Wikipedia:Reference desk/Archives/Science/2011 December 10

= December 10 =

Coherence Length of Lasers / Lunar Ranging Lasers
I was doing some research on the coherence length of lasers and was wondering: does anyone know what kind of lasers are used for lunar ranging experiments? NASA has a laser ranging experiment and some universities also do.

What kind of lasers do those experiments use? Does their coherence length make a difference in experiments like lunar ranging?

On a somewhat related note, I know that diode lasers have particularly long coherence lengths for being such small devices (as compared to He-Ne lasers whose coherence length is approximately their laser tube size). What are the coherence lengths of other kinds of lasers, and how does they compare to the overall size of the laser? For example, coherence length of a laser might be important for holography experiments--if I want to save space for my holography experiment, which kind of laser would give me the largest coherence length/laser size ratio?

Inasilentway (talk) 04:39, 10 December 2011 (UTC)


 * The coherence length isn't really important for lunar ranging experiments; the important factors in that application are good collimation and high intensity. Coherence is pretty much irrelevant by the time the beam gets back to Earth&mdash;even the best current lunar ranging facility (APOLLO: the Apache Point Observatory Lunar Laser-ranging Operation) only sees about one photon returned per pulse sent out, and that's tens or hundreds of times better than most previous projects.  APOLLO employs a YAG laser emitting 100 ps pulses of (green light) at 532 nm:, . TenOfAllTrades(talk) 15:06, 10 December 2011 (UTC)

Plants and LEDs
How many normal sized (5 mm) LEDs would be required to provide enough light for a plant to grow in an environment where the LEDs are the only source of light? 82.43.90.142 (talk) 11:27, 10 December 2011 (UTC)


 * This depends on the specifications of the LEDs, and what voltage you run them at (increasing the voltage gives more light but reduces the lifetime). There is some information in the article Grow light.    D b f i r s   12:29, 10 December 2011 (UTC)


 * In case that article isn't clear, many/most? modern LED grow lights don't use 5mm LEDs. There's a slight chance they may use the dies (I've never been entirely clear what dies the cheap Chinese arrays use) but even then I'm guessing not. Even if you're DIY using 5mm LEDs seems questionable to me. This is the same of course as with many LED lighting products. Nil Einne (talk) 18:32, 10 December 2011 (UTC)

Thanks for the answers so far, but I am not asking about professional grow lights. I want to know how many of these would be needed to provide enough light for a plant to grow. I don't understand what you mean by "dies" 82.43.90.142 (talk) 19:46, 10 December 2011 (UTC)


 * A specification would be more useful than a picture. You also need to tell us what size and type of plant.  Are you growing sativa, indica or ruderalis? ;)  As Nil Einne mentions above, your LEDs might not be ideal for purpose because of the wavelength of their light output.    D b f i r s   09:13, 11 December 2011 (UTC)
 * I don't know the specifications, that picture is just off google images, and I'm just looking for a very general answer. It doesn't have to be perfectly accurate. I am not growing illegal plants, I am not growing any plants at all, this is just something I was curious about after reading an old question from 2007 (hence why I don't need a accurate answer). I think everyone is getting distracted by the LED specifications, when the core of the question is "approximately how much lux does a small LED emit, and how much lux does a small potted plant need to survive". I did try to look this up on google but couldn't find it 82.43.90.142 (talk) 11:05, 11 December 2011 (UTC)
 * That will depend greatly on the light wavelengths emitted by the LED/s, the brand, the age, the current you run it at, since you're running a lot of them how good the heatsinking is etc, in other words your question is way too non specific to be able to give a meaningful answer. A recent Nichia white LED running at 20 mA will be quite different from a cheap Chinese blue LED from 2007 running st 10mA. As I said, it doesn't matter much whether you're talking about professional lights or DIY, it simply doesn't make much sense to design a grow light with 5 mm LEDs. In fact I think one of the article mentions that some grow lights tried that, it didn't work very well.
 * When I referred to die, I meant Die (integrated circuit). A cheap Chinese high power LED is always an array LED with lots of dies e.g. although now that I think about it more carefully, I think these are normally '1W' dies which are quite far from 5mm dies.
 * Note that you'll also have to be more specific in the rest of your question. What sort of area are your plants contained in? Clearly if you're growing a single basil plant you're going to need much less then if you're growing a 10 square meter room full of whatever. Note that if you have a specific type of plant in mind, you may also be able to tailor the wavelengths better then if you want a general purpose growlight.
 * Nil Einne (talk) 13:00, 11 December 2011 (UTC)
 * BTW if you really want to see how much light a 5mm LED emits, you can start here . However that only covers white LEDs and the newest LEDs are from 2009 (probably partially because few care about 5mm LED efficiency nowadays) and you'd be pretty dumb to design a grow light with a generic white LED anyway IMO. (Even if you wanted to replicate sunlight most white LEDs don't do a good job of that in general. I'm not even sure if a high CRI white LED would be good for that purpose since CRI is inherently dependent on human vision. In any case I'm not aware of many 5mm LEDs that are high CRI.)
 * For colour, I haven't see any tests of 5mm LEDs but you can look at official specs for a reliable manufacturer. E.g. Cree  or Nichia       . (I excluded white LEDs although I think  is their current best.)
 * P.S. Perhaps I didn't make clear earlier but if you aren't using a LED module designed for a grow light I think you'll generally use at least two colours for a grow light. Also I think one of the disadvantages of 5mm LEDs is that they aren't really designed for efficient heat transfer (you basically rely on the leads) unlike higher power LEDs (or even many SMD LEDs) so heat transfers is likely to be a more accute problem, consider that in relation to my heatsinking comment.
 * You may also be interested in these old threads   . Even in 2006-2008 most people who had any idea what they were doing didn't seem to consider 5mm grow lights a good idea.
 * Also in case it isn't obvious from the results, high power LEDs are generally rated in Luminous flux, except for royal blue LEDs (and I guess UV and infrared and others likes that) where that doesn't work very well. 5mm LEDs and similar are generally rated in Luminous intensity. Illuminance doesn't really make much sense since it depends on how far the LED is from the surface etc.
 * Nil Einne (talk) 13:41, 11 December 2011 (UTC)

blood groups
if a mother blood group is o+ve and her son blood group is o+ve what is the blood group of father — Preceding unsigned comment added by 101.2.3.121 (talk) 17:01, 10 December 2011 (UTC)


 * It could be anything except AB. Dauto (talk) 17:05, 10 December 2011 (UTC)
 * And if you take rare genetic variants into account, it could even be AB, see Cis AB. --NorwegianBluetalk 20:52, 10 December 2011 (UTC)


 * You will find more information at ABO blood group system.--Shantavira|feed me 17:59, 10 December 2011 (UTC)

Liquid metal cooled nuclear reactor
How can a hot liquid metal cool a nuclear reactor? It seems counter intuitive because liquid metal is usually hot unless they use mercury, which they don't anymore. ScienceApe (talk) 19:29, 10 December 2011 (UTC)


 * Temperature is ALWAYS a relative thing. Yes, the metal (Likely sodium) is going to be too hot to touch, but it is still cooler than the reactor core, so it will remove heat from the core, cooling it. Dauto (talk) 19:57, 10 December 2011 (UTC)


 * It doesn't sound like a good idea, to me, though. I picture something going wrong where the liquid stops circulating, then cools and forms a solid in the pipes far from the reactor.  Meanwhile the metal in the pipes near the reactor would get so hot it would melt the pipes.  At that point, even if you managed to avoid a disaster, your primary cooling system would need to be replaced.  Doesn't sound very reliable. StuRat (talk) 21:37, 10 December 2011 (UTC)


 * Sodium solidifying. If one looks at a sodium bulb from a street lamp one will see little beads of sodium. They melt at about boiling point of water– way, way below the the idling temperature of a reactor. So, not much of a problem – and it conducts heat very well so the melt travels. Which is why its being studied as a coolant. Primaries don't get replaced (radiations too high), instead the reactor gets scraped.--Aspro (talk) 22:04, 10 December 2011 (UTC)


 * Even worse, then. And there's always the risk that some water would leak into one of the cooling lines, or, sodium would leak into where there's water, with explosive results.  This seems especially likely if the secondary coolant loop is filled with water. StuRat (talk) 22:55, 10 December 2011 (UTC)


 * If water could leak in, that would mean coolant could leak out, which is a much more dangerous situation than a potential sodium fire. It's possibly the most important system in the reactor, and so is protected by layers and layers of backup and safety features. If you get to the point where coolant is leaking (a very unlikely event, it took the worst earthquake/tsunamis on record to do it, and even that obsolete design with fewer safety features than modern reactors did not fail until long after workers were evacuated from the facility), you should be more worried about a dangerous radiation situation than a sodium fire.- Running On Brains (talk) 07:16, 11 December 2011 (UTC)


 * With a water-cooled system, a few drops leaking out occasionally wouldn't be the end of the world (presumably they can detect any pressure loss and add more water while they locate and repair the leak). A few drops of sodium could ignite and expand the leak, causing a rapid catastrophic failure of the cooling system, leading to that radiation situation. StuRat (talk) 17:00, 11 December 2011 (UTC)


 * The laws of thermodynamics demonstrate that the hight the temperature differential the more efficient the power station will be at converting heat energy to electrical power. As temperatures increase water becomes more corrosive. Superheated_water. Once the core assembly and primary cooling system has been built and commissioned it is not feasible to do repair and maintenance upon it (which is why every bit is x-rayed and tested during construction, to ensure that it will last (?) the whole of its operational life). Liquid sodium,  although pyrolytic, it will allow  core to operate at a higher temperature that is currently possible with water as a 'primary' coolant over the economic life of the station. Thus, it promises higher efficiency – i.e., more KV hours per gram of fuel than with water cooled primary circuits. For comparison, if one is looking for safety over efficiency, then the the old British Gas Cooled Reactor was top-dog. It was very stable. So in answer to the OP's question, the primary liquid sodium cooling circuits is indeed a lot hotter but in terms of bother temperature and heat transfer [calories]  it does indeed  do a splendid job of conducting heat energy away to the secondary circuits, where the stream is produced for direct use by the turbines. Anyone with a high performance car will be familiar with their engine valves having liquid sodium cooling.--Aspro (talk) 21:47, 10 December 2011 (UTC)


 * (ec) Our article on liquid metal cooled reactors may shed some light. (Remarkably, there was at least one early experimental reactor that did use mercury coolant&mdash;Clementine was a research reactor, in service from 1946 to 1952.)  Liquid metal reactors can be operated safely at higher temperatures and at atmospheric pressure, because there's no need to contain (or prevent the formation of) high-pressure steam in the reactor vessel; this offsets some of the safety concerns associated with the use of liquid metal coolants.  Our article on sodium-cooled fast reactors provides links to more than a dozen past and current sodium-cooled nuclear plants operated by a number of countries.  Soviet Alfa class submarines (the fastest military submarines ever built) used lead-bismuth cooled power plants for maximum power output from a (comparatively) tiny and lightweight reactor. TenOfAllTrades(talk) 22:05, 10 December 2011 (UTC)

Killing dust mites
When I store clothes for the winter and bring them out in the spring, they smell foul, presumably because they are full of dust mite poop. So, I have to wash them all again. Would storing them in space bags kill them, or at least keep them dormant, or is the partial pressure of air inside enough for them to continue pooping happily all winter long ? How about if I used bleach in the wash just prior to putting them inside ? Would that kill them ? Or do I need to suck all the air out and replace it with nitrogen (which sounds far more expensive than rewashing everything) ? StuRat (talk) 21:48, 10 December 2011 (UTC)


 * The article suggest that you put the cloths in the freezer for a while. A few sachets of silica gel in the bag may also not go amiss either.--Aspro (talk) 22:11, 10 December 2011 (UTC)
 * So THAT's what those little beads are for... Heck froze over (talk) 16:43, 11 December 2011 (UTC)
 * I can see freezing to kill off the adults, but I would think eggs could survive freezing. If not, how did the species survive winter before they had nice warm homes to infest ? StuRat (talk) 22:51, 10 December 2011 (UTC)


 * The article doesn't mention whether washing at 60 degrees kills the eggs. Can they survive a hot wash?  Is is possible that the smell is from some form of mould rather than dust mites?  Mould spores seem to get everywhere and grow if there is significant moisture.  Silica gel would help reduce moisture, as would space bags because of the seal (not because of partial pressure -- the inside pressure is the same as the outside).  Try a combination of both!    D b f i r s   08:52, 11 December 2011 (UTC)


 * I'd have more confidence in the bleach killing the eggs than the hot water. StuRat (talk) 19:15, 11 December 2011 (UTC)


 * If you don't mind bleaching your clothes, then bleach will also kill mould spores, but space bags or any sealed container will help prevent more spores reaching the clothes, and silica gel, or a dry storage area will discourage growth of mould (sorry, mold on your side of the pond).   D b f i r s   21:01, 11 December 2011 (UTC)

jj thompson's third experiment
jj thompson measured the mass charge ratio of cathode rays in his third experiment. can someone explain in detail, including the simplifications, how he did this. the jj thompson article just states that "when Θ = φ and Fel/mv2= Hel/mv. This can be simplified to give m/e = H2l/FΘ" but doesnt say how it can be simplified. — Preceding unsigned comment added by 109.152.109.222 (talk) 22:24, 10 December 2011 (UTC)


 * Thomson's (no "p") 1897 article is online here. It's actually pretty readable even though I'm not sure the equations are in the same terms as we'd use today. He explains his experimental setup in very clear terms, as well as his mathematical reasoning from the data. (It's also pretty interesting as a read because you can see that Thomson thinks he has discovered something very different from what we today consider the "electron" to be, something he signifies by calling it the "corpuscle," even though the term "electron" was widely being used as a hypothetical negative charge particle by numerous theorists, like Lorentz. Thomson thought he had discovered the Proutian protyle, a very different sort of beastie.) --Mr.98 (talk) 22:59, 10 December 2011 (UTC)

thanks. exactly what i was looking for. — Preceding unsigned comment added by 109.148.92.178 (talk) 21:16, 11 December 2011 (UTC)

Blood transfusion
Were they sometimes performed before people knew about the ABO blood group system? It sounds dangerous, and it certainly was, but sometimes, if death was inevitable without a transfusion, why not take chances and maybe get the right blood type? 88.9.210.149 (talk) 22:42, 10 December 2011 (UTC)


 * Yes. "Experiments with blood transfusions ... have been carried out for hundreds of years. Many patients have died ...". Blood transfusion has more details. Clarityfiend (talk) 23:04, 10 December 2011 (UTC)


 * See http://www.watchtower.org/e/hb/index.htm.
 * —Wavelength (talk) 23:29, 10 December 2011 (UTC)
 * I'm not sure how a link to a Jehovah's Witness' website explaining why blood transfusion is a sin, or offering misleading and deceptive misrepresentations of the hazards associated with transfusion, is a constructive thing to provide here. This is the Reference Desk; proselytizing is unhelpful and unwelcome. TenOfAllTrades(talk) 00:34, 11 December 2011 (UTC)


 * People can choose to skip the religious portions. How does the page "Blood Transfusions—How Safe?" misrepresent the hazards?  It says that incompatibility involves more than "the relatively few blood types that hospitals seek to match", and provides medical references.  The page "Quality Alternatives to Transfusion" answers "why not take chances and maybe get the right blood type?" by discussing bloodless surgery with volume expanders as alternatives.  That page also provides medical references.
 * Possibly the information at http://www.noblood.org/content/ will be found to be more acceptable to some readers.
 * —Wavelength (talk) 01:48, 11 December 2011 (UTC)


 * I think the words "the relatively few blood types that hospitals seek to match" are an insult to modern medical practice and the thousands of dedicated medical professionals involved in it. It's probably dangerous too, in hinting to people that the system is not as good as it really is. Please drop the God driven crap! HiLo48 (talk) 01:55, 11 December 2011 (UTC)


 * I would also suggest that your imaginary friend is driving you to break our rule about no medical advice. Please keep your fringe personal values to yourself. HiLo48 (talk) 03:04, 11 December 2011 (UTC)


 * What does this have to do with the question on historic transfusions? I quite doubt other safety risks are a big concern when you don't even know the ABO type. And it's unlikely most of the techniques mentioned in bloodless surgery and volume expanders were available or understood by people who didn't even know of ABO blood types.Nil Einne (talk) 15:28, 11 December 2011 (UTC)


 * The article on the ABO blood group system describes several American ethnic groups, such as that Mayas, with entirely O blood type. For them, incompatibility would not have been a concern.  I think it is interesting that such groups have been widely reported to have engaged in extensive "ritual" bloodletting in the past; unfortunately, it doesn't seem like technological explanations are ever considered for such activities. Wnt (talk) 20:11, 11 December 2011 (UTC)
 * I actually did quite a bit of research into this because of people I know who are Jehovah's witnesses. As far as risky medical procedures go, blood transfusions these days are NOT high on the list and it is one of the few procedures that survives from medieval times due to the advances that have been made. The same can not be said for many "pre scientific" medical procedures, like trepanning, which were horrific by our standards. The are several BIG problems with the JW position, I know this isn't the proper place for it but I just can't help myself: They overstate the risks, as if blood transfusions are somehow unique in this regard. Every medical procedure has risks, but refusing any medical procedure has risks too. Yes there may now be "safer" alternatives in some cases, but JW position assumes that the alternatives are in every case available and/or more suitable, which is ridiculous. Secondly, the "bloodless" alternatives (products AND procedures) very certainly would not exist if not for the generations of work done by people who COULD use blood. Before blood transfusions, surgeons had literaly minutes after cutting a patient open before they'd have to sow them back up, or else the patient would bleed to death. Without blood transfusions, surgery as we know it today WOULD NOT EXIST, we certainly wouldn't have organ transplants, let alone "bloodless" organ transplants, no one would have dreamed of it without the additional time blood transfusions provide: So is "God" happy for his pious followers to benefit from the hard work of people who could ONLY be "sinners"? Third, isn't the act of forsaking your everlasting soul to save your own child's life just about the most selfless act conceivable? How could a God that punishes parents for saving the lives of their children fit the definition of benevolent? It's ridiculous. Vespine (talk) 21:52, 11 December 2011 (UTC)
 * In his novel Whipping Star, Frank Herbert described a fictional "Bureau of Sabotage" tasked with the job of preventing government from becoming too efficient. In the real world, religion is that bureau.  People decide they have to carry ceremonial daggers on planes, can't have pictures taken for driver's licenses, have to keep their hijab or their magic underwear on during the strip search, have to have peyote or wine for ceremonies, and yes, even turn down perfectly sensible medical advice.  Life would be so easy for bureaucrats if they could just decide what was right for everybody and make them do it without question.  But it would be life unexamined, without respect for individual cussedness, without the imagination to accept that maybe the faith these people have is worth something, even if we don't believe its expression.  And the bureaucratic strictures it allowed would be no less irrational - indeed, no doubt, the basis of some future religion when people continue to follow them despite a shift of power.  Of course, this doesn't deny a very real conflict of moral beliefs when the innocent really are at risk, but perhaps if we can find in ourselves a little more respect we could be a little more persuasive. Wnt (talk) 04:36, 12 December 2011 (UTC)

The novel Dracula describes Van Helsing trying several transfusions on Lucy Westenra to try to stop the effects of Dracula's bite. The novel was published in 1897, about 3 years before our article on ABO blood types says they were discovered. The transfusion is presented as a new, cutting edge procedure that may help Lucy, and if I remember right, all the other characters take turns giving her blood. Bram Stoker's characters don't seem worried about incompatibilities in the blood and appear to be unaware that such a thing exists. They are looking at it as a last resort to save her. So maybe if you were sick enough to need a transfusion, if you died due to blood type mismatches, everyone assumed your illness/injury caused your death, not the transfusion. Tobyc75 (talk) 22:04, 11 December 2011 (UTC)

Light bulb dimmer switch
Why do they buzz? Kittybrewster  &#9742;  22:58, 10 December 2011 (UTC)


 * That's the sounds of it rapidly connecting and disconnecting the power, to get the dimming effect. An older type, a rheostat, would burn off the excess as heat, so made less noise, but wasted more electricity.  According to that article "a light dimmer uses a potentiometer to control the switching of a TRIAC and so indirectly controls the brightness of lamps". StuRat (talk) 00:42, 11 December 2011 (UTC)


 * They can also cause a buzz on AM radio if you have the radio on and have the switch somewhere between "off" and fully "on". ←Baseball Bugs What's up, Doc? carrots→ 01:13, 11 December 2011 (UTC)


 * As for what actually causes the buzz, it's probably the choke used to slow down the switching edges in the triac to reduce RFI. A triac is a solid-state device that is unlikely to make any noise, but a choke is made from wire wrapped around a ferrite core. When the current in the choke changes rapidly, as it does twice in every cycle of the AC power supply, the magnetic field around the wire exerts a mechanical force on the wire, causing it to twitch slightly. This is what causes the buzzing. Sometimes 'potting' the choke in epoxy can reduce this noise. --Heron (talk) 11:58, 11 December 2011 (UTC)
 * See also Magnetostriction. Besides movement of the wires in a coil, ferromagnetic material can change its dimensions slightly when the magnetic field changes. Edison (talk) 19:50, 11 December 2011 (UTC)