Wikipedia:Reference desk/Archives/Science/2019 March 14

= March 14 =

Wastewater treatment
Another Infra-inspired question...

In a (real-life) sewage treatment plant, which unit stinks the worst? Is it the primary clarifiers, or the aerators, or the secondary clarifiers, or maybe something else? 2601:646:8A00:A0B3:4C6C:BBA:4ED1:76B8 (talk) 00:49, 14 March 2019 (UTC)

Cleaning contact lens.
We don't have an article for soft contact lens, only contact lens and general, and there is no section on cleaning them. So my question is on cleaning contact lens hard vs. soft. Are there solutions that are only meant to clean hard contact lens and not soft, and clean soft contact lens only but not hard? I say that because, I used to use hard contact lens and later changed to soft. The hard contact lens solution had some % of hydrogen peroxide. 1 time, I accidentally used it for the soft contact lens, and had to immediately take them out as it stung my eye (so most likely hydrogen peroxide is not easy to rinse off soft contact lens). But when, I see these multi-purpose contact lens cleaner that can do hard and soft, and also claim to have hydrogen peroxide in it?

Btw I have a off-topic question, and I think that soft contact lens cleaners are purposely weak in concentration due to economics: so you can continue to buy more contact lens. Does that mean people can make their own contact lens cleaner, at stronger concentrations, and therefore keep contact lens longer? Or is that not really possible due to safety. We're generally suppose to dispose soft contact lens after 30 days, so. Thanks. 67.175.224.138 (talk) 01:57, 14 March 2019 (UTC).
 * I'm sure there's all manner of advice on this subject via Google, but you should start by talking to your eye doctor. ←Baseball Bugs What's up, Doc? carrots→ 02:54, 14 March 2019 (UTC)


 * There is probably no section on cleaning them because Wikipedia is not a "how to ..." See What Wikipedia is not.196.213.35.147 (talk) 06:10, 14 March 2019 (UTC)


 * "Do not soak soft contact lenses in solutions meant for rigid, gas-permeable lenses. Rigid gas permeable contact solutions are much stronger than solutions meant for soft lenses, and will quickly ruin your soft contacts", American Academy of Ophthalmology, 2019. Alansplodge (talk) 11:28, 14 March 2019 (UTC)
 * See Contact_lens, List of contact lens complications and Effects of long-term contact lens wear on the cornea. Complications from contact lens wear affect roughly 5% of wearers yearly and improper cleaning and disinfection of the lens can lead to infections by various microorganisms including bacteria, fungi, and Acanthamoeba (Acanthamoeba keratitis). The OP should follow only FDA-approved (in USA) manufacturer's handling and care procedures. DroneB (talk) 11:42, 14 March 2019 (UTC)

LS coupling vs jj coupling
The difference between light and very heavy atoms reflects itself in these two schemes. My question is why one scheme for the vector sum is necessarily the right & suitable sum model for one case, and the 2nd scheme manifests the 2nd case ? In other words, why & how the relative magnitude of the respective terms in the Hamiltonian, namely for LS & the central field (residual), results in the order the summing of the angular momenta (orbital & spin) ? In my eyes, this is a fundamental question regarding the physical picture & the respective appropriate vector sum. בנצי (talk) 16:04, 14 March 2019 (UTC)
 * You can find this answer in any textbook on the atomic physics. If the spin-orbit interaction interaction is much weaker than the electrostatic repulsion between electrons then you should use LS coupling scheme. In the opposite case it is jj-coupling. However spin-orbit interaction is a relativistic effect and is strong only if electrons have large kinetic energy, which happens when the central charge is large i.e. in case of inner electrons or electrons in multi-charged ions. Ruslik_ Zero 18:35, 14 March 2019 (UTC)
 * This is well-known, and is reflected in my question. To pinpoint the issue: why a particular sum matches only one case and not the other ? An explanation to this isn't described in many textbooks I've gone through. If you know one which does, for sure, I'll be glad to hear. בנצי (talk) 20:54, 14 March 2019 (UTC)
 * In other words, why does the stronger interaction precede in the order of addition ? בנצי (talk) 21:38, 14 March 2019 (UTC)
 * The best read for this is the Theory of Atomic Structure and Spectra book by R. D. Cowan, but it's not an easy read at all. To your original question, the issue of ls vs jj coupling is essentially the issue of choosing a basis. When you look at an atom -- any atom -- you can use different sets of angular and total orbital quantum numbers to describe any many-electron (or single-electron, if the atom or ion is hydrogen-like) state. The problem is, these bases (that is, these sets of quantum numbers: nlsLS, nlsjJ, etc. that you choose) do not  diagonalize the Hamiltonian perfectly; you always end up with off-diagonal terms.   In other words, your true eigenstates are always mixtures of different nlsLS or nlsjJ states.  What happens, however, is that in the jj-coupling model the eigenstates are closer to the basis states in the heavier atoms and ions, where spin-orbital coupling (a relativistic effect) dominates; and in the ls-coupling model the eigenstates are closer to the basis states in the lighter atoms and ions, where other effects dominate. This is highly oversimplified, so I encourage you to read the book if you want to gain a proper understanding of this.  Hope this helps. Dr Dima (talk) 01:05, 15 March 2019 (UTC)
 * Eventually, someone's touching the issue I've been trying to raise. Thanks, indeed. I'll 1st repeat reading your reply because of the relevant direction/s it provides, then get a more profound view using Cowan's book (I'm using Eisberg & Resnick's book, for example, and been trying to get Bransden's, which, I assume, you know). Thank you again, until further comments / questions. בנצי (talk) 11:25, 15 March 2019 (UTC)
 * You are welcome! All the best, Dr Dima (talk) 22:37, 15 March 2019 (UTC)
 * What do you mean in designating nlsLS (or nlsjJ) by adding LS or jj ? Also, what's the difference between angular & total orbital quantum numbers ? Clearly, L is the total orbital angular momentum, but, then what's the 2nd ? בנצי (talk) 22:05, 26 March 2019 (UTC)

I'm trying to address this thread, but it wouldn't appear in the current board. I hope there's an elegant way to bridge between the two.

Anyway, my question regards these two coupling models and their respective resulting selction rules. The selection rules for LS coupling is quite clear - it's based on calculating the compatible electric dipole matrix element. However, in the case of jj coupling we end up with different selection rules, which implies that the appropriate calculation here is basd on otherwise. What is the right (or approximate) matrix element, if not the electric dipole's ? Thanks, בנצי (talk) 13:48, 3 April 2019 (UTC)

Compass navigation in a non-dipole magnetic field
I've read that Earth's magnetic field consists of a dipole term that is weakening, and may cross zero within the next 200 years in the process of flipping, and some non-dipole terms that are not weakening. Have any studies been done on how compasses might still be used for navigation, in combination with other data sources, if that happens? (For example, if we knew we were within range of exactly one specific magnetic pole's non-negligibility, we could still find the direction to that pole; and if we knew its strength in enough different places to filter out artificial magnetic fields, we could estimate the distance.) Neon  Merlin  22:27, 14 March 2019 (UTC)

GPS will still work Greglocock (talk) 04:35, 15 March 2019 (UTC)
 * On that topic, our article Geomagnetic reversal could use a section dedicated to consequences on human systems.
 * Geomagnetic_secular_variation tells us the non-dipole part is shifting westwards in time, so I would speculate that using it for navigation etc. would be difficult, as one would have to take into account time variation effects over the course of a year. Tigraan Click here to contact me 09:19, 18 March 2019 (UTC)

regarding selection rules
Selection rules in quantum physics are based on calculating the respective electric dipole matrix element & finding out under what conditions we get non-zero results. However, physically, forbidden transitions aren't strictly zero, but rather of very low probabilty. My question is where does lie the differense between the mathematical zero (vanishing integrals) & the very minute transition probabilty ? Is it a result of the due integrals being only approximate themselves ? בנצי (talk) 13:21, 3 April 2019 (UTC)
 * This question is here by mistake, and is moved to 3 April 2019. All repliers are kindly requested to address the question there. בנצי (talk) 13:26, 3 April 2019 (UTC)