Wikipedia:Reference desk/Archives/Science/2022 November 7

= November 7 =

Lens related question
The complete range of n for which a convex lens comprises of ( 2n-1) different transparent materials forming (2n-1) images ( n is a positive integer). Options: (1) n≥0 (2) n≥2 (3) n≥1, (4) n≥ 1/2 Exclusive  Editor  Notify Me! 13:13, 7 November 2022 (UTC)


 * AndrewWTaylor (talk) 14:14, 7 November 2022 (UTC)

What insect is this?
My brother randomly capture it in a water plastic bottle? We are in Indonesia if that helps.

https://i.postimg.cc/KcsC631Y/20221108-002246.jpg

https://i.postimg.cc/y8053fvJ/20221108-002339.jpg

https://i.postimg.cc/FKm6yjBT/20221108-002347.jpg Salbazier (talk) 17:39, 7 November 2022 (UTC)


 * Looks like an ant. ps last image is the clearest. Graeme Bartlett (talk) 21:30, 7 November 2022 (UTC)


 * Vesbius purpureus? See, for example, the Biodiversity of Singapore. But make sure you compare with other images too, including photographs. We have no article on the species or on the genus Vesbius which belongs to the Harpactorini, a tribe of the Harpactorinae, a subfamily of the assassin bugs. ---Sluzzelin talk  21:46, 7 November 2022 (UTC)

White dust around air purifiers
I've tried multiple air purifiers in the living room, brand name ones and highly rated models, with the correct CADRs for the room, and they, probably because of tall the air circulation around the units, all result in white dust on surfaces around the units. If these machines are so good at cleaning the air, why do they make their surroundings so dusty?

I tend to operate the machines at just below the top speed. The air quality indicators on the new machines all indicate good quality in general, although the current Winix does turn red from time to time, usually when I use the stove, so I'd turn it up when it does. 74.64.73.24 (talk) 21:01, 7 November 2022 (UTC)


 * In general, there is a very big difference between a room that has appropriately-filtered air (possibly maintained by a moveable or installed air filtration system); and a cleanroom that is engineered to have very few or zero airborne particulates.
 * Air purifiers remove dust; but they do not prevent dust from existing. Dust floating around in the air can be conceptualized as a mixture at some sort of stoichiometric equilibrium: we can remove dust, and new dust can be added, and the relative rate of adding and removing dust yields a sort of equilibrium with some amount of dust floating around, just like those rate-equations we used to do for wet chemistry.  Each "specie" of dust has its own rate-equation; an air filter machine just nudges that equilibrium around a bit, with a different effect-size for all the diverse species and sizes of dust.
 * To summarize this in a dramatic and oversimplified fashion: if your room has a floor (in other words, if - when you stand - you are not standing on the steel grates that cover your laminar flow exhaust ports), you shouldn't expect a dust-free volume of air - no matter how many large, powerful air purification appliances you employ.
 * Here's an array of fun further reading:
 * Health and Environmental Effects of Particulate Matter (PM)
 * Particle Pollution and Your Health - "The size of particles is directly linked to their potential health concern ... Larger particles are of less concern"
 * Indoor Air Pollution: Introduction for Health Professionals (in which our esteemed experts tell us how cleaning the air can actually stir up extra dust - hey, we were just talking about turbulence and rate-inversion and sign-flipping in sensitive systems a few days ago!)
 * And, of course, one of my favorite picture-books: Wolf's Microchip Manufacturing, which has a whole lot of cool photos of air filters —
 * Long story short - a very large majority of the dust you can see floating around is physically too large to harm your health. These particles can't get into the tiny spaces inside your lungs; they can't get inside your bloodstream.  Air filters (the kind of appliances you buy at the store - whether they are built to the "HEPA" standard or to any other standard) aren't necessarily trying to get rid of "all the dust you can see" - they are trying to get rid of those airborne particulate materials that are most harmful to human health.
 * If you need to get rid of airborne dust for some other reason — (let's say that you want to make sure there is no radioactive dust that could contaminate your fragile low-noise amplifiers, or avoid any airborne particulates that could smudge up your photolithography mask, or embed in your aluminum sputterer, or any of the other unusual things that you might do in your lab, and absolutely should not do at home, no matter how tempting — but these are scenarios where we say, "yes, we really want exactly zero dust in the air and we will pay any amount of money to do it") — then your air filtration system is going to look and behave a lot differently from even a professional-grade air filter designed to improve healthy air-quality.
 * Nimur (talk) 21:54, 7 November 2022 (UTC)


 * Thank you. The big dust/small dust contrast is important. Nitpick: It'd be clearer to say, "The size of particles is inversely related to their potential health concern." Imagine Reason (talk) 12:05, 11 November 2022 (UTC)


 * More specific to the original query, unless there is some other source of "white dust", it is most likely dead skin cells. Logically, the reason there is an accumulation near the filters, is that the filter's fan attracts the particles in the air, but some are heavy enough to fall before being sucked in, or they bounce off the grating in front.  (Just guessing -- no source).  136.56.52.157 (talk) 19:22, 8 November 2022 (UTC)


 * What? Why would the effective power of the fan be lessened as the particle got closer? If it was strong enough to haul a skin cell from across the hall, how could it be too weak to finish the job? Matt Deres (talk) 21:35, 9 November 2022 (UTC)


 * Yeah, that's my question as well. Perhaps these home purifiers blow filtered air up. As it circulates downward, perhaps more (larger) particles become stuck to horizontal surfaces? Imagine Reason (talk) 12:05, 11 November 2022 (UTC)


 * It's my understanding that dust often accumulates on surfaces, especially wherever the air is circulated such as with box fans, because of static cling. Indeed, according to the Lifehacker website:  which cites the Readers Digest,   spraying a diluted solution of antistatic fabric softener on surfaces ought help reduce the dust from accumulating in your home, but I don't know how effective it is. -Modocc (talk) 23:17, 9 November 2022 (UTC)


 * That'd be an interesting experiment. Imagine Reason (talk) 12:05, 11 November 2022 (UTC)