Why are some things darker when wet? • Hacker News Board

Re: Why are some things darker when wet?

#101

Post by improv32 » Thu, Jan 09, 2020, 8:36 PM UTC

tigershark wrote: post #30

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…

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?

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.

Re: Why are some things darker when wet?

#102

Post by aryankashyap » Thu, Jan 09, 2020, 8:42 PM UTC

elcomet wrote: post #55

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…

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.

Thanks!

Re: Why are some things darker when wet?

#103

Post by c1ccccc1 » Thu, Jan 09, 2020, 8:42 PM UTC

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.)

Re: Why are some things darker when wet?

#104

Post by phaemon » Thu, Jan 09, 2020, 8:44 PM UTC

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 - Schrödinger 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.

Re: Why are some things darker when wet?

#105

Post by throwaway_tech » Thu, Jan 09, 2020, 8:59 PM UTC

samatman wrote: post #96

throwaway_tech wrote: post #50

Earlier quoted context omitted.

> 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.

>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.

Re: Why are some things darker when wet?

#106

Post by anamexis » Thu, Jan 09, 2020, 9:04 PM UTC

lonelappde wrote: post #64

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. 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?

Because the object only reflects photons of certain wavelengths.

Re: Why are some things darker when wet?

#107

Post by sebastianconcpt » Thu, Jan 09, 2020, 9:17 PM UTC

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.

Re: Why are some things darker when wet?

#108

Post by samatman » Thu, Jan 09, 2020, 9:28 PM UTC

throwaway_tech wrote: post #105

samatman wrote: post #96

Earlier quoted context omitted.

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.

>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…

You're doing it again.

Humans do not radiate visible light.

Also colloquially referred to as light.

Re: Why are some things darker when wet?

#109

Post by cameldrv » Thu, Jan 09, 2020, 9:40 PM UTC

tyre wrote: post #38

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?

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.

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.

Re: Why are some things darker when wet?

#110

Post by zamadatix » Thu, Jan 09, 2020, 9:54 PM UTC

tsbinz wrote: post #91

empath75 wrote: post #72

Earlier quoted context omitted.

> 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.

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".