Wikipedia:Reference desk/Archives/Science/2015 October 28

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

October 28

Washing hands in 3% hydrogen peroxide solution

I noticed the label says it's good for gargling, but doesn't list washing hands as a use. Is this because it would dry them out ? StuRat (talk) 01:30, 28 October 2015 (UTC)

H2O2 can oxidize surface skin cells and leave white spots on your skin. Doesn't usually happen with 3% if you wash it off promptly, but if you leave it on, it will. I certainly wouldn't gargle with undiluted 3% H2O2 — surely it says to dilute it first? --Trovatore (talk) 01:53, 28 October 2015 (UTC)

Actually, now I see that our article claims that the white spots are due to "a local capillary embolism", whatever that means. I'm a little skeptical; my impression is that the whitening is more superficial than that. I'd be interested to see a specific reference either way (the citation in the article points to a page that has four separate articles; doesn't say which article or page). --Trovatore (talk) 02:10, 28 October 2015 (UTC)

It's very strange that gargling is recommended. Our own article on hydrogen peroxide says, Large oral doses of hydrogen peroxide at a 3% concentration may cause irritation and blistering to the mouth, throat, and abdomen as well as abdominal pain, vomiting, and diarrhea. Intravenous injection of hydrogen peroxide has been linked to several deaths. Akld guy (talk) 02:02, 28 October 2015 (UTC)

Yes, vomiting, diarrhea, etc. are expected for an "oral dose", i.e. ingestion, but I think gargling is not a dose or ingestion. SemanticMantis (talk) 02:08, 28 October 2015 (UTC)

But if it can blister your mouth if you swallow it, I don't see why it wouldn't also blister your mouth if you rinse with it. Someone please check the bottle — I'm pretty sure the "gargling" usage is diluted, not full 3%. I don't think I'm likely to try it even diluted. --Trovatore (talk) 02:17, 28 October 2015 (UTC)

The bottle says to dilute with an equal quantity of water, so 1.5% concentration. StuRat (talk) 16:54, 28 October 2015 (UTC)

You made no mention of dilution in your original post. We have all been commenting on the basis that the concentration was 3%, which is what you did state. Akld guy (talk) 19:03, 28 October 2015 (UTC)

Well, my Q was about hand washing, which isn't mentioned at all on the label, so obviously there's no mention of concentration either. StuRat (talk) 01:58, 29 October 2015 (UTC)

Not only that, but if gargling were recommended with a strict admonition that it must always be spat out and not swallowed, it would only be a matter of time before someone would swallow a mouthful. Any instance where a product is intended to be placed in the mouth must cater for the fact that it will eventually be swallowed. The makers of chewing gum are mindful of that. Akld guy (talk) 05:33, 28 October 2015 (UTC)

A few drops, yes, but if somebody drinks a gallon of a gargle, they can't blame the manufacturer. A few drops should be nicely diluted and not pose any problem in the stomach. StuRat (talk) 06:05, 28 October 2015 (UTC)

Lots of websites seem to recommend diluting OTC H2O2 3% down to 1.5% or 1% for gargling and other oral care, but that other products equivalent to 3% or stronger is not uncommon for tooth whitening. A capillary embolism is fun, if your idea of fun is an embolism in your capillaries (tiny bubbles of oxygen under your skin). DMacks (talk) 02:19, 28 October 2015 (UTC)

Is that for real though? The spots seem very superficial, not deep enough that a cut that deep would bleed. Maybe they're sort of disused capillaries, passages where red cells used to travel back when that part of your skin was alive? --Trovatore (talk) 02:24, 28 October 2015 (UTC)

I am similarly skeptical. The epidermis doesn't have blood vessels (as our article says). The living parts obtain nutrients through diffusion, as do other avascular tissues like the cornea. --71.119.131.184 (talk) 04:59, 28 October 2015 (UTC)

Plenty of sources say the same, although some call it micro-embolisms Ssscienccce (talk) 10:08, 28 October 2015 (UTC)

Australian Dental Association study reports[1] long-term oral use of ≤3% was generally safe. DMacks (talk) 02:31, 28 October 2015 (UTC)

How much light can you put through a lens?

Say you had a large parabolic reflector aimed at a lens designed to focus the light into a beam. How much light could you send through the lens before the lens itself started overheating? Cpergielx (talk) 02:21, 28 October 2015 (UTC)

Let see. Some factors would be:

1) How close to perfectly transparent the lens is. If it really was 100% transparent, then an infinite amount of light could pass through (unless we count photons occasionally spontaneously decaying).

2) How small the lens is. A smaller (and in particular thinner) lens should be able to cool off more quickly.

3) What surrounds the lens. Some materials will carry heat away faster than others.

4) How hot the lens can get before it fails. If glass, melting is one possible failure mode, but uneven heating might also make it crack. A material with a zero coefficient of thermal expansion would resist cracking best, as long as the material to which the lens was mounted also didn't expand or contract.

5) The duration of the light beam. For use in lasers, it can be extremely short, and very little heating will occur in that time.

6) Note that some form of active cooling system, like oil circulated through the center of the lens, might keep it much cooler (although then you would also need to worry about the boiling point of that fluid). StuRat (talk) 04:57, 28 October 2015 (UTC)

Re 1) At 150,000 to 300,000 photons to the electron mass and Avogadro's number of atomic masses to the gram, the mass-energy of finitely bright light in the 100% transparent lens would exceed the supertanker in a pinhead density of a neutron star and therefore the maximum possible luminosity and the entire light-path will collapse into a black hole. Presumably, far before that you could put supertankers in orbit around the light beam if this was in deep space? Sagittarian Milky Way (talk) 05:26, 28 October 2015 (UTC)

Transparency of glass to sunlight is the key question. I'm not finding much. Cpergielx (talk) 11:53, 28 October 2015 (UTC)

The technical term is "Laser Damage Threshold" - see, for example, this datasheet for details on how it's calculated and results for various lenses. Tevildo (talk) 13:22, 28 October 2015 (UTC)

It depends on what you mean by "glass". I recall someone telling me that the glass used in optical fibers is so transparent that a kilometer of the stuff has the same transparency as a single sheet of window glass. So there is a massive variation possible between the best and the worst and "glass" becomes an impossibly vague term. SteveBaker (talk) 15:33, 28 October 2015 (UTC)

I'm someone who spends time putting LOTS of light through small lenses every day, (my wife's business involves laser cutting - and I maintain our two Lasersaurs) - our CO2 lasers put out 120 watts of infra-red light in a ~10mm diameter beam and a 25mm diameter gold-coated zinc/selenium lens focuses the beam to an 0.03mm spot. As you might guess from the exotic nature of the materials - getting that much IR light focussed though that much lens is difficult.

As a practical matter, there are three issues:

1. What percentage of the incoming light is reflected away, back towards the source? This is wasted energy (because it didn't end up where you wanted it) - but that reflected energy has to go someplace - and there is a lot of incoming energy, that might be problematic (it certainly is in my case). A lens may scatter the light back out through a wider angle, effectively diluting it to the point where it may not matter - but when the amount of incoming energy is really high - that may not be enough to prevent problems.
2. What percentage of the incoming light is absorbed by the lens? This is also wasted energy - but where the energy goes is almost certainly in heating up the lens. If the lenses I used were (say) 99% transparent - then the lens would heat up and shatter in a matter of seconds because 1% of the incoming energy is still a lot of energy! So I need something like 99.9% to allow the heat to radiate off of the lens as fast as it accumulates. Overheating the lens can cause a variety of problems - the lens might overheat and shatter or melt - the thing holding the lens in place might suffer - any fancy coatings on the lens might boil away or corrode in some fashion - it's certainly not a good thing!
3. What contaminants or other damage is likely to occur on the lens? Any kind of scratch or scuff will wreck the transparency of the lens...(see (2), above) and any kind of contaminant (dust, water condensation, etc) will also absorb the incoming light, get hot, etc, etc. So keeping the lens clean and avoiding it being touched by anything that might scratch it becomes a big deal. Ordinarily, when you care about something being contaminated by the environment, you encase it with something. But in this case, the "something" has to transmit light at least as efficiently as the lens - and then it has to be protected too - so you just get into an infinite regress.

There are certainly theoretical limits to what you can manage - but the practical issues are much more of a problem. Our lenses last only a couple of months before they have to be replaced. SteveBaker (talk) 15:29, 28 October 2015 (UTC)

What is the typical failure mode for your lenses ? As for keeping dust off them, there's a clean room or an evacuated chamber for the entire apparatus, but, in addition to being prohibitively expensive, if you are using the laser to cut, that will generate lots of gas and/or dust. StuRat (talk) 16:52, 28 October 2015 (UTC)

Laser cutting wood (which is what we do most of) produces a lot of smoke - which we pull away from the lens with some seriously powerful fans. There is also a small air compressor that blows clean air both over and under the lens in an effort to divert more smoke away from it. However, despite all of those efforts, it does slowly get contaminated - which soon becomes be fatal for the lens because the dirt collects laser energy, heats up and wrecks it. So we have to clean the lens every dozen or so operating hours - but because the surface of the lens is coated with a layer of gold that's just a few atoms thick - and the zinc/selenium stuff is about as hard as candle way - it's terrifyingly easy to scratch or scuff the surface. It winds up being a trade-off between the benefits of cleaning it versus the hazards of cleaning it! So no matter how careful we are, we end up replacing the lens every once in a while. We used to pay $300 for them - but with the popularity of these small laser cutters growing, we now find we can get them for around $60...which is some small fraction of a dollar per hour for a machine that can easily earn $1 to $2 per minute while it's running. SteveBaker (talk) 20:27, 28 October 2015 (UTC)

Wouldn't enclosing the lens in a glass box solve the problem ? The smoke would accumulate on that, which you could clean without fear. StuRat (talk) 01:52, 29 October 2015 (UTC)

You've entirely missed the problem though - how does the light get through the glass box (and out of it again)? Glass is much less transparent (and more reflective) to light at this frequency than my lens is - so the glass box would heat up and pretty soon shatter. Also contamination would wind up on the surface of the glass - with much the same result. The point is that the lens is already made of the best material for the job - so all you could do would be to surround it with more of that same material - so you'd be no better off! The best we can do (and what we actually do on the more vulnerable underside of the lens) is to enclose it in an opaque box with holes just large enough to permit entry and exit of the light beam. Then we pump filtered air into the box to create enough positive pressure inside to keep the smoke out. Sadly, that's not 100% effective - there is inevitable turbulence in the air leaving the box, and those swirls are enough to pull small amounts of smoke and other contaminants into the box and onto the lens. There is no easy way out of this - which is why high transparency and low reflectivity of the lens material and the avoidance of contamination impose practical limits on the amount of light you can push through a lens. SteveBaker (talk) 15:41, 29 October 2015 (UTC)

Well, the glass box could be a compromise, less expensive than the lens, if for no other reason than being flat. And, if you could skip that fragile coating, then you could clean it without ruining it. If the glass box occasionally fractures, just replace it, it should be cheap. You could also put all your air handling schemes into effect around the glass box, too. StuRat (talk) 00:45, 1 November 2015 (UTC)

@SteveBaker: Hmmm... There's no chance you could enclose the chamber where the smoke is generated, and have the beam come into it from directly beneath after reflecting off a pair of flat mirrors set at 45-degree angles underwater? The water would block smoke from getting back at the laser... supplying it with a small, constant flow from a tap would keep the system clean and cool... by having the beam perpendicular to the surface you'd reduce, though not eliminate, the havoc wrought if something knocks the apparatus and kicks up waves in the tank. Wnt (talk) 18:19, 31 October 2015 (UTC)

A "lens" may be more than a convex piece of glass, where the failure mode would be cracking due to uneven heating, or in the extreme literal melting. A multi-element lens may have multiple types of glass in it (if I recall correctly, crown and flint glass) to correct for certain aberrations. It may have groups of such elements to achieve the best image quality, if used as a telescope, binocular or camera lens. Lenses which are ideal for use in a view camera, for instance, may overheat and fail when used in a projector or photo enlarger. A similar focal length of lens made for use in a video projector, slide projector or movie projector would not have cement which would fail when light from a high power lamp passes through it.A camera lens might also have an iris or Waterhouse stop in it to reduce the light transmission and increase the depth of field, and it could overheat and warp r catch fire if made of a flammable substance. The shutter itself could similarly fail form overheating before the actual glass/plastic lens elements failed. A projector system I'm familiar with used a "cold mirror" in front of the lamp which selectively diverted visible wavelengths of light from the longer infrared wavelengths, which contributed to overheating of the film and optical path without contributing to image brightness. Edison (talk) 17:19, 28 October 2015 (UTC)

• The National Ignition Facility (a giant US government experiment to create nuclear fusion by blasting pulsed lasers at very small targets) puts over 3 GW/cm² through its lenses (which are made of fused silica) – that would be equivalent to putting the entire power output of Hoover Dam through your little fingernail. Luckily, they only operate for a very brief period of time, so the total energy input into the lens is small: this article says 9 joules of ultraviolet per square centimetre, which wouldn't even give you a suntan if you absorbed it in a second... but absorbing it all in a single nanosecond can have explosive results. Smurrayinchester 14:16, 30 October 2015 (UTC)

DHT and Masturbation

Does everyday masturbation increases DHT production thereby increasing hairloss ? — Preceding unsigned comment added by 175.101.24.136 (talk) 09:11, 28 October 2015 (UTC)

Probably not on your palm. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:32, 28 October 2015 (UTC)

The general consensus is that there is no such effect. Most studies that have looked at the effect of masturbation on testosterone levels in men have found it to be small and inconsistent. Looie496 (talk) 20:32, 28 October 2015 (UTC)

"Everyday" meaning daily, or "everyday" meaning non-exotic? Enquiring minds would like to know. {The poster formerly known as 87.81.230.195} 185.74.232.130 (talk) 14:23, 29 October 2015 (UTC)

What is the element with least amount of weight that follow all those rules?

What is the element with least amount of weight that follow all those rules?
1-Its not ultra rare
2-Can be used safely, its not radioactive, not burn easily......
3-Is found on solid state on normal temperature.
201.79.56.139 (talk) 16:36, 28 October 2015 (UTC)

Carbon ? Note that "burns easily" and "can be used safely" are somewhat at odds. Carbon has both radioactive and nonradioactive isotopes, as do most elements (there are three naturally occurring isotopes, with ¹²C and ¹³C being stable, while ¹⁴C is radioactive, decaying with a half-life of about 5730 years). "Light weight" is somewhat subjective, as different molecules will have different weights. Diamonds are a fairly heavy form of carbon, while carbon nanotubes and buckyballs are fairly light. StuRat (talk) 16:39, 28 October 2015 (UTC)

I read item #2 as "not burn easily", in keeping with the "can be used safely" theme. While carbon can be ignited and then it burns easily (at least in some forms), it's pretty easy to simply not ignite it (unlike pyrophoric materials). DMacks (talk) 16:50, 28 October 2015 (UTC)

Yes, we need a clarification from the OP on whether it should burn easily or NOT burn easily. As for how simple it is to avoid igniting it, that rather depends on the use they had in mind. If it's going to be used for the walls of a kiln, that's not so easy (although still possible, say by coating it with something fireproof). StuRat (talk) 16:57, 28 October 2015 (UTC)

Also, he needs to clarify "least amount of weight". Are we talking density or are we talking molar mass? By molar mass, the lightest "shelf stable", non-radioactive, non-flammable, common solid element is probably aluminum. It is the Third most abundant element on earth, while it is nominally very chemically reactive, any sample of aluminum exposed to air produces an impenetrable coating of protective aluminum oxide, which is why aluminum is stable enough for use in cans, food wrapping, cooking dishes, etc. It doesn't burn, it isn't overly radioactive, etc. In terms of density, however, the element that matches the OP's requirements is likely [silicon]. Every element less dense than silicon is either a gas, or highly chemically reactive, or flammable. Silicon is fairly inert stuff. The reason why silicon is less dense, even though its atoms weigh more than aluminum is because the crystal structure of silicon means that its atoms are more spread out, making the bulk material "lighter" than aluminum. I hope that helps the OP get what they are looking for. --Jayron32 17:05, 28 October 2015 (UTC)

Aluminium does burn, aggressively and hotly. Powdered aluminium is used in pyrotechnics. Solid aluminium is hard to ignite, but once it burns, it burns. --Stephan Schulz (talk) 19:42, 28 October 2015 (UTC)

Almost every element burns. Iron burns. Aluminum, in any form the OP is going to work with it, is not dangerously inflammable. You can safely use aluminum cookware on your stovetop, and stand no chance of it bursting into flame. --Jayron32 20:10, 28 October 2015 (UTC)

If the OP's "burn easily" is meant literally, calcium is probably the best candidate (1.55 g/cm³, lighter than graphite at 2.27) - lithium and beryllium are lighter, but probably fail the "used safely" test. Tevildo (talk) 17:12, 28 October 2015 (UTC)

And yet, graphite is used to line kilns and crucibles, as lubricants even at very high temperatures, and as electrodes in furnaces. DMacks (talk) 17:15, 28 October 2015 (UTC)

Please do your own homework.

Welcome to Wikipedia. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know.--Aspro (talk) 19:29, 28 October 2015 (UTC)

In what sort of school is "not ultra rare" considered an appropriate way of specifying a homework question in science? (Never mind, don't answer that.) --70.49.170.168 (talk) 04:51, 29 October 2015 (UTC)

I'm not sure why people haven't considered boron here. Yeah, it's expensive - $5 a gram, or about 1/8 the cost of gold (though also 9x less dense) - in its crystalline state. But you can buy a bottle for a couple of bucks in oxidized form as a roach killer, so the cost is basically purification, and of course if you purify anything enough it costs ... question said rare, not cost. Rare in the cosmos, not so much in certain dried-up lakes. Wnt (talk) 21:42, 28 October 2015 (UTC)

It's too heavy - 2.34 g/cm³, as against silicon at 2.33. Tevildo (talk) 21:57, 28 October 2015 (UTC)

Some clarifications, nope its not a homework, burn easily is a typo the real thing is not burn easily, with weight I mean the density201.79.56.139 (talk) 15:24, 29 October 2015 (UTC)

In that case, the best answer is probably carbon, in the form of graphite - density about 2.2 g/cm³ (lighter than silicon), and "difficult to ignite" (according to our article). Tevildo (talk) 22:52, 29 October 2015 (UTC)

Space news websites for kids?

Hi all, with the recent developments in space (the blood moon and other eclipses, water on Mars, the photographs of Pluto's surfaces, etc.) I would like to be able to share them with a group of kids (~6 to 8 years old). I'm not confident enough to adapt a source that's aimed at adults on the fly, and the NASA Kids Club doesn't seem to be a news service. Could you please help me find a reputable news service for kids which covers outer space topics? If not a service, just some recent kid-focused articles with pictures that I can use to help explain what's new in outer space. Thank you. --211.30.17.74 (talk) 21:43, 28 October 2015 (UTC)

A good space-related news site for kids may be http://www.timeforkids.com/minisite/space, though some of the articles are quite technical, and better suited for older children. http://www.planetsforkids.org/news/ seems nice but outdated, and http://www.funkidslive.com/?s=nasa&x=0&y=0 may also be interesting. - Lindert (talk) 22:52, 28 October 2015 (UTC)

The European Space Agency runs a space news website, ESA Space for Kids, but it also looks a bit outdated, the newest item is from September. --Jayron32 23:32, 28 October 2015 (UTC)

Found a kids news website called "Kids Ahead" which has a space news section: [2] It seems to be more of a news-and-press-release aggregator however, and doesn't seem to have its own content geared towards children. --Jayron32 23:36, 28 October 2015 (UTC)