e6czsd/e...