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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?

#141

Post by JoBrad »

CrazyStat wrote: post #51
maskedinvader wrote: post #45

Earlier quoted context omitted.

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)

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?

This video addresses that, as part of a longer discussion.

https://youtu.be/CiHN0ZWE5bk

Also this one:

https://youtu.be/YW8KuMtVpug

Re: Why are some things darker when wet?

#142

Post by viklove »

chemeng wrote: post #59
whatshisface wrote: post #49

Earlier quoted context omitted.

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…

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…

But that's just not true. The electron will only get excited if the incoming photon has a very specific wavelength, so most of the light that hit your house's walls, for example, is reflecting. The photoelectric effect is real, but it only applies in specific circumstances...

Re: Why are some things darker when wet?

#143

Post by mr_toad »

hanoz wrote: post #129
ngvrnd wrote: post #119

Earlier quoted context omitted.

it's all the same photon

For those missing the reference: https://en.m.wikipedia.org/wiki/One-electron_universe

I wonder if there’s an antimatter universe going backwards in time from the Big Bang.

Re: Why are some things darker when wet?

#144

Post by mr_toad »

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.

Momentum is a vector. The sum of the vectors is preserved.

Re: Why are some things darker when wet?

#145

Post by amluto »

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…

> The actual photons that hit an object are not the same ones that eventually enter your eyes.

This part is philosophy, not physics. Photons are “indistinguishable particles”. You cannot tell two photons apart. This has a rigorous meaning: if you swap two photons, the state of the universe is unchanged. (This is in contrast to fermions — if you swap two fermions, the state of the universe has its phase shifted 180 degrees.) If photons we’re distinguishable, then swapping two photons would swap them, and the resulting state would be different. The matters from a statistical perspective. Look up Bose-Einstein statistics if you’re interested.

As for whether photons are absorbed and re-emitted or merely reflected, this surely depends on the surface on question.

Re: Why are some things darker when wet?

#147

Post by zamadatix »

dandelany wrote: post #138
zamadatix wrote: post #131

Earlier quoted context omitted.

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 wha…

I'll try again in another way and if this doesn't work I may need to come back to trying to communicate what I mean.

When you encounter an XY problem question/statement only stating "this doesn't make any sense. Computers are very complicated and don't work like that in the first place" is useless to everyone except maybe a comment or that wants to boost their ego. Taking the XY question, explaining why it's wrong, and trying to explain how things actually work at a deeper level is helpful. I think your comment about them being field interactions and needing to think about them that way is a good start on that kind of answer rather than the ones above.

Re: Why are some things darker when wet?

#148

Post by thelittleone »

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…

If I recall a high school physics class correctly the colour of the photons that are emitted from the object (the colour we see) are the ones the object does not absorb. If so, is the object the colour we see or is it the colour it absorbs.

Re: Why are some things darker when wet?

#149

Post by sjg007 »

amluto wrote: post #145
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…

> The actual photons that hit an object are not the same ones that eventually enter your eyes. This part is philosophy, not physics. Photons are “indistinguishable particles”. You cannot tell two photons apart. This has a rigorous meaning: if you swap two photons, the state of the universe is unchanged. (This is in contrast to fermions — if you swap two fermions, the state of the universe has its phase shifted 180 de…

What surfaces reflect?

Re: Why are some things darker when wet?

#150

Post by amluto »

sjg007 wrote: post #149
amluto wrote: post #145

Earlier quoted context omitted.

> The actual photons that hit an object are not the same ones that eventually enter your eyes. This part is philosophy, not physics. Photons are “indistinguishable particles”. You cannot tell two photons apart. This has a rigorous meaning: if you swap two photons, the state of the universe is unchanged. (This is in contrast to fermions — if you swap two fermions, the state of the universe has its phase shifted 180 de…

What surfaces reflect?

Metals and, generally, homogenous materials (dielectrics, conductors, etc). Light reflects off of a conductor or a dielectric due to a relatively straightforward solution to Maxwell’s equations.

In contrast, anything phosphorescent most definitely absorbs and re-emits, sometimes hours later. Imagine glow-in-the-dark pigment.

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