[HN Gopher] Why are some things darker when wet?
___________________________________________________________________
Why are some things darker when wet?
Author : aryankashyap
Score : 298 points
Date : 2020-01-09 14:41 UTC (8 hours ago)
(HTM) web link (aryankashyap.com)
(TXT) w3m dump (aryankashyap.com)
| Robotbeat wrote:
| 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
|
| 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 wrote:
| 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 wrote:
| Well, we may well be able to simulate it perfectly, but it
| might not fit into Frozen 2's limited budget for rendering.
| ryanmercer wrote:
| 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.
| mlurp wrote:
| 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.
| mannykannot wrote:
| 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.
| rahuldottech wrote:
| Related Q/A on physics.SE:
| https://physics.stackexchange.com/questions/30366/why-do-wet...
| Lxr wrote:
| 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.
| 20191224234044 wrote:
| Does this have anything to do with red look darker when there's
| less light?
|
| https://en.wikipedia.org/wiki/Purkinje_effect
|
| edit: maybe not. Looks like one is physiological, the other
| physical
| robobro wrote:
| Is this a gimmick to boost your seo?
| [deleted]
| aryankashyap wrote:
| No, but the fact that you said it shows you've done something
| like that in the past.
| aphextron wrote:
| 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".
| 8bitsrule wrote:
| >Everything in this essay comes from this source.
| https://fermatslibrary.com/s/why-some-things-are-darker-when...
|
| It's less hand-wavey. Also, the reflectivity of water is very
| low at low angles of incidence, rising to 100% at normal
| angles. Less light reaches the wetted material.
| https://en.wikipedia.org/wiki/File:Water_reflectivity.jpg
| mrlala wrote:
| Does that mean like, all the time?
|
| For example.. if I'm looking at a star, and thinking it's the
| 'same' photon that came off of that star billions of years
| ago... that might be true in the vacuum of space, but since
| it's passed through the medium of the atmosphere, does that
| mean it's constantly been absorbed/new photons being emitted?
| whatshisface wrote:
| 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 of events.
| Instead, they describe an _instantaneous_ flow of amplitude
| between different quantum states. I would only go along with
| the absorption and re-emission interpretation if there was
| always a time delay between the two steps (which there cannot
| be if the mirror does not have any energy level deltas suitable
| for storing the energy).
|
| The other side of the debate would be that if the mirror is
| moving towards or away from the light, the reflection will be
| Doppler-shifted to a higher or lower frequency. Does this mean
| that the reflected photons are not the same photons as the
| incident photons, or does it mean that the same photons have
| had their energy changed? I think there is no meaningful
| distinction because every two particles with the same name in
| quantum mechanics are identical anyway. There's no telling
| which are which. If I showed you a photon, then took it back
| and showed you another, you would never be able to tell whether
| I had opened the same box twice or if I had taken the old one
| out and captured a new one from my desk lamp.
| chemeng wrote:
| 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...
| empath75 wrote:
| > They are not the same photons.
|
| How do you define "the same" for two photons?
| tsbinz wrote:
| This is the question that should be higher up in the
| thread. This whole discussion makes no sense. Photons
| carry no hidden identity.
| zamadatix wrote:
| A photon doesn't need to carry a UUID for people to
| conceptualize a difference between a photon going through
| free space and being absorbed and re emitted. The parent
| comment asks the right question, what is being meant by
| "the same" is required to know before being able to say
| the discussion makes no sense.
|
| Taking your first literal interpretation as the only way
| the discussion could have value is no more helpful than
| this exchange: "I'm going to the store, do you want the
| same hot dogs we have in the fridge?" "That doesn't make
| sense, the store can't have the same hot dogs we have in
| the fridge".
| zelphirkalt wrote:
| Who knows, maybe they actually do, but we do not know it
| yet. I mean, how do you know they do not?
| rosybox wrote:
| The absence of evidence doesn't make an idea more
| plausible, it makes it less plausible.
|
| I can't disprove that a celestial teapot is circling the
| sun between Earth and Mars, so how do you know it isn't?
|
| https://en.wikipedia.org/wiki/Russell%27s_teapot
| zelphirkalt wrote:
| Less plausible != impossible.
|
| "Knowing" for me means, that there is no way, that we can
| be wrong about something. Otherwise it's only "believe",
| not "knowledge".
|
| It would be good to say the following, instead of "it is
| so": "So far experiments did never show any sign of
| identity of photons." (Were there any such experiments?)
| Oh and what about quantum entanglement of photons? Could
| that not be interpreted as a sign of identity?
| ngvrnd wrote:
| it's all the same photon
| whatshisface wrote:
| Hydrogen atoms can't store photons that do not have the
| same wavelength as any of its spectral lines. Nonetheless,
| hydrogen gas has an index of refraction that is a
| continuous function of frequency, and as a result it can
| reflect light of any wavelength. Therefore, hydrogen does
| not store any energy for any length of time in the course
| of typical reflection. Even if the Feynman diagram for
| reflection looks like a combination of absorption and re-
| emission, if the absorption and emission process can't
| occur at separate times, then it is not right to say that
| one is followed by the other.
| boomboomsubban wrote:
| Isn't hydrogen gas almost exclusively H2, giving it
| plenty of possible electrons to interact with photons?
| whatshisface wrote:
| That's a good catch, but H2's spectral lines are still
| discrete. It's not that gasses can't absorb photons, it's
| that they can only do so at very specific frequencies.
|
| To simplify the argument, you could use Helium instead.
| Here is a (discrete!) list of its spectral lines: https:/
| /physics.nist.gov/PhysRefData/Handbook/Tables/heliumt...
|
| And here is a chart of its (continuous!) index of
| refraction: https://refractiveindex.info/?shelf=main&book
| =He&page=Mansfi...
| BeetleB wrote:
| You didn't address the concern. A given atom cannot absorb
| any photon. They can only absorb photons of certain energy
| levels/wavelengths. Electrons can occupy only certain
| energy levels. So they can only absorb photons that match
| the difference if two energy levels.
| marcosdumay wrote:
| > 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".
|
| That's how electrons and photons interact. It doesn't make
| sense to say that they are different photons or the same ones,
| since you can't observe the interaction.
| elcomet wrote:
| 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.
| aryankashyap wrote:
| Thanks!
| dan-robertson wrote:
| It doesn't really make sense to talk about "the same" photon
| because photons don't really have identity. Even in a vacuum
| you can send a photon in one end and see one on the other and
| not really know that the one going in was "the same" as the one
| going out. It can turn into other particles on its way and then
| recombine into a photon and there is no way to know this based
| on the observation that a photon came out of the end of your
| vacuum.
| zelphirkalt wrote:
| That's one of the things, that annoys me with questions about
| physics, which me might not know well enough yet: When
| someone asks about whether something "is", people answer with
| what we can in general know at our current level of
| understanding and then make it seem as though that is the
| same, as answering the question, what really "is".
|
| > Even in a vacuum you can send a photon in one end and see
| one on the other and not really know that the one going in
| was "the same" as the one going out.
|
| I don't really care, if _I or current physicists_ cannot
| _distinguish_ them, when I ask, whether they _are_ the same.
| I would rather have an answer like as follows:
|
| "We do not know this yet. We do not have the means of
| telling, whether it is really the same photon or not.
| However, even if they were not the same, it would make no
| difference (according to our current understanding of
| physics!), as their effect on the surroundings would still be
| the same, because ..."
| keithnz wrote:
| no, what dan said is better, it might annoy you, but
| physics is such that wording is important. Even then,
| nearly all wording is losing information and in some cases
| changes it from the reality. You are asking a question
| which you want an answer but the question itself has a
| troubling fit with reality as you try to relate to things
| in terms that seem familiar.
| melon_madness wrote:
| I don't think this is quite the photoelectric effect. The atoms
| absorb photons and re-emit photons--they aren't emitting
| electrons.
| tigershark wrote:
| 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?
| melon_madness wrote:
| Conservation of momentum, I think
| whatshisface wrote:
| 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.
| samatman wrote:
| Only if the momentum wasn't transferred to the mirror.
|
| Which is, in fact, what happens. Solar sails exploit this
| fact:
|
| https://en.wikipedia.org/wiki/Solar_sail
| whatshisface wrote:
| If momentum was transferred to the mirror then the
| emitted photon would not have the same momentum as the
| absorbed photon.
| melon_madness wrote:
| Conservation of momentum doesn't mean that the momentum
| of the emitted photon has to be equal to that of the
| absorbed photon. It means the sum of the momentum of the
| emitted photon and the mirror must equal the momentum of
| the absorbed photon, assuming the mirror had no momentum
| to begin with.
| samatman wrote:
| > _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?_
|
| That is, the equal and opposite direction, exactly as
| predicted by conservation of momentum.
| improv32 wrote:
| The photon has some amplitude for reflecting in all possible
| directions, it happens that for certain directions these
| amplitudes are very close to the same value and so reinforce
| and become more probable, and at wider angles tend to cancel
| out and become less probable. When this is not the case and
| wider angles instead reinforce you can observe this directly
| in for instance a diffraction grating.
| mac01021 wrote:
| I think the two slit experiments indicate that a photon does
| exactly have a direction of travel?
| whatshisface wrote:
| It has enough direction of travel for laser pointers to
| work.
| lonelappde wrote:
| Only after it arrives, so the situation's a lot more
| nuanced than that.
| tyre wrote:
| One constant in quantum electrodynamics is that it doesn't
| look right to me, but it seems to work!
|
| Richard Feynman's "QED" is a great read on this.
| cameldrv wrote:
| You can also watch the original lecture. This is one of my
| very favorite explanations of anything:
|
| http://www.vega.org.uk/video/programme/46
|
| This explanation starts at about 30 minutes, but I highly
| recommend watching from the beginning.
| yesenadam wrote:
| Thanks! I hadn't seen these. There are 4 Feynman lectures
| on this page[0], the Douglas Robb Memorial Lectures, of
| which you linked to the second.
|
| http://www.vega.org.uk/video/subseries/8
|
| 1. A gentle lead-in to the subject, Feynman starts by
| discussing photons and their properties. 2. What are
| reflection and transmission, and how do they work? 3.
| Feynman diagrams and the intricacies of particle
| interaction. 4. What does it mean, and where is it all
| leading?
|
| The site has many other science videos too:
|
| http://www.vega.org.uk/video/series/5
| aryankashyap wrote:
| Could you please elaborate on how it's hand-wavey. Happy to
| take constructive criticism and improve :)
| [deleted]
| ignoramous wrote:
| This might interest you: https://www.quora.com/What-are-the-
| physics-behind-colour/ans...
| lonelappde wrote:
| > The actual photons that hit an object are not the same ones
| that eventually enter your eyes.
|
| For anything opaque, you should expect this, because otherwise
| how could you see something's color if you only saw the same
| photons as the light source it reflected?
| anamexis wrote:
| Because the object only reflects photons of certain
| wavelengths.
| CrazyStat wrote:
| 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?
| kurthr wrote:
| Because the electro-magnetic field polarization is preserved
| in many cases (specular). That doesn't mean there isn't a
| plasmon generated in between or that the sign of the electric
| field is maintained. What is interesting is that in the case
| of metalic (from low to high index of refraction) there is an
| inversion of the electric field. This does not happen at the
| reflection from a high index to low index (e.g. under water
| looking up to see total internal reflection the polarization
| remains the same). You can see this same effect bouncing
| water waves off of a concrete wall.
| maskedinvader wrote:
| 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)
| CrazyStat wrote:
| 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?
| georgeburdell wrote:
| I can recommend Hecht's Optics textbook for this. I found
| it at my local library. I have had a number of
| electromagnetics courses over my life and none explained
| the unifying scattering principles of reflection,
| transmission, and refraction concept so clearly as that
| book
| dhimes wrote:
| Hecht is an amazing author IMHO. Even his intro physics
| book is remarkable for introducing Emmy Noether at the
| outset of dynamics.
| hn3333 wrote:
| 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.
| wizzwizz4 wrote:
| WikiBooks really does have everything: https://en.wikiboo
| ks.org/wiki/A-level_Physics_%28Advancing_P...
|
| This example is _wrong_ , but useful, and it's (probably)
| less wrong than what you've been talking about so far. At
| the very least, it's more useful.
| hn3333 wrote:
| Thanks for correcting me and thanks for the link. But
| what have I been talking about so far? I don't
| understand. Also I just watched this video I'd recommend
| that explains the idea:
| https://www.youtube.com/watch?v=NumSE2LvSmQ
| lonelappde wrote:
| They don't! That's why the thickness of th glass of
| affects it's reflectivity, and not the simple way you
| might guess. Read Feynman's QED book or watch the YouTube
| vidoes for a lay intro.
| thaumasiotes wrote:
| The 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 wrote:
| "The Science Asylum" explains it:
| https://www.youtube.com/watch?v=cep6eECGtw4
| lonelappde wrote:
| That's almost exactly the same as Feynman's QED lecture
| from the 1980s(?), but with MSPaint animation instead of a
| chalkboard.
| sieabahlpark wrote:
| Except light is a wave and not a particle so reflection is
| explained when you consider that.
| reubenmorais wrote:
| Light is not a wave. Photons are particles. Light as a wave
| is a convenient abstraction that explains a lot of light's
| behaviors, but not all of them. I recommend Feynman's QED
| lectures (or the book), where he explains this.
| hutzlibu wrote:
| This debate is very old. Newton and Huygens fought long
| over it .. and eventually it was solved by: it is both or
| rather neither. It is a quant.
|
| https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality
| #...
| lonelappde wrote:
| That's half right.
| robbrown451 wrote:
| 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-do-damp-items-such-as-fabrics-appe...
| ngvrnd wrote:
| Why are some things not darker when wet?
| mattb314 wrote:
| 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).
| sebastianconcpt wrote:
| 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._
| marban wrote:
| https://pictures.abebooks.com/RECYCLE2/22655932905.jpg
| aryankashyap wrote:
| ?
| Zenst wrote:
| Short version: It changes the refraction index and less photons
| bounce of in the direction of the observer, so it looks darker.
| k__ wrote:
| How does it manage to direct the photons away from the
| observer?
|
| I mean, it obviously doesn't know where they stand.
| Khoth wrote:
| The 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
| skykooler wrote:
| It directs them into a more specific spot from which the
| material appears bright. This is why wet things can look
| "shiny".
| k__ wrote:
| Thanks, that makes sense!
| mrtweetyhack wrote:
| photons get wet and don't travel as fast
| asimjalis wrote:
| So why is a swimming pool not darker when there is water in it?
| hammock wrote:
| 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."
| hn3333 wrote:
| 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.
| jimmyseoul wrote:
| 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 wrote:
| >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 wrote:
| >> 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 wrote:
| > 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 wrote:
| 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 wrote:
| >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 wrote:
| You're doing it again.
|
| Humans do not radiate visible light.
|
| Also colloquially referred to as light.
| throwaway_tech wrote:
| 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.nationalgeographic.com/science/phenomena/2009
| /07...
|
| https://journals.plos.org/plosone/article?id=10.1371/jour
| nal...
| Ceezy wrote:
| Water is not transparent, not 100%. So it just absorb light
| enought that you would noticed it whent something is wet
| klodolph wrote:
| 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.
| raphlinus wrote:
| 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).
| c1ccccc1 wrote:
| 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.)
| fbelzile wrote:
| 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?
| ryanmercer wrote:
| I think it is more what mannykannot said, combined with the
| fact that chlorine/salt/sand will also irritate skin, as well
| as the fact that sun reflecting off of water/glass/metal is not
| just reflecting visible light (sometime in the past few months
| there was a great thread on here with a guy talking about how
| they use mirrors to reflect sun onto solar panels where he
| works in Canada I think).
|
| Snow sunburn is also a thing, IIRC snow can reflect as much as
| 90% of the UV. You can actually go 'snowblind' from this uv
| reflection.
| tartoran wrote:
| Yes, the water drops on the surface of the skin act as mini
| lens.
| [deleted]
| RealityVoid wrote:
| 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 wrote:
| If the drops or any layer of water were staying still you'd
| probably get some patters, but they don't.
| tartoran wrote:
| 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...
| morsch wrote:
| 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...
|
| 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.
| true_religion wrote:
| It does happen, but due to inflammation and the fact that
| the "lens" wobbles since it is made out of water, you get
| enough consistency that it's not noticeable without a
| detailed inspection.
|
| It's possibly still noticeable via the naked eye, but you'd
| have to compare and contrast to notice the slight mottling.
| mannykannot wrote:
| Maybe you just do not notice because the water keeps your skin
| cool?
| kyberias wrote:
| The original article: https://fermatslibrary.com/s/why-some-
| things-are-darker-when...
| phaemon wrote:
| 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 - Schrodinger 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 wrote:
| I'm sorry what
| phaemon wrote:
| Google it
| SamBam wrote:
| > 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 wrote:
| 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?
| thaumasiotes wrote:
| Does it become heat?
| arh68 wrote:
| 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 wrote:
| Wet clothes just blur light!
| Asterate wrote:
| 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 surfaces, the reflection will be diffuse,
| i.e. visible from all directions. 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.
|
| Conductors are a completely different beast. The reflection off
| of metals are not solely dictated by the refractive index.
| SamBam wrote:
| > 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
| never seen a damp rag be shiny in any direction.
|
| (...and it shouldn't be that hard to see, if that effect is
| really true. With any other shiny object (polished car, CD,
| balloon) you see the specular reflection frequently.)
| danans wrote:
| > 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.
| aryankashyap wrote:
| Thanks for your feedback. Will make the changes shortly!
| mjd wrote:
| 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) 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.
| kinow wrote:
| Sounds interesting! Thanks for the link!
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