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

71–80 of 171 posts

Re: Why are some things darker when wet?

#71

Post by georgeburdell »

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?

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

Re: Why are some things darker when wet?

#72

Post by empath75 »

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…

> They are not the same photons.

How do you define "the same" for two photons?

Re: Why are some things darker when wet?

#73

Post by boomboomsubban »

whatshisface wrote: post #67
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…

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 com…

Isn't hydrogen gas almost exclusively H2, giving it plenty of possible electrons to interact with photons?

Re: Why are some things darker when wet?

#74

Post by klodolph »

Ceezy wrote: post #58

Water is not transparent, not 100%. So it just absorb light enought that you would noticed it whent something is wet

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.

Re: Why are some things darker when wet?

#75

Post by whatshisface »

boomboomsubban wrote: post #73
whatshisface wrote: post #67

Earlier quoted context omitted.

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 com…

Isn't hydrogen gas almost exclusively H2, giving it plenty of possible electrons to interact with photons?

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...

Re: Why are some things darker when wet?

#76

Post by whatshisface »

mac01021 wrote: post #35
tigershark wrote: post #30

Earlier quoted context omitted.

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?

I think the two slit experiments indicate that a photon does exactly have a direction of travel?

It has enough direction of travel for laser pointers to work.

Re: Why are some things darker when wet?

#77

Post by reubenmorais »

sieabahlpark wrote: post #37
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…

Except light is a wave and not a particle so reflection is explained when you consider that.

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.

Re: Why are some things darker when wet?

#78

Post by tartoran »

tartoran wrote: post #41
RealityVoid wrote: post #15

Earlier quoted context omitted.

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.

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...

Re: Why are some things darker when wet?

#80

Post by BeetleB »

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…

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.
Post reply on HN