Wikipedia:Reference desk/Archives/Science/2014 September 2

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September 2

Light, momentum, and refraction

We had the "momentum of light" topic ago: When a beam of light hits a mirror, it transfers momentum, so at least a very light near-perfect mirror could be pushed around by bright light.

Now, what about refraction? Let's say a beam of light, coming in from the left, hits a prism and gets refracted:

     /\

=>==/  \

   /____\\

          \\

            \\

This time, the momentum doesn't change sign, but still changes direction: from horizontal to diagonal. Does the "missing momentum" end up in the prism? (I suspect it does; it has to go somewhere. With enough beam power, the prism could levitate - at least in theory.)

That momentum change would point up and slightly to the right. However, there are two refraction events: one, when entering the prism, and two, when leaving. Both times, the only thing to act on is the part of the surface through which the beam enters/exits. What exactly does the light act on? The glass molecules?

For a conducting medium, I'd guess the free electrons. However, the conductors I know (metals, graphite) are either opaque or reflective, not transparent. The transparent materials I know (glass, plastic) do not provide free electrons. Does the light act on the bound electrons instead?

Also, entering a prism, the light will get slowed down. Does the loss of speed translate into another momentum transfer, pushing the glass surface in (which would be the only momentum transfer if the light hits at right angles)?

     ___

    |   |

=>==| = |======

    |___|

Would this glass cube get pushed to the right by the momentum transfer alone (i.e. even if we assume that it doesn't reflect any light) ? - ¡Ouch! (^{hurt me} / _{more pain}) 06:40, 2 September 2014 (UTC)

It wouldn't push the cube; when exiting, the original momentum would be restored. One question solved. - ¡Ouch! (^{hurt me} / _{more pain}) 06:48, 2 September 2014 (UTC)

I thinking it would be noted that the resonance of the light is always been only when the light had a mirrory (зеркальность), so the absolute reflection and absolute refraction always had the mirrory (зеркальность).--Alex Sazonov (talk) 07:15, 2 September 2014 (UTC)

If in nature had an absolute optical environment, so that the light is been able to reach their absolute values!--Alex Sazonov (talk) 15:49, 2 September 2014 (UTC)

Note: The original thread was [1]. I'd been thinking I ought to finally set up a proper VPN before ... [this space intentionally left blank] ... but procrastinated. But I'd also welcome further discussion/explanation of what was said there. Wnt (talk) 14:53, 2 September 2014 (UTC)

Optical Refraction that causes the path of a light beam to bend where it crosses the interface between two media (e.g. air and glass) in which it has different velocities and therefore different wavelengths is more intuitively understood from the wave-view of light than the particle-view. Thus the analogy: "Imagine a marching band as they march at an oblique angle from pavement (a fast medium) into mud (a slower medium). The marchers on the side that runs into the mud first will slow down first. This causes the whole band to pivot slightly toward the normal (make a smaller angle from the normal)." The particle-view though arguably valid tends to lose sight of the finite width of every light ray, making refraction harder to explain. However it is useful when considering the momentum of light as the OP is doing. The prism gains Angular momentum in the direction opposite to the bending of the beam that passes through it. The mechanical action and reaction occur in the electron clouds of the surface molecules of the prism. This would also be the case in a prism made of a solid transparent conductor such as Indium tin oxide. 84.209.89.214 (talk) 15:27, 2 September 2014 (UTC)

The light do transfer momentum to the prism when it is refracted. The photons are mainly interacting with the electrons, but since they are bound the momentum is transfered to the atoms and to the prism. This phenomenon is the working principle of optical tweezers, so you will find a more thorough explanation in that article. Ulflund (talk) 16:20, 2 September 2014 (UTC)

For light entering a different refractive index, it is going to have some reflection back off the surfaces. For your perpendicular case the light will be reflected directly back, and this will increase the momentum transferred to the glass block. Graeme Bartlett (talk) 20:41, 2 September 2014 (UTC)

Yes, I should have phrased the answer to my last question differently, like "Refraction wouldn't push the cube". - ¡Ouch! (^{hurt me} / _{more pain}) 06:38, 4 September 2014 (UTC)

I have a question, does the light can do reflecting and refracting himself by himself?--Alex Sazonov (talk) 16:14, 2 September 2014 (UTC)

Photons do not interact directly with other photons, but in a non-linear optical medium they can indirectly do so. In high power lasers the phenomenon of self-focusing, where the high intensity changes the refractive index differently in different positions, thus resulting in refraction. Finally two-photon physics is the field studying interactions between photons and photons in vacuum, although this only occur through higher order where one photon first turns into a fermion anti-fermion pair. Ulflund (talk) 16:31, 2 September 2014 (UTC)

The double-slit experiment start reading here is fundamental to understanding how light can interfere with itself. 84.209.89.214 (talk) 03:47, 3 September 2014 (UTC)

Thanks, everybody. I started thinking about momentum transfer because of the marching band analogy.

When they get into the mud, they have to transfer momentum to the mud... So they push the mud forward a bit when entering and back when leaving... *lightbulb* So the sum of momentum transfers is zero, they won't move the muddy patch except compressing it slightly!

But wait... what if the muddy patch is shaped like a prism? Won't they displace it sideways as they go? Do dirt track racers start to be a good analogy at that point?

The Optical tweezers#Ray optics paragraph mostly confirms my thoughts (and I thought optical tweezers worked using vapor pressure... well, that's another misconception cleared up!), and the topic Wnt provided above is even better. - ¡Ouch! (^{hurt me} / _{more pain}) 06:38, 4 September 2014 (UTC)

Sense of the nature of physical-chemical phenomena

Could the nature of a physical-chemical phenomenon to refute anther nature physical-chemical phenomenon?--Alex Sazonov (talk) 11:10, 2 September 2014 (UTC)

The verb to refute means to contradict in words someone's argument, which I thinkfirst thought is not what you are asking about. I understand understood your question as "Could one physical-chemical system in equilibrium disturb another system in equilibrium?" The first issue to answer is whether there is a way to expose the systems to each other without disrupting the equilibrium of either. For example, the Mechanical equilibrium of two children on a Seesaw and the Dynamic equilibrium of a certain reversible chemical reaction in seawater are necessarily independent equilibriums that cannot be tested against one another. Generally speaking, if A and B are elements in equilibrium and they are exposed to another pair C and D of elements in equilibrium, then a reaction in any of these pairs may disrupt both equilibriums: AC, AD, BC or BD. (For "elements" substitute whatever force, concentration, rate, etc. is in equilibrium.) There is also a special case in chemistry of Catalysis where the rate of a chemical reaction, which could be a constituent of some greater equilibrium, is abruptly increased when the substance called the catalyst (itself in self-equilibrium) is introduced. 84.209.89.214 (talk) 14:43, 2 September 2014 (UTC) I apologise for misunderstanding your question!

I believe that, the paragraphs (sections) of physics and paragraphs (sections) of chemistry could never deny each other, both in their unison and as among themselves, because the physics and chemistry always had the mutual of universal knowledge.--Alex Sazonov (talk) 15:27, 2 September 2014 (UTC)

Physics and chemistry are studies that overlap, as demonstrated in the articles Physical chemistry and Chemical physics. 84.209.89.214 (talk) 20:58, 2 September 2014 (UTC)

What is been Alex Sazanov's Native Language?

The name seems like some sort of mock Slavonic, but the "user's" grammar corresponds in no way to any Slavic language. Is been this some sort of joke? μηδείς (talk) 19:54, 2 September 2014 (UTC)

I thought at first that the questions were trolling, then that they were being translated from Russian, but I assumed good faith. It would be interesting to know. Dbfirs 21:37, 2 September 2014 (UTC)

I wonder if it is an Eliza program with the bad grammar to cover up any problems. Dmcq (talk) 22:19, 2 September 2014 (UTC)

язык Стол-ичная ВСЕГДА лучше будет возможно назад в будущее рассмотреть?--Digrpat (talk) 22:25, 2 September 2014 (UTC)

I think we're simply dealing with a troll or two here, probably the same person. Digrpat seems to be a very infrequent one-purpose account. The "Russian" above is very poorly constructed apparently meaning "Capitalized language [is] ALWAYS better will it be possible to consider from the back to the future?" Neither of these users is writing anything that would make half the sense even a word-for-word calque would. Probably the closest thing that a good faith bad translator of Russian would write that might look like "is been" is "Было бы" which is the construction for the conditional, like "Было бы хорошо?" Would it be good... I have two large off-wiki projects, So I am not prepared to invest the time to start a complaint at ANI, but I suggest our Russian speakers here be pinged and I will gladly testify to my suspicions. μηδείς (talk) 21:35, 3 September 2014 (UTC)

Assuming good faith even if only, as Douglas Adams might have said, for the sheer mental exercise of it, it strikes me as not completely impossible that Sazonov might have learned just enough English to be dangerous at some point, extrapolated who-knows-what to invent his own grammatical rules, and these are now fixed ideas that are somewhat refractory to evidence or observation. I do find that his contributions have gotten somewhat less impossible to understand that they were at one time, although whether this is a function of an actual change or merely my exposure to them is difficult to be sure. --Trovatore (talk) 23:12, 2 September 2014 (UTC)

(For what it's worth, this user is blocked on ruwiki. His contributions there appear to be in perfect Russian, but make absolutely no sense: he's spouting scientific gibberish.) -- 79.233.115.170 (talk) 03:57, 3 September 2014 (UTC)

This seems the logical place to apologize for saying he's "being rubbing off on me" in a recent edit summary. What I meant to say, of course, was he "is been rubbing off". I meant to make a dummy edit to this effect, but couldn't think of the right minor change. InedibleHulk (talk) 06:49, 3 September 2014 (UTC)

I assume that's not rubbing one off on you, in which case immediately contact your local police.

No, thankfully. I read it that way, too, after posting. In Soviet Russia, local police contact you, but there's a universally understood response to that sort of questionable behaviour. InedibleHulk (talk) 03:00, 4 September 2014 (UTC)

Why do I get a feeling we're being subjected to a Turing test. The bad grammar, rubbish "science" and a consistent failure to actually respond to any question or comment directly addressed to him/it, seems to me to add up to some kind of AI test. Roger (Dodger67) (talk) 11:55, 3 September 2014 (UTC)

What would be the opposite of artificial intelligence? Edison (talk) 13:17, 3 September 2014 (UTC)

I used to joke about artificial stupidity, but then expert systems were invented, and they are precisely that. --jpgordon^{::==( o )} 01:16, 5 September 2014 (UTC)

C++. I hope. Unless ...Asmrulz (talk) 18:45, 3 September 2014 (UTC)

Which of these Two Kidney Situations Is Better?

Out of curiosity--serious question: Is it better for someone (who, for whatever reason, previously lost one kidney) to only have one kidney of his/her own (with his/her own DNA) or to have one kidney of his/her own and one kidney which was donated to him/her from someone else (with someone else's DNA)? Futurist110 (talk) 20:57, 2 September 2014 (UTC)

Organ transplantation is major surgery, with all the associated risks. The transplant recipient will likely need to remain on antirejection meds for the rest of his life, with the associated risks and side effects. In contrast, individuals with just one fully-functional kidney are generally just fine. Heck, in unilateral renal agenesis, individuals get by with just one kidney for their entire lives, typically without major medical consequences.

So really, one is asking whether the very tiny risk that an infection, malignancy, or injury could damage a lone kidney is greater than the risks associated with major surgery and long-term immunosuppressant use—and the answer is almost certainly not. (And in many circumstances, there is still the possibility of arranging a transplant after the first kidney starts to go south.) The other issue which arises as well is that donor kidneys are a finite and limited resource; no ethical transplant physician or surgeon is going to 'waste' a donor kidney on someone who doesn't need one. TenOfAllTrades(talk) 21:51, 2 September 2014 (UTC)

And no ethical funeral director would call death the "better option", but it does pay the bills. Of course, you specified better for the person with the kidney, but you didn't specify whether it's better to give or receive. If giving's better, take a kidney. If receiving is, let it be. InedibleHulk (talk) 06:45, 3 September 2014 (UTC)

I can't actually tell what you're trying to ask, and the OP's question seems perfectly clear.... TenOfAllTrades(talk) 13:48, 3 September 2014 (UTC)

Mine's not a question. But yeah, the OPs was almost perfectly clear. He probably thinks people should live. InedibleHulk (talk) 03:02, 4 September 2014 (UTC)

You'd have to consider the reason why they lost a kidney in the first place. If whatever condition caused that is still present, then losing the other kidney may be far more likely. Also note that people can live for quite some time with no functional kidneys, while on dialysis, making waiting to get a kidney transplant until medically necessary a better option. There's also the ethical problem that getting an unnecessary kidney transplant now may deny somebody else that kidney, who will die as a result. And finally, in a few years we may be able to clone our own kidneys to come up with exact matches, allowing for much more successful transplants, so holding out until then makes sense, if you can. StuRat (talk) 17:49, 3 September 2014 (UTC)