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Why are some things darker when wet?

aryankashyap.com

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Re: Why are some things darker when wet?

#91

Post by tsbinz »

empath75 wrote: post #72
chemeng wrote: post #59

Earlier quoted context omitted.

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…

> They are not the same photons. How do you define "the same" for two photons?

This is the question that should be higher up in the thread. This whole discussion makes no sense. Photons carry no hidden identity.

Re: Why are some things darker when wet?

#92

Post by mrlala »

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?

Re: Why are some things darker when wet?

#93

Post by samatman »

whatshisface wrote: post #53
melon_madness wrote: post #42

Earlier quoted context omitted.

Conservation of momentum, I think

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

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

Re: Why are some things darker when wet?

#94

Post by morsch »

tartoran wrote: post #78
tartoran wrote: post #41

Earlier quoted context omitted.

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

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...

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.

Re: Why are some things darker when wet?

#95

Post by whatshisface »

samatman wrote: post #93
whatshisface wrote: post #53

Earlier quoted context omitted.

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.

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

If momentum was transferred to the mirror then the emitted photon would not have the same momentum as the absorbed photon.

Re: Why are some things darker when wet?

#96

Post by samatman »

throwaway_tech wrote: post #50
thaumasiotes wrote: post #28

Earlier quoted context omitted.

>> 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…

> 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, the…

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.

Re: Why are some things darker when wet?

#97

Post by samatman »

whatshisface wrote: post #95
samatman wrote: post #93

Earlier quoted context omitted.

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

If momentum was transferred to the mirror then the emitted photon would not have the same momentum as the absorbed photon.

> 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.

Re: Why are some things darker when wet?

#98

Post by kyberias »

matt-attack wrote: post #48

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.

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

Re: Why are some things darker when wet?

#100

Post by hn3333 »

wizzwizz4 wrote: post #68
hn3333 wrote: post #54

Earlier quoted context omitted.

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.

WikiBooks really does have everything: https://en.wikibooks.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.

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
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