Wikipedia:Reference desk/Archives/Science/2009 March 15

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March 15

Melatonin activity and the human eye

hi guys... do you know where i can find info on melatonin and race/ eye colour? i mean, it definitely has something to do cause melatonin depends on light, since for example blue eyes are more sensitive to light i imagine melatonin in the pineal gland is different in diferent races.

also, does the sensitivity of the melatonin receptors in the eye have anything to do with eye colour? like for example, are the melatonin receptors more sensitive in blue eyes than they are in brown eyes?

thank you! hopefully I can find some info.

oh and also haha, does eye colour affect eye sight? like does a really light blue eyed person see things 'differently' than a brown eyed person?

—Preceding unsigned comment added by 75.69.27.198 (talk) 01:10, 15 March 2009 (UTC)

Have you read eye color? That will probably answer most of your questions. --Tango (talk) 01:21, 15 March 2009 (UTC)

Seems like you're confusing melatonin with melanin. Dauto (talk) 01:25, 15 March 2009 (UTC)

Or may be you're not confusing them, but why should eye color play a role in the circadian regulation? Dauto (talk) 01:37, 15 March 2009 (UTC)

I'm not confusing melatonin with melanin...i know they are different but they are both related and affect each other. and why would eye color play a role in circadian regulation? because colour is related to light 'intake' and circadian rythm is also. —Preceding unsigned comment added by 65.183.130.151 (talk) 16:20, 15 March 2009 (UTC)

No, they're not related. One is a pigment, the other is a hormone. Melatonin itself isn't affected by light, melatonin levels in the blood are, through a complex set of mechanisms, which are not related to your eyes. Even blind people have lower melatonin levels in daylight. And eye color doesn't change 'intake' of light one bit. The light you see is passing through your pupil, not the colored portions of your eye. --130.237.179.182 (talk) 20:47, 15 March 2009 (UTC)

Pain in the...eye

So after reading arc eye, snow blindness and flash blindness, as well as perusing through eye disease, I'm still in the dark as to how bright lights cause pain and permanent loss of vision. Why do our eyes hurt when we see very bright lights? The retina allegedly has no pain receptors...is it strain on the iris dilator muscle trying to contract very hard? Or an interpreation by the brain of intense light? And aside from UV damage to the cornea, what is the mechanism by which very intense light (I'm talking nuclear flash or laser in the eye kind of intense) can blind a person, or is that just hollywood dramatization? Thanks. Someguy1221 (talk) 07:14, 15 March 2009 (UTC)

Retina has many light receptors. These receptors never grow in number, but instead, may deplete over time. this is why older people have weak eyesight. Intense light may destroy these light receptors and may cause blindness. the pain in the eye is because of the fact that iris, that controls the amount of light entering the eye, tries to reduce the amount of intense light entering the eye.--Lightfreak (talk) 08:27, 15 March 2009 (UTC)

A nuclear flash - at a distance where it's not going to kill you - won't blind you. Richard Feynman was the only person to watch the world's first nuclear explosion with his naked eyes (well, actually, 'eye' - he closed the one eye just in case he had a slipup in his math). In several of his autobiographies - he explains why he knew that was a safe thing to do. On the other hand, even a relatively small laser (such as a laser-pointer) will damage your eye if you stare at it for more than a half second or so. The iris is intended to shrink to exclude harmful amounts of light - but there is a limit to how small it can get. A laser beam can easily get through that hole. In addition to the pain from the iris muscles contracting - there definitely is a pschological element to it. If you use computer graphics to display all of the artifacts of a bright, blinding light on a computer screen - in a situation when they are 'immersed' in the on-screen action - you can convince people that the light is painfully bright, when in reality even if the entire screen were lit up at maximum brightness, it would not give you a moment's thought. SteveBaker (talk) 13:10, 15 March 2009 (UTC)

Just a reality check on the supposed devastating amount of energy a laser pointer could pout through the iris. A laser pointer says "Class 3a laser product. Max output < 5 mW. Wavelength 630-680 nm." How many mW of solar energy would be reflected through the pupil if you looked at a white surface large enough to fill the visual field illuminated by full sunlight, which would be bright but would not be expected to cause blindness? Pupil (eye) says it can narrow to as small as 3 mm. Solar energy issaid to be about 1 kilowatt per square meter. White paper is said to reflect about 85% of the light hitting it. So how many mW of energy would enter hit the 3mm pupil when you look at a white painted house, or car, or white paper, or snow (albedo 96% per [1]) in full sunlight? Woul'nt it be somewhat more than 5 mW? Edison (talk) 15:13, 15 March 2009 (UTC)

According to the US laser health and safety guidelines: a class 2 red laser will damage your eye with less than a quarter second of exposure. The concern is that this can be less than your blink response. Hence all legal laser pointers (in the US at least) are class 1 or (at most) 1M. Those can still do bad things to your vision - but not if you know to quickly close your eyes and turn your head away because you have more time to react. With a class 3 or above - you have no chance. Pretty much any exposure is enough to blind you...these things are dangerous! SteveBaker (talk) 01:27, 16 March 2009 (UTC)

And thus the question, how do they blind you? Someguy1221 (talk) 05:45, 16 March 2009 (UTC)

I think the key detail is that light scattering off a piece of paper spreads out, a laser doesn't (significantly). I'm not sure how to calculate how much light would getting into the eye from those things, but there is a simple reality check - compare with looking directly at the sun. If solar energy is 1kW/m² then a 3mm diameter disc would receive 7mW. Comparable to shining a Class 3a laser at your eye. However, I was having a discussion over dinner just last week with a physicist and a chemist who each use big lasers (Class 3 and 4) and apparently how dangerous a laser is doesn't just depend on its power. A powerful laser in the visible part of the spectrum will make you blink and look away very quickly, so the danger is minimal (still not a good idea to try, though!). A slightly less powerful IR laser will burn straight through your retina before you realise what is happening. (At least, I think the key detail was wavelength, I don't remember everything they said - the bit about power not being all important is definitely true, though.) --Tango (talk) 15:34, 15 March 2009 (UTC)

Tango - Yow. Both interesting and shudder-inducingly informative; nice response.

Edison - from a couple of different tacks, explaining the same principle of why concentrated low energy may have a lot more effect than diffuse high energy:

"Sharpness" represents the fact that when you concentrate force into a very small area, it has a much greater effect. Look at the tip of a pin, or the edge of a razor blade - they have very tiny surface areas. One pound of force concentrated into a square 0.1 mm wide (100 lb/mm²) may easily cut through something, while 100 pounds of force may have no effect whatsoever if dispersed over a comparatively "huge" square 1 cm wide (1 lb/mm²).

Similarly, consider a space heater that puts out 10,000X amount of energy per second into a cubic 10 m wide room (10 X/m³). It keeps the room warm, but never comes close to burning you, because the energy is diffused into a large volume. But if you touch a spot on the heater for one second, absorbing a tiny 1X fraction of the energy into a "cube" of flesh 1 cm wide (1,000,000 X/m³), you get burned.

Does this make it any clearer, or just more confusing? arimareiji (talk) 20:22, 15 March 2009 (UTC)

Incidentally, the US government did do many studies on how much light and at what distances from a nuclear explosion would be needed to do eye damage. --98.217.14.211 (talk) 15:34, 15 March 2009 (UTC)

Back to basics: explain why solar energy reflected from a snowbank or white painted wall filling the visual field is so much less dangerous than the 5 mW laser pointer. I scent mumbo-jumbo. Edison (talk) 02:20, 16 March 2009 (UTC)

Short answer: Because it is more spread out. (Long answer, see above!) (Although, I'm not actually sure it is more dangerous - snow can cause temporary blindness and I'm not sure a 5mW laser pointer is going to cause much more than that unless you are really stupid with it.) --Tango (talk) 12:12, 16 March 2009 (UTC)

One factor that has been missing from this discussion is how "spread out" (more technically, collimated) the light is. This factor is important because the eye is not just a single large photoreceptor with some area; it has a focussing system - a lens. A perfectly collimated light source shining on a perfect lens would focus the light to a single point, which would burn anything on that point regardless of the irradiance (power per unit area) of the light. Of course no light source or lens is perfect, but laser light is much more collimated than sunlight. The relevant unit here is radiance, sometimes referred to as brightness. The higher radiance of a laser is why even a low power laser is much more dangerous than sunlight. Someone42 (talk) 12:19, 16 March 2009 (UTC)

That's been missing from the discussion? I'm sure I mentioned it about half a dozen times... --Tango (talk) 15:32, 16 March 2009 (UTC)

Death

Here is the first scenario. A healthy person is "locked away" in a room, with no chance for escape or outside assistance/intervention/communication. The room has an infinite supply of air/oxygen pumped in ... and the room has an infinite supply of good drinking water available. Would that person be expected to die? Or can he live like this indefinitely (up until his inevitable and expected "natural death")? If the former, how long would it take to die? And what would be his cause of death? The second scenario is exactly as the first, minus the water. Same questions. In these scenarios, both the person's physical body and mind are completely healthy ... so that he would not die of any (pre-existing) disease, nor commit suicide, etc. Also, the room is perfectly comfortable, so that heat/cold/etc. is not an issue. If it makes any difference, we can also throw in good working plumbing/toilet facilities in the room to remove waste. Thanks. (Joseph A. Spadaro (talk) 18:40, 15 March 2009 (UTC))

He would die of starvation in a few weeks in the former and of thirst in about three days in the ladder. — DanielLC 19:24, 15 March 2009 (UTC)

1. Yes. No. Depends on a wide variety of factors, including how much fat and muscle he has. Starvation.
2. Yes. No. Generally, one to two weeks. Dehydration.

This PDF may shed more light on the subject. arimareiji (talk) 19:38, 15 March 2009 (UTC)

The Scottish used to wall enemies up like this in cupboard sized spaces in their castles, although they may in some cases have left a small hole to allow food and water in. 84.13.169.19 (talk) 22:10, 15 March 2009 (UTC)

In some of the above replies, you have answered that the person dies of starvation / dehydration / etc. I know that that is what happens to the person's body (i.e., he becomes starved and/or dehydrated) ... but is that what he actually dies of? In other words ... does he really die of, for instance, "multiple organ failure" (or some other medical terminology) ... if indeed the starving leads to vital organs failing, that is? Thanks. (Joseph A. Spadaro (talk) 01:06, 16 March 2009 (UTC))

Multiple organ failure is an option, sure. Our article on starvation outlines the various effects of starvation pretty well. It's impossible to say what exactly would kill the person in such a situation, but it could be a heart attack, or it could be the result of an infection, or scurvy, or... there are lots of things, but the point is that if the body runs out of energy, it will eventually break down one way or another. (Also, you mention that they wouldn't commit suicide, but psychological symptoms -- including depression -- are generally associated with starvation, and a person trapped in a room with nothing to eat might well commit suicide.) Likewise, dehydration can be fatal pretty quickly (and can also be associated with starving, which can cause diarrhea). Dehydration tends to result in kidney failure, but other internal organs will also stop working. Regardless of whether the problem is starvation or dehydration, if you take away the body's fuel, it'll start to shut down. The exact order this happens in probably varies, depending on the patient. -- Captain Disdain (talk) 02:07, 16 March 2009 (UTC)

A friend who went through "survival school" said that they teach the rule of "threes:" you can survive three minutes without oxygen, three hours without shelter, three days without water, and three weeks without food. Your scenario suggests death in about three weeks if food is denied while air, shelter and water are provided. Milage may vary. Edison (talk) 02:17, 16 March 2009 (UTC)

Why would three hours without shelter kill anybody except under extreme weather conditions? Dauto (talk) 03:28, 16 March 2009 (UTC)

The sky is falling! Clarityfiend (talk) 03:40, 16 March 2009 (UTC)

I'd like to second Dauto's question, this seems interesting... could you elaborate, Edison? --Ouro (blah blah) 07:33, 16 March 2009 (UTC)

Indeed, how long you can last without shelter depends on the circumstances. It could vary from anywhere between a few minutes (you're in the Arctic and your tent blows away while you aren't wearing your Arctic clothing) to pretty much indefinite (in the tropics, say, as long as you have a decent hat). Shelter was probably included in there because that is the order you are generally taught to deal with your primary needs in. Obvious, if you don't have air, you need to sort that out pretty quickly! After that, you worry first about shelter, then water, then food. Unless you are already seriously dehydrated, that will pretty much always be the best order. Even if you don't need shelter in order to live it has massive psychological benefits. --Tango (talk) 15:30, 16 March 2009 (UTC)

I'd just like to add that one morbidly obese person who was quoted on the BBC lived for over a year on water and vitamins - so this might suggest someone could live longer than three weeks (until the vitamin part took effect, in which case a disease like anemia or scurvy would kill them). One source suggests eight to twelve weeks. - Jarry1250 ^{(t, c)} 20:47, 16 March 2009 (UTC)

If you are in a place where it is not too hot and sunny and not too cold, then shelter might be a less acute need than water, (but still following air!). If it is extremely cold, then you need to find or build shelter pretty quickly to avoid dying of heat loss. See [2]. This apparently dates back to U.S. military survival training. Edison (talk) 17:25, 17 March 2009 (UTC)

Irish republicans who were on hunger strike in British jails in Northern Ireland survived for 94 days without food. Of the nine who set out to do that, three died - but the rest survived - suggesting that 94 days isn't just a one-off flook. SteveBaker (talk) 02:35, 18 March 2009 (UTC)

Tesla's wireless electricity

According to this article, Tesla tried to create a world-wide, aerially-broadcast energy network. Would this have had health or environmental implications? Would the electricity mess anything up, or would we be safe? 86.8.176.85 (talk) 18:53, 15 March 2009 (UTC)

It would mess _everything_ up. The amount of RF interference would be ridiculous. --130.237.179.182 (talk) 20:44, 15 March 2009 (UTC)

RF interference with what? Modern technology, certainly, but the question asked about health and environmental issues. I'm not aware of any proven RF hazards in that area until it gets so intense as to directly cause heating (like in a microwave oven). --Anonymous, 21:20 UTC, March 15, 2009.

Some people have expressed concern about the health effects of high-voltage pylons near homes. But there seems to be no basis for their apprehension.

For significant distances and significant amounts of power, there is no known method of transferring electrical power without the use of wires.

A transformer will do it by employing a magnetic field. But that involves only an extremely short distance.

I once read a newspaper article about a farmer who had a metal fence that ran alongside an overhead power line. The farmer was picking up and using electrical power from the fence. He was taken to court for it! I don't know how much power he picked up, or what the voltage was, but the distance was small.

However, a crystal radio set does receive a tiny amount of power from a rather distant transmitting station and put it to use (legally!) to actuate headphones. It is amazing that such a small amount of power will do it. The power must be in the microwatts.

In Wikepedia, the article Crystal radio, (in the section "Attempts at recovering RF carrier power") says that claims have been made that power from a radio station carrier wave can be used to amplify the output of a crystal radio. But no details or circuits are given. (A crystal radio itself does not amplify.) The "Construction and operation" discussion which follows is also interesting.

However, the obvious problem with sending significant amounts of power through space is that anyone can pick it up for free (like a radio signal) unless some means can be devised to make it available only to paying customers. – GlowWorm. —Preceding unsigned comment added by 98.17.37.175 (talk) 22:31, 15 March 2009 (UTC)

For anything but very short distances (less than WiFi can transmit), it would be wildly impractical using current technologies. If we can focus and recapture extremely coherent EMR that's minimally absorbed by intervening matter, it may be possible, but if you promiscuously radiate energy in all directions, you lose the overwhelming majority of it. Entropy's a bitch. arimareiji (talk) 23:05, 15 March 2009 (UTC)

In some cases, radiated power that is not expended externally is periodically returned to the source. A coil with AC flowing through it exhibits that property. The collapsing magnetic field returns energy to the source unless a conductor intercepts the field. The conductor would need to be part of a complete circuit for any appreciable amount of power to be taken by it. Obviously, an AC magnetic field cannot be used to transmit power for any great distance - every conductor in its path would absorb some of the energy. At present there is no known method of transferring a significant amount of electrical power broadcast (as Tesla planned) or by a directional beam. – GlowWorm. —Preceding unsigned comment added by 98.17.37.175 (talk) 01:19, 16 March 2009 (UTC)

Tesla claimed that electromagnetic radiation consisted of longitudinal waves of compression and rarefaction, rather than transverse waves as called for by Maxwell. He built a tower which was intended to send energy to great distances, but which was not shown to do so. He made some major contributions to electrical technology, but also made periodic wild claims. Edison (talk) 02:13, 16 March 2009 (UTC)

I agree with GlowWorm; as far as I know, it has never been proven that radiation from those huge electrical pylons right over your head is harmful, but somehow I think a huge amount of wireless energy traveling through someone wouldn't be good for people (it's like the previously mentioned radiation, except ALL of the energy traveling through the pylons being transmitted), especially for those with pacemakers and such. Not to mention the ridiculous amount of interference and possible damage to anything electrical that passed in it's range. -Pete5x5 (talk) 17:57, 16 March 2009 (UTC)

Many normal items can act as a (rather inefficient) antenna. This can include metal clothes hangers, wire fences, even dental work. Normally the amount of energy received is insignificant (although, in rare cases it's enough to hear a radio station on these items). However, if you had a billion times as much energy in the air, you would then get a billion times as much received by all these accidental antennae, causing fires and exploding people's heads from time to time. StuRat (talk) 02:37, 17 March 2009 (UTC)

This is all true - but over shorter ranges, there is a TON of research into the idea of broadcasting power to things like laptops and cellphones. Here for example. You can broadcast power over shorter ranges with much less danger and trepidation. The idea being that instead of having all of those low voltage chargers with the ubiquitous wall-warts, you'll have just one WiTricity generator build in somewhere like under your desk, for example - and any appliance placed on top of or nearby (eg in your pocket) would automatically get charged up. Since connectors are the least reliable parts of most small devices, this make a lot of sense. This stuff is not far off into the future - there are plans for actual WiTricity products to appear in just a couple of years. SteveBaker (talk) 02:29, 18 March 2009 (UTC)

Right. But, of course, the energy levels from such a system are far lower than those Tesla imagined, making it much safer. StuRat (talk) 17:02, 18 March 2009 (UTC)

Wadeaminnid. Is "WiTricity generator" and whatnot just marketing chitchat for the induction chargers already present in every electric toothbrush? And... would not a "WiTricity generator" continue to radiate power even if there were no devices soaking it up? And, what advantage would such a scheme have over a standarized charger with wires? Y'know... One wallwart to rule them all, and in the darkness bind them. ;) -- Fullstop (talk) 14:21, 19 March 2009 (UTC)

Chargers with wires and induction chargers still require that you place the device in a specific location and maybe plug it in. Forget, and your device isn't charged. Plugs also tend to fail. I lost a laptop that way. There is one advantage to the current chargers, though (besides electrical efficiency). It's hard to lose an object if it's plugged into the wall. StuRat (talk) 15:08, 19 March 2009 (UTC)

Roses: meaning of "perpetually" or "repeat" flowering

The gardening book I'm looking at distinguishes between perpetually and repeat flowing roses. What is the difference, and which is better? I have read the Rose article, and the gardening book does not define the difference. 84.13.169.19 (talk) 22:05, 15 March 2009 (UTC)

The perpetual flowering roses will have a small number of flowers all through the growing season, spring, summer and autumn. The repeat will come out in a second burst with a blank period in between. Better is hard to say as each kind will be more attractive at different times. Graeme Bartlett (talk) 02:53, 16 March 2009 (UTC)

Thanks. 89.243.72.130 (talk) 20:49, 16 March 2009 (UTC)

Remember to deadhead the roses after the first bloom. (cut off the dead flowers), so that energy goes into new flowers rather than rose hips.

When is certain pet article content a "How To"

I'm pretty new, so I'm sorry if this is something that would be common sense for more established wikipedians.... I am looking at small animal pet articles such as house rabbit, syrian hamster, fancy mouse, fancy rat and so on... some of these contain information on the care of the species as a pet (caging, feeding, handling, bedding etc). At what point does go from being informative about the article's subject to being How To? If these topics are appropriate, what would be best way to address the somewhat controversial issue of what kind of bedding(s) are safe vs toxic. Currently, the topic is avoided in some articles, and presented one-sided in others. Should each species' article be edited to have the different POV's supported with article references, and then watch every article to make sure that edits are reflected across all of them; or should "pet bedding" be made into an article, and each pet species article then point to that article? (I'm leaning towards this being too much of a How-To that should be deleted, or if it is appropriate to the articles, then I'd rather see "pet bedding" being its own article..., but I don't want to step on any of the many toes of the different small pet article editors!) --6th Happiness (talk) 23:16, 15 March 2009 (UTC)

Your best bet for questions of this nature would be to run them by folks at either the Help Desk (if you're trying to find the relevant policy pages) or the Village Pump (if you're looking for a more free-ranging discussion about what the policy should be or how it ought to be applied). TenOfAllTrades(talk) 23:45, 15 March 2009 (UTC)

Thanks, will do that. --6th Happiness (talk) 23:47, 15 March 2009 (UTC)

Just a couple of suggstions, if you see them before leaving: At least in the hamster article, there are a few sentences that are too close to How-To and a few that are just downright regrettably-shallow advice. That type of info can and should be pulled out of the articles. Specifically wrt bedding, I would think it's completely appropriate to briefly note in specific animals' articles why certain bedding types are bad based on biology - but not to go on at length about how often to change it, or whether your hamster likes paper or hemp bedding, etc. As to whether a longer article on pet bedding would be viable, I would think it can be. But I'm also a lazy bum, so I'll leave the researching and writing to you. ^_^ arimareiji (talk) 23:54, 15 March 2009 (UTC)

Thanks for your input arimareiji (I've reposted to Wikipedia:Help_desk if you want to see if there are any further replies there) —Preceding unsigned comment added by 6th Happiness (talk • contribs) 00:11, 16 March 2009 (UTC)