Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Why are some things darker when wet?

aryankashyap.com

Post reply on HN
Print view View original

131–140 of 171 posts

Re: Why are some things darker when wet?

#131

Post by zamadatix »

izietto wrote: post #127
zamadatix wrote: post #110

Earlier quoted context omitted.

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 th…

> "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" Yeah but hot dogs belong to classical mechanics, while photons to quantum mechanics, and in quantum mechanics you can actually have the same hot dogs in different places, until you observe them

That a photon can be in superposition is even a better example than my hot dog example. Nobody is impressed if you can find a corner case interpretation that makes the question sound dumb they are impressed if you can try to figure out what the person really wanted to know and answer it to the best of current understanding.

In the hotdog example the point is the person is actually trying to figure out if you want them to buy you more hotdogs not whether or not the exact same hotdogs can be found at the store and fridge at the same time. In this actual discussion about the photon it's not about whether or not the photon actually carries an ID card with it it's about what actual interactions are happening to photons when the light is reflected or refracted in different ways.

Sometimes what a person is trying to figure out may not line up with how things actually work, in fact take any science and add 100 years of knowledge and pretty much all discussions are this way. That does not mean it was nonsensical to talk about it just means the details are more exact and complicated than the asker knew how to phrase (and again, it's always like more exact and complicated than any of us know as we don't know everything there is to know about the universe).

Re: Why are some things darker when wet?

#132

Post by StuffedParrot »

keithnz wrote: post #123
zelphirkalt wrote: post #114

Earlier quoted context omitted.

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…

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.

No, physics is such that semantics are important. What’s the observable difference between absorption/emission and reflection? If the latter doesn’t happen at all the distinction is meaningless.

Re: Why are some things darker when wet?

#134

Post by ggggtez »

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…

I think this is called, "knowing enough to be dangerous". You clearly have some understanding of light, but you are missing some key facts that would lead you to the correct understanding.

>Light can reflect off of transparent crystals

No, the point of the above post is that a photon is not "reflected", but captured and re-transmitted by photo-electric effect.

Light travels at maximum speed C, only when in a vacuum. Otherwise it travels through a medium. We can see that the different speeds of light cause dispersion such as when it enters and leaves a clear prism (changing mediums, thus spreading out the different speeds/wavelengths of light). Since we know that light is "traveling in a medium" when it travels in air or in the prism, then what do we mean by this? It is traveling slower, so it must have some information about the medium...

So how does the light "know" it's in a material, without interacting with the atoms of the material?

Of course, it couldn't know. It is interacting with the material. The behavior of the particle is fully explained by the photo-electric effect.

Re: Why are some things darker when wet?

#136

Post by 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.

Re: Why are some things darker when wet?

#137

Post by H8crilA »

SamBam wrote: post #9

> 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 wa…

Let's follow energy, after all darker = less energy. For simplicity let's consider the sum of energy over all observer angles, to get rid of the "it's shiny only from one angle" problem.

Reflection on the surface of the water is "free" both ways (from air to air bounce, and from water to water bounce), no energy loss. Travelling through water - similar, lite energy loss, though not exactly zero. The only way we can really lose energy is on the cloth reflection. Therefore (almost) all of the darkness effect should be attributable to increased number of cloth reflections, via the total internal water reflection mechanism.

This also explains why colors are more vivid when the cloth is wet - after all the same ray of white light bounces off the cloth several times, increasing the "color filtering properties" of the cloth.

Re: Why are some things darker when wet?

#138

Post by dandelany »

zamadatix wrote: post #131
izietto wrote: post #127

Earlier quoted context omitted.

> "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" Yeah but hot dogs belong to classical mechanics, while photons to quantum mechanics, and in quantum mechanics you can actually have the same hot dogs in different places, until you observe them

That a photon can be in superposition is even a better example than my hot dog example. Nobody is impressed if you can find a corner case interpretation that makes the question sound dumb they are impressed if you can try to figure out what the person really wanted to know and answer it to the best of current understanding. In the hotdog example the point is the person is actually trying to figure out if you want the…

The point is, if A) photons carry no individual identity, and B) the interaction happens instantaneously, then there is no meaningful difference between saying "the photon bounced off like a ball" and "the photon was absorbed and a new one was instantly re-emitted". It's literally a distinction without a difference.

As humans living in the classical world, we hear that and think "oh, that just means we don't know what's really going on, it either bounces or re-emits, it just looks the same to us." But no - reality is just an interaction of quantum fields and we're putting these human stories on top of it. It's like debating whether or not submarines swim.

Re: Why are some things darker when wet?

#139

Post by elihu »

SamBam wrote: post #9

> 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 wa…

I guess the internal reflection bit explains why less total light energy might escape; if it hits the shirt and bounces off the water-air boundary and hits the shirt again, then it had two chances to get absorbed by the shirt instead of one.

But yes, you're right that shiny things tend to seem darker from most angles, because the observer is usually not in line with most of the reflected light.

In computer graphics, Blinn highlights are a pretty decent approximation for shiny surfaces. (Interestingly, the highlights generally don't take on the color of the object unless it's metallic.)

https://en.wikipedia.org/wiki/Blinn%E2%80%93Phong_reflection...

Car headlights on wet pavement are kind of a worst case. You don't get much benefit from your own lights because the vast majority reflects off at a useless angle, but it does reflect back up at oncoming traffic.

(Impractical startup idea: headlight drones, that fly ahead of your car in rainy weather and illuminate the road in front of you from a more useful angle. Powered by microwaves beamed from the car.)

Re: Why are some things darker when wet?

#140

Post by empath75 »

mrlala wrote: post #92
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…

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?

It’s not even true in the vacuum of space. It might have been created and annihilated billions of times on the way.
Post reply on HN