Wikipedia:Reference desk/Archives/Science/2007 July 16

= July 16 =

psi for knockdown
How much PSI [pressure per square inch] would it take to send a man flying backwards about two foot if the force was only applied to around 11.8 square inches of the mans body, he is 5.11 feet tall and weighs 155 pounds, can anyone calculate this? Thank you
 * This sounds suspiciously like homework. In any case, we'd still probably need to know the coefficient of friction between his shoes and the ground. --YbborTalk 00:44, 16 July 2007 (UTC)
 * I don't think the coefficient of friction means much here - there is enough of a moment between frictional force and center of pressure of the body to rotate the guy backwards and then the friction in the feet will essentially go away. So neglecting the friction of his shoes...Well, how fast to we have to accellerate him to get him to "fly" backwards for two feet?  Actually, if he just fell backwards, he'd end up with his center of gravity two feet from where his feet were so the answer might strictly be zero.  But let's suppose you actually wanted his feet to move back two feet in the time it took him to fall flat on his back.  That would mean moving his center of gravity (CofG) maybe 4 feet backwards in the time it takes his CofG to fall maybe 2 feet.  That's going to need an accelleration of about 2 g's - so a force equal to maybe twice his weight?  If he weighed 115lbs - then we need 230lbs of force spread over 11.8 sq inches - so the pressure is 230/11.8 or about 20psi.  That's a LOT! SteveBaker 02:21, 16 July 2007 (UTC)
 * Is there a way of converting between wind speed and the effective pressure one would feel (average) if standing directly in the path of the wind? Someguy1221 03:07, 16 July 2007 (UTC)
 * Look up Drag (physics). Mathematically, it's the same weather the body is moving and the fluid is at rest or if the fluid is moving and the body is at rest. — Daniel 03:51, 16 July 2007 (UTC)

Why can the sky be blue or red, but not green?
If the red sky during sunrise and sunset is due to refraction, why aren't there times when the angle is such that a different wavelength, such as green light, hits where you're standing and causes the sky to turn that color? — Daniel 00:08, 16 July 2007 (UTC)


 * It's not. See: Rayleigh scattering and diffuse sky radiation.  The sky away from the sun looks blue because blue light is more easily scattered back towards you.  If you look towards the sun at sunrise or sunset, then there is so much atmosphere in the way that all the blue is scattered off and what you see is the leftover reds.  Dragons flight 00:17, 16 July 2007 (UTC)

Green Flash? -- L augh! 00:26, 16 July 2007 (UTC)


 * How come there isn't a time between a blue sky and red sky when there is enough atmosphere to filter out most of the blue light but not most of the green light, and more green light gets scattered towards you then red light? Also, why is the green flash only green, or occasionally blue? From what I read of the article, it seems like it should have a rainbow look. How come the effect I mentioned doesn't happen to the moon during a lunar eclipse? It's red in the middle of the shadow, and blue at the edges, but it's purple in between, rather than a rainbow containing every color but purple. — Daniel 00:51, 16 July 2007 (UTC)


 * I think some of the green flash types are a full rainbow, as seen in the pics, but the "normal" colors just aren't worth mentioning, as they're so common. Also, I've seen pale green skies as a result of severe thunderstorms (and, I imagine, hurricanes), perhaps due to some matter that has been drawn into the air, like droplets of sea water. StuRat 00:57, 16 July 2007 (UTC)


 * Green flash has this covered. The deal is that the human eye doesn't see the pure frequency of a colour - it sees the amount of pure red, pure green and pure blue - and anything between those three primary colours seems to us to look like a mixture of the two.  We can't tell the difference between 'yellow' and a mixture of 'green' and 'red'.  In physical terms, those are different things - but not to our eyese.  The sky isn't ever truly "blue" (the intense blue our retinas respond to) - it's more of a 'cyan' - which is about a 50/50 mixture of blue and green.  Similarly, the 'red sky' of a sunset is more often yellowish/orange - which is a mixture of red and green.  So if there is never a period in which there is neither red nor blue light in the sky - but instead it's always a mixture of colours - then all you'd see would be a muddy transition from yellow to cyan - without there ever being a 'pure' green. SteveBaker 02:28, 16 July 2007 (UTC)


 * Technically, you can't even see pure green, because green light excites red and blue receptors, though less then green ones. I'd expect you'd at least see a distinctly greenish gray. — Daniel 03:06, 16 July 2007 (UTC)


 * To amplify Daniel's point here, it is not correct that the eye "sees the amount of pure red, pure green and pure blue". There are three kinds of color receptors and it sees the amount of light each one detects, but each responds to a range of frequencies.  The three primary colors are only relevant to technology that wants to trick the eye into thinking it is seeing a full range of colors without actually producing them all -- i.e. printing, photography, and video.  And I don't believe Steve is correct to suggest that the details of human color vision are relevant to the main question.  The question can be recast as, how come it doesn't seem to ever happen that of the range of frequencies in sky light, the dominant one is green?"  And I suggest that the answer is just that there are only so many ways that color gets into the sky, and it happens that none of them enhances the ratio of green light to other frequencies.  --Anonymous, July 16, 2007, 07:52 (UTC).


 * Anonymous makes an important point here. The frequency responses of the various pigments in the human eye don't correspond very well to blue-red-green and IMHO this is not that relevant to the question anyway. See Colour vision. However several points which need to be clarified from anonymous... There is a large variety of possible primary colours. Red-green-blue (RGB) is the most common additive combination by far and is therefore relevant in monitors, cameras, scanners etc. CMY (cyan, magenta, yellow) is the most common subtractive combination and is therefore relevant to printers, paiting etc. Nil Einne


 * This is not about printers or technology or TV or anything like that. Allow me to explain in more detail - I'll split the explanation into little bite-sized chunks:
 * Each type of receptor in the eye has a roughly gaussian response curve centered on whichever frequency it is responsive to - these being the colours we label as 'red', 'green' and 'blue' - the precise 'center' frequency isn't particularly important here - so let's just label those three center frequencies 'red', 'green' and 'blue' for the purposes of these discussions.
 * In order for our eye/brain to see 'green', there must be very little red or blue light present. If there is green light AND red light entering the eye - we see yellow or orange because we are totally unable to distinguish a mixture of red light and green light from (say) a true orange light from a sodium lamp - even though there is a very, very clear physical difference between the two.
 * Since the light from the sun is pretty much evenly spread over all of the visible spectrum, if the sky at sunset were to filter out (refract away/absorb/whatever) all of the light within the range of frequencies close to the center of our blue response - then we'd see the sky as being a yellowish orange even though the light is actually a mixture of all of the frequencies from red through green. That's because when both red and green sensors are being adequately stimulated - we can't tell the difference.
 * Similarly - if the sky is getting rid of all of the red light (as during most of the day) then we get a mixture of all of the frequencies from green through to blue - and we see the colour that computer graphics folks call 'cyan' (a paler, baby-blue..."sky blue" to use a common name for it).
 * If the frequencies that are filtered out by the sky never include both red and blue simultaneously - then we'll never percieve the resulting light as 'green' even though there is tons of green light present, the addition of either red or blue in sufficient quantities will result in us seeing yellow or cyan respectively.
 * So - if the sky filters out red light ("all of the frequencies close to red") during the middle of the day - and blue light ("all of the frequencies close to blue") at dawn and dusk - leaving green light always present - then it can be (and actually *is*) the case that green light may never be present by itself.
 * What all of that long explanation means is that you'll only ever see the sky transition from cyan (blue+green) to yellow/orange (red+green). Halfway between the two, you are seeing red+blue+green = white or grey.  If you actually watch a sunset sometime, you'll see that between the blue sky and the orange sunset there is indeed a band of grey right where our questioner expected there to be a pretty green rainbow-effect. The 'green flash' phenomenon is something else. SteveBaker 17:17, 16 July 2007 (UTC)


 * I say again, all the details of the eye's color perception are irrelevant. The only important issue is in order to see the sky as green, the dominant color of the light would have to be green, and for the reasons explained, that doesn't happen. --Anon, July 18, 02:52 (UTC).


 * Well, you can say it all you want - but this (====>) image proves you are wrong. The cyan and yellow swatches (because they are being displayed on a computer monitor) are not 'true' monochromatic colours - light of wavelength ~480nm (cyan) and ~580nm (yellow) are not present in the light from your monitor. I deliberately painted those swatches so that they have more green than either red or blue. In your terms "green is the dominant colour" in both colour swatches.  Yet they don't look green - they look sky-blue and yellow...just the kinds of colour you'd see around sunset.  This is all the more surprising because our eyes are more sensitive to green light than to red or blue.  A spectrometer reading (or a super-human being with perfect colour vision) would immediately reveal that these are mixtures of green/blue and green/red - and not cyan and yellow at all.  Such a being/instrument would be able to tell you that there is lots of green there just as there is lots of green light in the sky at sunset.  We poor humans can't see that because our eyes just can't do that.  So details of human perception are entirely relevent to this explanation. SteveBaker 23:42, 18 July 2007 (UTC)


 * I said "in order to see the sky as green, the dominant color of the light would have to be green", not the converse. And anyway, we are talking about natural light, which does not consist of a mixture of just two or three widely separated frequencies like light from a monitor. --Anon, July 18, 02:44 (UTC).


 * Not to take sides (because you both lost me a ways back), but I just came across this little piece of doggerel last night, which seems somehow apposite:
 * Cold-hearted orb that rules the night
 * Removes the colours from our sight
 * Red is gray, and yellow white
 * But we decide which is right
 * And which is a quantization error.
 * —after The Moody Blues ("Nights in White Satin"), as found on the NetPBM man page for ppmtopgm
 * —Steve Summit (talk) 15:45, 22 July 2007 (UTC)


 * (unindent)
 * The OP asked why there is no green in the sky around sunset. We agree that sunlight contains (more or less) all light frequencies.  Your statement is of course true - the sky won't look green unless there is green light in there somewhere - but we're not being asking why the sky looks green.  The OP asked why it DOESN'T look green - which makes your statement useless (although it's undoubtedly true).  My point is that there are two possible (and very different) interpretations of the observation no part of the sky looks green in a typical sunset sequence:
 * 1) There is no green light present around dusk because it's somehow being filtered out.
 * ...or...
 * 2) There is plenty of green light present - and it might even be the dominant frequency band - but because sunlight contains an almost complete spectrum, the presence of blue or red light in the mix will trick our eyes into seeing cyan or yellow/orange and make the perception of a green band a virtual impossibility.
 * My explanation for the reason for not seeing green in the sky is NOT (1) - it's (2). I happen to know for sure it's true because I've studied the sky in order to render it in computer graphics for flight simulation.  There is in fact plenty of green light in the sky throughout sunrise and sunset - and frequencies close to green might even dominate the frequencies in some parts of the sky (between the orange sunset and the cyan zenith) - but we can't see that as an obvious green band because there is still enough blue and red present in the sky to fool our eyes.  The fact that our eyes can be fooled into not seeing green even though it's dominant is elegantly demonstrated by the colour swatches (above).  That statement is only explained if our eyes work the way they do - which is why my discussion of how human vision works is entirely relevent to the discussion.  A hypothetical creature with better colour perception than humans (a goldfish or certain species of freshwater shrimp) would be perfectly able to see a green band in the sky. SteveBaker 12:07, 19 July 2007 (UTC)

You say the colour picture is supposed to be sky blue? You must have a very weird sky then, because it looks distinctly turquoise to me.


 * Actually, it does on my LCD monitor here at work too - but not on my CRT at home...well, whatever. It's not green. SteveBaker 19:04, 19 July 2007 (UTC)

plant leaves turning bright red
I noticed a few species of plants are already turning bright red. I wonder why. (It's a normal summer here in Iowa.) Is some evolutionary advantage conferred by this early shutdown of photosynthesis? --Halcatalyst 03:59, 16 July 2007 (UTC)


 * Are you referring to leaves which have started senescencing earlier then normal or plants which always undergo leaf senescence fairly early on? If it's the former, there are a variety of reasons why this may happen. Check out and  for some discussion but I'm sure I've read better discussions somewhere, perhaps it was on here even. If it's the later, the first thing to consider is what plants are you talking about. Are they natives? If not, they may very well be adapted to a different environment and there is no evolutionary advantage to them to start leaf senescence so early, indeed if they survive naturally in their current environment in the next say 50 generations their leaf senescence may be likely to evolve to a later period. Of course, if the plants have a wide range, then they may always exhibit a fairly early senescence in certain areas. Late senescence is I presume far worse then early senescense and the competiting pressures over the range means it will always be a balance of both... Nil Einne 12:26, 16 July 2007 (UTC)
 * Thanks for your response. Leaf senescence is a concept new to me. I poked around a little on the web to find out more, but the articles I found were beyond my competence, or at least beyond my time availability. ;-)
 * Not knowing much about botany, I can't answer your questions very well. I assume they're native plants; I found them in the wild; I don't know how to identify the species. What struck me was the bright red, brighter than any maple I've seen. The red would be produced by one of the substances remaining in the leaf when photosynthesis ends. Are there classes of plants that produce especially bright red leaves early in the season? --Halcatalyst 14:34, 16 July 2007 (UTC)


 * We don't have an article on leaf senescence but we do have a (rather poor) article on plant senescence. You don't really need to understand leaf senescence (I briefly studied it once but have now mostly forgotten what I learnt) but it's important I think to appreciate, and I think you already did that the leaves of decidious plants turn red and eventually fall off during autumn by a tightly regulated process (which we're only just beginning to understand). Also, perhaps I was a little too technical in my answer. What I was trying to say is, are you referring to a case where the leaves turned red earlier then would normally be expected or plants whose's leaves always turn red very early on? From your later responses, it's appears you may not be sure. Nil Einne 23:12, 17 July 2007 (UTC)


 * A cold snap can cause leaves to turn colors earlier than usual (senescence), basically by making the plants think fall is here. Have you had a cold period there ? StuRat 14:18, 16 July 2007 (UTC)
 * Hi StuRat. We had some unusual cold earlier this year, but it's been a month or two. There was a hot period, and it's been fairly dry, so that could have stressed the plants. But the rest of the plants in the particular place where I noticed the red leaves seem to be thriving as usual. --Halcatalyst 14:37, 16 July 2007 (UTC)


 * The principle is pretty straightforward - the plant needs to use some energy to keep the leaves alive - once those leaves produce less energy than they are consuming - the tree is better off without them - so it dumps them and shuts down until spring. It's not about heat - it's about incident sunlight.  As the days get shorter and the angle of the sun lower to the horizon, the amount of energy produced by the leaves gets less and less.  So even if there are periods during the day when the sun is bright and the weather is hot, the leaves may still be producing less energy than they consume as an average over each 24 hour period. SteveBaker 15:39, 16 July 2007 (UTC)


 * Here's what I'm really interested in knowing: Are there classes of plants that produce especially bright red leaves early in the season? If so, what might be the evolutionary advantage? Thanks, Halcatalyst 18:24, 16 July 2007 (UTC)
 * Evolutionary advantage, I wouldn't know. But if you're in chaparral country in the American West, there is one particular plant that notoriously tends to turn red early. Don't count on it being red, though -- there could still be green bushes of it quite late into the summer (maybe even fall). --Trovatore 18:43, 16 July 2007 (UTC)

Red leaves tend to indicate a state of stress, to much heat or lower leaves not receiving enough light will cause the plant to shut down leaves, people like red leaves so they put selective pressure on plants with red colored leaves - in the 'wild' they (red leaves) would be at a disadvantage. Hardyplants 09:39, 17 July 2007 (UTC)
 * Thanks all for your responses. I conclude from what you say that the red color has no particular evolutionary consequence. --Halcatalyst 13:55, 18 July 2007 (UTC)

research for a sermon   concerning flames
how far can a single flame be seen by the naked eye in the dark? miles? feet?

please email me at (deleted).


 * The custom is that questions asked here are answered here, not by email.
 * At this page is a table showing how far away a typical candle flame can be seen with different optical instruments. With the naked eye it says 1.4 km, which is just under a mile.  Similarly, if you look at the Galileo Galilei page, it talks about experiments he invented in an attempt to measure the speed of light, which were conducted using lanterns about a mile apart.  Presumably a greater distance would have been used if the lantern could have been seen.  So it seems the answer for a small flame is about a mile.  --Anonymous, July 16, 2007, 08:10 (UTC).


 * Obviously it depends somewhat on what you mean by a single flame. A burning tree could probably be seen for far greater then a mile, even though it's arguably a single flame. Nil Einne 12:56, 16 July 2007 (UTC)


 * Agreed. Oil well fires can even be seen from space, like these in Kuwait after the 1991 Gulf War: . StuRat 14:14, 16 July 2007 (UTC)


 * Though it's much easier to see smoke than flame in a large fire. A smoke plume can rise thousands of feet in minutes, while a flame rarely gets anywhere close to that altitude.  Nimur 15:20, 16 July 2007 (UTC)
 * The original questioner may also want to see Visibility, which has precise definitions of the atmospheric condition. Nimur 15:27, 16 July 2007 (UTC)


 * Remember we're talking about the dark here. Presuming we mean no other light at all, then won't see any smoke if you can't see any light from the flame. Nil Einne 22:58, 17 July 2007 (UTC)


 * There are lots of variables here. Firstly, are the viewer's eyes dark-adapted by being in darkness for at least 30 minutes beforehand?  Secondly how much other light is there?  You can see incredibly dim things with dark-adapted eyes in total darkness - but in full sunshine it would be hard to see a candle flame at 100 feet.  I believe I've heard the distance of about a mile for a candle flame on a dark (ie moonless) night, far from any cities with dark-adapted eyes.  But this is for a sermon?  You might as well make up a number - everything else is unlikely to be scientifically correct anyway. SteveBaker 15:32, 16 July 2007 (UTC)
 * WP:Don't bite the newbies, SteveBaker... we have no way to know what the contents of the sermon are, and the effort that is being made towards scientific fact should be encouraged. Nimur 02:17, 17 July 2007 (UTC)
 * See Candela. The candela is a modern unit of luminous intensity, light emitted by a point source in a given solid angle carefully set to equal the brightness of the 19th century unit the "candle" which was defined in terms of the composition and the grams burned per hour. "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian."  Now relate that to the threshold sensitivity of human vision and you have your answer. Certainly I would assume full scotopic vision after a half hour or more of dark adaptation. In lab experiments the brightness of a distant candle could be simulated for test purposes with a point source of light closer to the observer. I would not assume it was foggy, or that there was dust in the air, or that it was broad daylight or any other assumption that diminished the distance, as the goal seems to be to state the maximum distance. I seem to recall that under ideal conditions and lab conditions the threshold was just a few photons. The observer would not see a bright flash, but would see a slight change in a small area of the random visual noise generated by the retina and nervous system. This absolute threshold would only show up in a forced-choice psychophysics experiment, and an observer would typically want intensity to be several deciBels above it to confidently say he "sees" the light. The Theory of signal detectability provides a theoretical framework for measuring the detection of signal in noise, which is always the case near threshold. The range of stimulus intensity for human vision is large. The most intense light that can be viewed without pain is 1016 more intense than the weakest light that can be seen, which is a range of 160 deciBels. 10-10 lambert has been stated as the visual threshold.(P. Lindsay and D. Norman, Human information Processing, Academic Press, New York, 1973, p153). Per Graham and Veniar, 1949,  "photons=(10/π)* (pupil area in square millimeters)*(brightness in millilamberts). Per Pupil the dark adapted human pupil has a 8mm diameter when dark adapted, or an area of 50.3mm. This means the stated threshold would equal .00016 photon, which is pretty surprising! Perhaps others can work from the one candela, the 8mm pupil, and find another statement of brightness threshold to yield a max distance. Hecht in the 1940's developed a night vision adaptation testing device which flashed a light repeatedly 4 degrees from the center of fixation, where night vision is most sensitive.  reprints his data graph showing the threshold to be about 3.2 log micromicro lamberts, which should equal 16 *10-10 Lambert. This was apparently not a forced choice modern psychophysics experiment, so it would have yielded a higher threshold.  Still have to work this back to distance.  Edison 16:10, 16 July 2007 (UTC)
 * I find luminance questions tedious to calculate because of the numerous different bases, and this at least makes me more sympathetic to people who can't grasp the various electrical units and measurements. At leastI found an apparently authoritative answer to the original question. A source at Electro Optical Industries says "At the threshold of vision the dark adapted observer can see a flash if it contains on average 90 photons at the cornea or 9 at the retina. This is equivalent to a candle at 30 miles on a clear night." The difference between this answer and the previous 1.4 kilometer answer is a factor of 67.6, and assuming an inverse square law for the brightness decrease, the light at 1.4 km would be 4573 brighter than the same light at 30 miles. Perhaps the greater distance would be during careful forced choice psychphisics experiments in a lab, and the 1.4 km figure would be for seeing it as a strong clear unmistakable signal, but the difference is 36 decibels, and I would expect only about 5 decibels of signal strength above a TSD threshold would make it pretty clear to an observer. Such a tiny, faint light would work in a sermon as an analog of the "still small voice" in 1 Kings 19:11-13 (King James Version). Edison 15:45, 17 July 2007 (UTC)
 * There is something seriously wrong with that answer. How long is the flash?  It doesn't say.  A candle that's 1000 miles away will EVENTUALLY produce 90 photons that hit your eye - and yet a candle that's only exposed for a few picoseconds at a distance of a meter might not happen to shoot 90 into your eye.  Relating candelas to number of photons makes no sense - you can only translate it into photons per second.  Worse still, what can your eye see?  90 photons - one entering your eye each night over three months?  No!  Of course not.  90 photons per picosecond?  Yes - that would be a bright light - much brighter than (say) 90 photons spread over a second.  Without stating the duration of the flash and the period within which the eye must get hit with those photons just stinks of oversimplification of the problem.  Furthermore, the size of a candle flame at 30 miles means that it would subtend such a tiny angle at your eye that there is a fair chance that none of the incoming photons would actually hit a light receptor - your eye doesn't have infinite spatial resolution - it's not just a matter of brightness. SteveBaker 19:09, 17 July 2007 (UTC)
 * The source said "flash" which would be a defined brief period such that there is time intensity reciprocity. Even more than the reciprocity failure in photographic film, the eye only integrates perfectly for a small fraction of a second. The small angular subtense of a distant candle is no problem at all, any more than the small angular subtense of the disc of a star. It can be treated as a point source, which will produce a blur circle, and there is no danger its light would fall in between rods in the retina. I expect that 90 photons in a  millisecond, would look remarkably like 90 photons in 1/30 second, but nothing like 90 photons in 1 minute or even in 1 second, because the the natural limits on temporal resolution in the human eye, which make 50 or 60 Hz AC lighting look like it is on continuously. See Persistence of vision and Flicker fusion threshold for related topics. Absolute visual threshold measurements use brief flashes. They also use an artificial pupil to remove pupil diameter effects. I could not satisfy myself with my calculations trying to calculate the distance from a 1 candela source to an 8mm pupil (7mm might be more realistic) of a fully dark adapted eye, with the claimed thresholds of 1 *10-10 to 16*10-10 Lambert.  Edison 20:55, 17 July 2007 (UTC)


 * Edison's reply is sound, and I think the various estimates can be reconciled. A source with a luminous intensity of I = 1 candela at a distance of r = 50 km (converting to SI units for sanity's sake) produces an illuminance of E = I/r2 = 4*10-10 lux (or lumen/m2). Over the area of the eye pupil, which is A = 5*10-5 m2, this translates to a total luminous power of P = EA = 2*10-14 lumen.  How many photons per second is this?  The most "efficient" candle would be one which emits all its photons at the peak of the eye's response, 555 nm or 540*1012 Hz frequency.  At this frequency, one watt corresponds to 683 lumen, so in terms of radiometric units the threshold is about 3*10-17 watt.  One photon at this frequency carries an energy of 3.6*10-19 joule (multiplying frequency by Planck's constant). Therefore the number of photons per second at the cornea is about 80. Now the average luminous efficacy (lumen/watt) will be less than 683 lumen/watt, but also a flash of duration of less than one second should be visible, so the minimum photon number of 90 looks to be reasonable.


 * The problem with the threshold figure of 1*10-10 lambert (= 3.2*10-7 cd/m2) is that the lambert is a measure of luminance L, which is power per unit area and solid angle, and as such it can only be applied sensibly to an extended source, rather than the point source that the candle presents at a distance of 50 km. We have to assume a limiting solid angle of resolution for the eye, corresponding to the minimum area of the retina that the photons need to fall within to register. There are various figures for this solid angle, depending on the duration of the flash, the brightness level, saccadic motions of the eye, etc, but the Laser Safety standard uses a figure of between 1.7 milliradians and 24 milliradians angular diameter to draw the line between a point source and an extended source. Taking an angle of 20 milliradians gives a solid angle of W = 1.3*10-3 steradian.  The corresponding illuminance is then E = LW = 4*10-10 lux, which agrees nicely with the first calculation.


 * So, 30 miles sounds like a good estimate. The obvious application is to Susan B. Warner's hymn.  --Prophys 14:11, 18 July 2007 (UTC)
 * A couple of engineering handbooks said that if the light source was less than one second of arc in diameter it could be treated as a point source. Another said of it was ten times the diameter away it could be treated as a point source (rather more tolerant). Further research shows that per Bloch's Law there is time intensity reciprocity in the human eye up to 100 msec (the shortest time found was 87 msec for threshold level illumination). The most sensitive wavelength is closer to 510 nanometers in scotopic (night time) vision rather than the 555 nanometer figure for photopic vision, per the Purkinje shift. See also the gospel song "This little light of mine, I'm gonna let it shine" . ps: I found a number of sites which state the "30 miles on a clear night" claim for candle visibility threshold, but others state that a certain kind of distress flare or even a lighthouse has that range. The candle, the viewer, or both would have to be at an elevated viewing point or the curvature of the earth would prevent seeing it. For an authoritative published source, there is  at Google books, see "Contemporary Color" By Steven Bleicher, Published 2004, Thomson Delmar Learning, ISBN 1401837409, which says on page 4 "...the minimum amount of light required to produce a visual entity is a candle flame at 30 miles on a clear night." Edison 01:35, 19 July 2007 (UTC)

Dead ants on the shore of Topsail Island
About several ago, I spent a weeks vacation at Topsail Island in North Carolina. And every afternoon, the creepiest thing would happen. thousands of large black ants would wash up on the shore of the beach and would lay there to die. When the sun set, crabs would come out and eat the ants. They'd all be gone by morning, but the same thing would happen the next afternoon.

Does anyone know how and why so many ants washed into the ocean ended up there? Is it some kind of natural cycle that happens all the time, or a freak accident? I have a phobia of ants (especially huge ones) so put a huge damper on my beach vacation, but I'm still curious about the phenomena. I haven't found any information on it, which is why I'm asking you.

There are some pictures of the ants here from someone who had a similar experience (they may have been there the same week as me): http://www.coasttocoastam.com/gen/page2030.html?theme=light

Please shed some light on this mystery! Thanks ^_^

--76.50.80.171 05:20, 16 July 2007 (UTC)


 * User:Dyanega, who apparently has a PhD in entomology, says:

''... those are dead ants from mating flights. The winged reproductive ants normally emerge by the millions, in synchrony, in many different ant species. They mate if they can, and most die. The only unusual thing here is that they should be concentrated in one small area, perhaps because of the wind or water currents. Otherwise, nothing odd or newsworthy about it. It happens all the time.''
 * --TotoBaggins 17:35, 16 July 2007 (UTC)

Flank bell?
What does it mean when a military naval ship or submarine performs a "flank bell"? 75.35.111.26 06:50, 16 July 2007 (UTC)
 * A "flank bell" refers to the position on the Engine Order Telegraph. It's position the bell, "AAIII", means "all ahead flank speed" and is essentially a signal to set the engines to the fastest speed the sub will go in the forward direction.  Rockpock  e  t  07:14, 16 July 2007 (UTC)

electrical and electronics project
I am an enginnering student and I need to do a project as part of my academics. My branch is electrical and electronics engineering. can anyone suggest me a good topic. i would be very grateful to u. i have been searching for weeks but i have never hit upon a suitable one. thank u for ur help.


 * What sort of thing are you interested in? And what kind of level are you studying? Suggestions:

GB 10:37, 16 July 2007 (UTC)
 * 1) something to do with mobile phones - can you make a readable LCD display that does not need lighting?
 * 2) Greenhouse gas reduction - can you design something to be much more energy efficient?
 * 3) build a terahertz radiation detecter and imager to see if someone is carrying a hidden gun
 * 4) Make a cheap quadrapole nuclear magnetic spectroscopic detector to detect smuggled substances - like alcohol for customs.
 * 5) Make a USB decoration different to anything else.
 * 6) make a television out of plasticine - that works.


 * Anything which solves world hunger or creates world peace is a good start. On a more serious note, why don't you try asking your course coordinator/lectures/academic advisors and friends/course-mates? Nil Einne 12:11, 16 July 2007 (UTC)

How about a system to turn window fans on and off automatically when the temp outside is lower than the temp inside ? I imagine two temperature probes on the end of wires, one hanging inside the house and one outside. For a bonus, also consider humidity. If you really feel ambitious, you could have one master control send an RF signal to turn all fans in the house on or off at the appropriate times. See for a description of how window fans can be used as an alternative or supplement to A/C. StuRat 14:00, 16 July 2007 (UTC)
 * Microwave death ray.
 * Oh, here's something I found in a "they oughta make" column in a (no kidding) 1944 Mechanix illustrated or popular mechanix or whatever: a bathtub that turns the water off when it's full. I'd enlarge the concept to include sinks. You could build something into the overflow hole. Sheesh, these days when you can spend $500 on a faucet, you'd think it would be an obvious option. They've had 60 years to think about it since that guy mailed it in to the magazine. Gzuckier 15:06, 16 July 2007 (UTC)
 * A television for hamsters? Powered by their excercise wheel? Capuchin 22:22, 16 July 2007 (UTC)
 * Ooh ooh even better! A little hamster camera that is wired up to their hamster TV so they can see themselves working out! Awesome! I wish I was your hamster! Capuchin 10:03, 17 July 2007 (UTC)

Freshwater and the Himalayas
In this article, it says: "The glacial retreat presents a double peril for those who live in the Himalayas and the populations of India and China, where the water flowing from the mountains accounts for 40 per cent of the world's fresh water." Is it really 40%?? (that figure seemed high and I was wondering if anyone could confirm it, I can´t find this info. on wikisites freshwater and Himalayas). Thank you. --AlexSuricata 11:40, 16 July 2007 (UTC)


 * I'm thinking they mean 40% of the fresh water which is used by people. Since the water from the Himalayas would flow into populous nations like China and India, as well as Southeast Asia, it is likely that a large portion of it is used, versus melt-water from Greenland, Northern Canada, Siberia, or Antarctic glaciers, which likely pours into the oceans unused. StuRat 13:48, 16 July 2007 (UTC)


 * "Water flowing from the mountains" would include rain that falls on the mountains, and especially in a place with India's climate, there's going to be a lot more of that than the amount of glacial meltwater, right? --Anon, July 17, 2007, 05:35 (UTC).

Equilibrim data of CO2 in MEA solution (10%)
I'm chemical engineering undergraduate and designing a stripping column for my project.

The Column have to separate CO2 gas absorbed into 10% aqueous MEA (mono ethanol amine) solution with the use of steam stripping. So i need vapor liquid equilibrium data on Steam-CO2-MEA(10%) system.

Is there is any method to calculate column/packed bed height without using equilibrium data?

What are to recommended references for steam stripping column design (web sites/Books) ?

Thank you Malinda

Well, maybe you could ask your lecturer? I'm not up to that standard yet. Cheers!!! -- Zachary crimsonwolf  13:35, 16 July 2007 (UTC)


 * Some clarification questions:
 * 1. Are you dealing with 10% MEA in water?
 * 2. Do you have to give a quantitative answer as to the amount of CO2 that was absorb in the sample?
 * Mrdeath5493 20:00, 16 July 2007 (UTC)

Can I hazard a speculation that the steam only adds heat and the presence of water vapour does not alter the partial pressure of carbon dioxide in the gas phase. Water only absorbs a small amount of carbon dioxide and you can see the figures on carbon dioxide (data page). So you could expect CO2 in the liquid phase to be proportional to the monoethanolamine MEA concentration. Next you will have to work out the equilibrium with bicarbonate ions and the weak base. One shortcut may be to measure the pH of your solution, or find the Ka for MEA. GB 21:45, 16 July 2007 (UTC)

Rheum
I'm saddened by the lack of article on rheum. What is this stuff? Mainly minerals from tear evaporation? Has anybody ever collected a large amount of it (Gypsies?)? How much? Are there any conditions where rheum does not form or forms too much? I remember as a child occasionally having my eyes welded shut when I woke up, was this due to rheum? What caused a large enough production of rheum to weld my eyes shut? These are the kind of world-changing questions that science should be answering. Capuchin 14:42, 16 July 2007 (UTC)
 * I'd guess it's got some mucus in it. What does it have to do with rheumatoid arthritis, is what I'd like to know. Gzuckier 15:10, 16 July 2007 (UTC)


 * The only time I've come across eyes welded shut like that is with conjunctivitis. Skittle 17:31, 16 July 2007 (UTC)
 * Definately didnt have conjunctivitus. It has maybe happened to me 5 times in my life. I remember my father having the same thing one morning too, with no particularly obvious illness going on. Capuchin 22:20, 16 July 2007 (UTC)
 * To answer yor question Gzuckier, from wiktionary definition of rheumatism: "First attested 1601, from Latin rheumatismus, "rheum", from Greek rheumatismos, from rheumatizesthai, "to suffer from rheum (which was thought to cause pain)", from Greek rheuma "a stream, flow," from rhein "to flow," from PIE *sreu-, "to flow". This shows the link between the two. Etymology is ridiculously interesting. Capuchin 09:56, 17 July 2007 (UTC)
 * Thanks. (every time I see the word rheum, I think of Inspector Clouseau). Gzuckier 14:34, 17 July 2007 (UTC)
 * I always think of Shylock -
 * Signior Antonio, many a time and oft
 * In the Rialto you have rated me
 * About my moneys and my usances:
 * Still have I borne it with a patient shrug,
 * For sufferance is the badge of all our tribe.
 * You call me misbeliever, cut-throat dog,
 * And spit upon my Jewish gaberdine,
 * And all for use of that which is mine own.
 * Well then, it now appears you need my help:
 * Go to, then; you come to me, and you say
 * 'Shylock, we would have moneys:' you say so;
 * You, that did void your rheum upon my beard
 * And foot me as you spurn a stranger cur
 * Over your threshold: moneys is your suit
 * What should I say to you? Should I not say
 * 'Hath a dog money? is it possible
 * A cur can lend three thousand ducats?' Or
 * Shall I bend low and in a bondman's key,
 * With bated breath and whispering humbleness, Say this;
 * 'Fair sir, you spit on me on Wednesday last;
 * You spurn'd me such a day; another time
 * You call'd me dog; and for these courtesies
 * I'll lend you thus much moneys'?
 * JackofOz 04:27, 19 July 2007 (UTC)

1800-1850 Medicine
What were the common practicing medical beliefs during the first half of the 19th century?


 * You may need to be a bit more specific in your question. Our articles on medicine and the history of medicine may be helpful, as might our timeline of medicine and medical technology.  The history of biology might also come in handy.  TenOfAllTrades(talk) 19:38, 16 July 2007 (UTC)


 * You might also read Madame Bovary, which contains a presumably plausible, but quite grim account of medicine in that period, as performed by a mediocre/bad doctor. --TotoBaggins 19:12, 17 July 2007 (UTC)

Popularity of home birth
Where can I find information about how many women in United States bear their children at home? I.e. with the help of a doctor or a midwife, but at home and not in the hospital. In percents to the total number of births. If you have any reliable data about other countries, please post it too. Thanks in advance! 83.237.160.59 19:29, 16 July 2007 (UTC)


 * The CDC keeps statistics on this. Go to this page and look for the term 'final data'.  Download the PDF and go to 'Medical Services Utilization, Attendant at birth and place of delivery'.  Statistics are available on that page from 2004 back to 1999.


 * In 2004 (PDF link),
 * 91.5% of births were delivered by physicians in hospitals.
 * 7.9% of all births were attended by midwives.
 * About 1% of all births took place outside of a hospital; a third of those were in a freestanding birthing center, most of the remainder were in residences. About two-thirds of the non-hospital births were attended by midwives.
 * Hope that helps.  TenOfAllTrades(talk) 20:09, 16 July 2007 (UTC)


 * One more: this site provides links to Office of National Statistics for Britain and the UK, broken down by region. The UK average is 2.4% for 2005. TenOfAllTrades(talk) 20:15, 16 July 2007 (UTC)

tungsten
How can I lower the melting point of tungsten?
 * Tungsten is an element. You cannot lower its melting point without changing the elemental composition to something other than tungsten. --Anonymous 20:45, 16 July 2007 (UTC)
 * If you wish to change the elemental composition of your sample to something other than tungsten (an alloy, for example), see freezing-point depression (note that freezing point is the same as melting point). --Anonymous 20:52, 16 July 2007 (UTC)


 * Hmm, well, speculating a little here -- the melting point might go down very slightly as you lower the ambient pressure. I wouldn't expect it to be lower by any significant amount, though. --Trovatore 20:54, 16 July 2007 (UTC)
 * In the melting point article, it mentions this phenomenon, but notes that "Unlike the boiling point, the melting point is relatively insensitive to pressure". --Anonymous 20:58, 16 July 2007 (UTC)
 * Interestingly, the graph there shows the melting point decreasing, rather than increasing, with increasing pressure. This had me confused for a moment, until I realized that that particular graph is for water, which (somewhat anomalously) expands when it freezes -- by Le Chatelier's principle, it therefore makes perfect sense that its melting point would decrease with increasing pressure. --Trovatore 21:16, 16 July 2007 (UTC)
 * High pressure physics gets no love, just because temperature is so much more accessible. Neat recentish result - negative melting curve in Na. For reference on their units, a million atmospheres is 100 GPa. -Eldereft 04:31, 18 July 2007 (UTC)

Texture of testicle
Hi there

My girlfriend seems to be concerned by my one testicle, because it seems to have a veiny mass attached to it, whereas the other testicle is perfectly smooth with nothing attached. Can someone possibly provide me with a reputable source to prove to her that my testicle is not abnormal? Thanks a lot

41.241.100.11 20:49, 16 July 2007 (UTC)


 * We cannot provide medical advice here on Wikipedia. Your best bet would be to seek the professional advice of a physician. TenOfAllTrades(talk) 20:57, 16 July 2007 (UTC)


 * I would add that testicular cancer is the most common form of cancer among young adult males, so if there is significant concern I would strongly encourage you to speak to such a medical professional. Dragons flight 21:04, 16 July 2007 (UTC)

Ok, in that case, let me rephrase. In terms of the anatomy of the human testicle, is it normal for one of these testicles to have somekind of chord/mass attached to it?


 * One's testicles should not be remarkably different in shape, texture, or configuration. If there is actually a difference, it should be checked by a physician (who, in any case, may be able to teach you what you're feeling, which may or may not be the epididymis or the vas differens). And you should learn to examine your own testicles: girlfriends may come and go, but your testicles are - hopefully - forever. - Nunh-huh 21:07, 16 July 2007 (UTC)


 * There is a helpful article at Testicular self-examination, but I would strongly re-state the advice above that you Consult your medical practitioner if you have any concerns about your testicles, or any other body-part. DuncanHill 00:24, 17 July 2007 (UTC)