Wikipedia:Reference desk/Archives/Science/2007 August 19

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August 19

Why do we go blue?

OK, got asked an interesting question the other day that I couldn't fully answer to my satisfaction. When we get really cold our hands, lips, etc can take on a bluish tinge. Now I know all about the blood supply to the surface reducing to retain heat, etc, but what I couldn't answer was, 'why blue'? I don't know of any body tissues that are really blue, and despite what diagrams typically show deoxygenated blood isn't blue, so what's with the blue colour? The best link I can find is this, but can anyone explain it better? Thanks. --jjron 05:01, 19 August 2007 (UTC)

I’m not sure about your question. However, for clarification, here’s a link that talks about the red/blue blood thing. --S.dedalus 05:19, 19 August 2007 (UTC)

Thanks for this, but can I just cut anyone else off before this degenerates into a side issue and loses the question I'm asking; I don't want a discussion on the relative merits of representing deoxygenated blood as blue in medical diagrams, or anything like that. --jjron 05:36, 19 August 2007 (UTC)

LOL! A.Z. 05:45, 19 August 2007 (UTC)

(Nice comment A.Z. :-) ) Well actually, I was adding the link to help others understand the red/blue blood thing. I think I have found your answer. This link suggests that a lack of blood in tissue is the cause of blue skin. I would guess that the body shunts blood away from the skin to conserve heat when we are could. I’m still not sure why this would created the blue color, but if you read my previous link it mentions that this is caused by seeing the skin itself. --S.dedalus 05:52, 19 August 2007 (UTC)

That's correct. Your previous link says "Your *blood* does not ever *look* blue. The blue things you see under your skin are veins. Veins are really whitish in color but because the blood is dark and the skin difuses the light, the veins *look* blue." That doesn't really do it for me. How does something whitish in color looks blue just because there's a dark think inside of it and light difusion by the other thing outside of it? A.Z. 05:57, 19 August 2007 (UTC)

So after asking that I realized I didn't know what light diffusion was in the first place, and I searched for the Wikipedian article about it, only to find out there is no article. A.Z. 05:59, 19 August 2007 (UTC)

OK, thanks for discussion so far, but at the risk of sounding rude, can we please read the question before answering - I quote from the original question: "...I know all about the blood supply to the surface reducing to retain heat..." which is what S.dedalus's second answer addresses. And at the risk of answering my own question, I think S.dedalus's first link, which AZ discusses above, is actually talking about dispersion, not diffusion as it apparently incorrectly says. In other words, it would look blue for much the same reason as why the sky looks blue. --jjron 07:45, 19 August 2007 (UTC)

We do have an article on cyanosis. Bendž|Ť 09:13, 20 August 2007 (UTC)

Thanks, nice link, I had never heard of that term before. Sadly that article still doesn't really explain it very well (it more says what it is, rather than why it is so, and in fact there were some errors in the article that I've tried to correct), but we may be getting closer. --jjron 10:09, 21 August 2007 (UTC)

A chemical bond

I was observing my glass of sparkling water (with a piece of lime in it) this lunch, and i saw gas bubbles adhering to and thus collecting on the surface of a seed. Can anyone please tell me what is the name for this type of chemical bond? Is it a hydrogen bond, or not really? Thanks!

The surface of the bubble has more highly ordered water than found in the bulk liquid.

It's probably hydrogen bonding yes.87.102.4.148 08:53, 19 August 2007 (UTC)

Studies on bubble adherence show that hydrophobic surfaces (less ability to hydrogen bond) get less bubbles stuck to them.87.102.4.148 08:59, 19 August 2007 (UTC)

http://en.wikipedia.org/wiki/Nucleation ?

Veins

Why are veins greenish in color? Thanks.

What veins are you looking at? Read Vein#Color for some information. Someguy1221 07:18, 19 August 2007 (UTC)

Check the question two above called "Why do we go blue?". It's basically the same issue. --jjron 07:53, 19 August 2007 (UTC)

mimimum pixel size of lcd display

I've already found that camera lcds have pixel width around 50mm/500pixels = 100micrometers..

What's the smallest currently available pixel size?

Anyone know what the theoretical limit of pixel size would be?87.102.4.148 08:51, 19 August 2007 (UTC)

Dot pitch lists current minimum as around 100 micrometres. As for the theoretical limit well I don't know. For LCDs I would guess it would be several times the size of a silicon atom. According to 65 nanometer a crystal of bulk silicon has a lattice constant of .543 nm so if it's say 20 times this that would be ~11 nm. However anything smaller then the wavelength of light is AFAIK going to be extremely complicated to get to work and also probably pretty pointless so I would say perhaps around 750nm Nil Einne 11:36, 19 August 2007 (UTC)

I remember one of my prof's claiming that a superlense could be used to lower the theoretical and practical limits on pixel size...can't seem to find it in my notes. Someguy1221 20:00, 19 August 2007 (UTC)

A while back, IBM produced an LCD that was (IIRC) 200 dpi rather than the more-normal 90-100 dpi. It was very expensive but reported to be an exceptionally beutiful display (closely approximating what we can accomplish with printing on paper).

Atlant 13:37, 20 August 2007 (UTC)

Night vision

Is there any way that you can do to improve on your night vision? Other than stuffing yourself full on vitamin A's, I mean. --Zacharycrimsonwolf 12:40, 19 August 2007 (UTC)

Night vision goggles but I guess that wasn't what you meant. The obvious things are vitamin A which you already mentioned and adaptation. For example, go to a very dark room and stay there for several hours and do not exposure yourself to light. In an extreme you could live in a very dark environment for weeks or months but I wouldn't recommend it. If you believe in reincarnation, you could pray that you're reincarnated as a cat Nil Einne 12:58, 19 August 2007 (UTC)

Our night vision article makes mention of atropine eye drops. (At your own risk)--Shantavira|^{feed me} 14:21, 19 August 2007 (UTC)

What you could do is spend a lot time moving around in the dark, etc, in effect training yourself to be more comfortable functioning at night (as Nil basically suggests). While it may not improve your night vision in an objective measurement, you can become far more functional in that environment. --jjron 14:58, 19 August 2007 (UTC)

One interesting phenomena is that your night vision is better at the periphery of your vision, so try not looking directly at what you want to see. Tomgreeny 15:39, 19 August 2007 (UTC)

OT but I had eye drops to dilate my pupils once during an eye examination. I guess tropicamide or phenylephrine or a combination of both. I didn't have any sunglasses and walking out into the sun was horrible, even after giving it a while Nil Einne 17:47, 19 August 2007 (UTC)

I am skeptical about the usefulness of using drops to dilate the pupil, since the image quality may degrade due to the image passing through the optically poor outer area of the lens. Enhancing the sensitivity of the scotpoic vision via the rods is a more likely avenue. For a half hour before going out in the dark avoid exposure to any light. If you need some task lighting, wear red goggles or use red bulbs. This was done in World War 2 in submarines before it surfaced at night to preserve the night vision of the persons who would be on lookout. Binoculars with a large objective lens can collect more light than the naked eye and make fainter objects visible, as well as magnifying, if you do not want to do the obvious and use night vision goggles. Edison 18:09, 19 August 2007 (UTC)

Wear very reflective clothing to help light up your enviroment. Trust me my police jacket lights up the room sometimes if the background light is sufficient. SGGH ^speak! 18:30, 19 August 2007 (UTC)

You don't need atropine to dilate the pupil; just stay in the dark long enough. Our article on eyepatch mentions that the eyepatch of the stereotypical pirate might not be an invention of modern movies but actually have been worn by sailors with two intact eyes, to help them see better below deck. Oh, and as far as I know, a lot of people with only very slight myopia who usually don't use their glasses find them quite helpful when driving at night. Simon A. 21:21, 19 August 2007 (UTC)

Night vison article says that atropine is used together with night vison goggles, from personal expirience - normaly atropine worsens sight for few days, it is used so that pupil remains dilated in light, but it is dilated in dark anyway -- Xil/talk 21:24, 19 August 2007 (UTC)

An ophthalmology technician who specialized in both pediatric surgery, and refractive surgery for over 15 years, I can assure you that the article on night vision is inaccurate. Atropine is a very strong dilation medication, and in no way increases night vision. Yes, it allows more light onto the retina, but it also, as mentioned above, blurs the vision quite significantly. It does this because atropine relaxes the lens of the eye, and lens "flexation" is what helps us to focus. For example, hold your hand about 12 inches from your face, and focus on it. Then look beyond it, and focus on the wall. Your hand becomes blurry, and you notice a specific change in vision. This is because your lens is flexing. If your lens is unable to do so, your vision is compromised. This is also why people report they are unable to see well after having an eye examination. The more common dilation compound is Mydriasil (tropicamide.) (While phenylephrine is used in some offices, its use should be done with caution as it does have cardiac side effects with some patients.)

Atropine in pediatric ophthalmology is used to treat conditions such as "lazy eye" (amblyopia) when one eye does not see as well as the other; we would often prescribe one drop of atropine each day in the good eye, thus burring it intentionally, and forcing the "bad" eye to be put into use. Often a couple months of this therapy, and the child's eye would be fairly equal. There is really no way to improve night vision without using night vision goggles. However, there are a number of things that make night vision worse: Contact lenses (most people have issues with halos, starbursts, or foggy vision at night), cataracts, dry eye symptoms, fatigue, and in some cases, refractive surgery. If you find your vision has gotten worse progressively with regards to night vision, it is time to go see an ophthalmologist. ^Ariel♥Gold 03:02, 20 August 2007 (UTC)

You don't need the lens to do any work when focusing on distant objects (e.g., astronomy). Perhaps atropine would be useful then? --Tardis 17:26, 20 August 2007 (UTC)

Practice indirect observation (lateral vision). This is an astronomers trick. The central location where you tend to focus your eyes is full of cones. Around it is rods. If you train yourself to look slightly off axis you can observe more. In fact, you can try this with the night sky. You can see stars around the one you are looking at but they disappear when you stare directly at them. --Tbeatty 06:37, 20 August 2007 (UTC)

Yep, that's a most excellent point, Tbeatty, and certainly is true and reliable. Good point! ^Ariel♥Gold 06:39, 20 August 2007 (UTC)

The peripheral vision trick gets you better perception of motion and of very dim objects - at the price of reduced image 'sharpness' and a total loss of colour perception. But colour vision is poor at night anyway - so it's probably no great loss. SteveBaker 16:22, 20 August 2007 (UTC)

TI-89 Titanium Units

Why does my TI-89 Titanium think that all four of its preset units of temperature are "inconsistent units" and refuse to convert between them/add them, etc.? Thanks! Steevven1 ^{(Talk) (Contribs) (Gallery)} 21:23, 19 August 2007 (UTC)

Try using the tmpCnv() function. An example is $tmpCnv(0^{\circ }C,^{\circ }K)$ (the units need underscores, but I can't figure out how to make them show up there). anonymous6494 22:51, 19 August 2007 (UTC)

The problem with temperature is that it is used in two different ways.

(1) temperature

(2) temperature difference

So 20 F is a different concept from diff_20 F.

If you convert to celsius you get

(1) 20 F is equal to -6.66 C

(2) diff_20 F is equal to diff_11.11 C

I hope this clears it up. 202.168.50.40 03:55, 20 August 2007 (UTC)

Bald-headed birds and the sun...

Silly question I just thought about now. Do birds with bare heads (vultures or the marabou stork, for instance) get suntans on their heads and necks? --Kurt Shaped Box 22:34, 19 August 2007 (UTC)

Bound to- unless they have a built in UV screen--DieselFitter 23:37, 19 August 2007 (UTC)

That would be for a sunburn. Suntan would imply a natural ability to overproduce melanin in response to sunlight. Someguy1221 01:20, 20 August 2007 (UTC)

Just a guess but I would think bird skin is very similiar to lizard skin. I don't think they tan. --Tbeatty 02:08, 20 August 2007 (UTC)

Lizards don't tan in the same way humans do. They do disperse melanin throughout their melanophores though (as described in the article) they do so in a physiological rather than morphological manner. The situation in birds is not well understood, but its safe to assume that the bare head of vultures do not tan like humans would, for the simple reason that humans are extremely unusual in their tanning response. With a few exceptions (notably pigs and hippopotamus), even mammals do not respond to UV in the same way as humans. Rockpocket 06:24, 20 August 2007 (UTC)

Do pigs tan then? Does any animal? I doubt it.--Shantavira|^{feed me} 08:12, 20 August 2007 (UTC)

A quick and dirty google search gives reference to pigs with pink skin being susceptible to sunburn, and I see multiple reccommendations to apply suntan lotion to them. Stands to reason that if they can get a sunburn they could also get a "tan". Shhh though, if the sunscreen companies get wind of this they might think to test their products on pigs or, god forbid, hippopotamus. ;| 38.112.225.84 09:36, 20 August 2007 (UTC)

As I mentioned above, the ability to burn does not imply the ability to tan. These are biologically distinct processes. Someguy1221 19:54, 20 August 2007 (UTC)

Pigs should always have access to shady spots and a mud wallow as they burn readily in the sun. A wallow, as well as being cooling, is used by the pigs to cover their skin with a protective layer. DuncanHill 11:18, 20 August 2007 (UTC)

Correct me if I'm wrong - but aren't pink, shorthaired pigs a human creation? --Kurt Shaped Box 18:55, 20 August 2007 (UTC)

Maybe "we can tan" is the long-sought-after attribute of humanity that separates us from the beasts. --Sean 13:59, 20 August 2007 (UTC)