Wikipedia:Reference desk/Archives/Science/2011 March 25

= March 25 =

An interesting light wave problem
Dear Wikipedians:

I have encountered the following interesting light wave problem:

Two waves of light in air, of wavelength 612.6 nm, are initially in phase. They then travel through plastic layers as shown in figure below, with L1 = 4.07µm, L2 = 3.60µm, n1 = 1.41, and n2 = 1.56. In wavelengths, what is their phase difference after they have both emerged from the layers? Do not enter units with your answer.



I have answered it in the following way:

For plastic #1, number of wavelengths is: $$\frac{L_1}{\frac{\lambda}{n_1}} = \frac{4.07 \times 10^{-6} \times 1.41}{6.126 \times 10^{-7}} = \frac{12129}{2042} $$

For plastic #2, number of wavelengths is: $$\frac{L_2}{\frac{\lambda}{n_2}} = \frac{3.60 \times 10^{-6} \times 1.56}{6.126 \times 10^{-7}} = \frac{9360}{1021} $$

So difference between two is: $$\frac{19129}{2042} - \frac{9360}{1021} = \frac{19129}{2042} - \frac{18720}{2042} = \frac{409}{2042} \approx 0.2 $$

Therefore the phase difference is one fifth of the wavelength.

Does the above solution look reasonable?

L33th4x0r (talk) 03:18, 25 March 2011 (UTC)


 * The solution is almost correct but you are missing the fact that the two waves do not emerge from the plastic layers at the same position because the layers have different thickness. BTW, you are not deceiving anybody. We know that's homework. Dauto (talk) 03:46, 25 March 2011 (UTC)


 * I think the easiest way to deal with this is to subtract 1 from the refractive indexes for each; that way you're not counting the total number of wavelengths but just the number of extra wavelengths (or the number of cycles while the light is held up by the refractive material). Wnt (talk) 08:02, 25 March 2011 (UTC)
 * The instruction "Do not enter units with your answer." goes against good teaching practice. Phase might be expressed in degrees, radians, wavelengths, gradians, myriogrades, distance at a given frequency and speed of propagation or something else. ALWAYS state your units! Cuddlyable3 (talk) 13:17, 25 March 2011 (UTC)
 * The problem states that the units should be wavelengths. I think that the problem requires units to be excluded because this is a problem to be solved online and graded by a computer and the program was not designed to deal with units and verify whether they are correct. Dauto (talk) 15:06, 25 March 2011 (UTC)

Thank you all for your response. I am greatly touched. 70.29.27.87 (talk) 16:18, 27 March 2011 (UTC)

7 log = ?
In the phrase "..surveys of commercial jerky have shown total viable counts up to 7 log cfu/g" what exactly does '7 log' mean?

tl;dr How many cfu's per g are there in "7 log cfu"?

220.244.35.181 (talk) 13:56, 25 March 2011 (UTC)


 * 107 or 10 million (and cfu is colony-forming unit). Gandalf61 (talk) 14:16, 25 March 2011 (UTC)


 * If I saw "Here is a table of the log cfu count" I would expect to see the common (base ten) Logarithm of the specified variable, with the"characteristic" power of ten as well as the "mantissa" representing the number multiplied by a power of ten, (if I remember the terminology correctly).  I have not seen 10,000,000 represented as just "7 log." How would, say 15,000,000 be presented in this notation as "7.18 log?" I see no reference to this in the article Logarithm. Edison (talk) 17:02, 25 March 2011 (UTC)
 * Biologists seem to use this "log" notation in a very imprecise and unpredictable way (by my standards as a numerical scientist). I have heard an M.D. colleague use weird terms like "one and a half - seven log" (for "15,000,000").  It seems more common to consider biologist's usage of the "log" as a shorthand for "order of magnitude" - in other words, "15,000,000" would be "seven log" ; you would write the exact value as 15 x 106 if you needed precision.  If you scan through microbiology literature, you can see similar usage.  Here's Basic Mathematics for Microbiology from the University of Leeds: "The number of cells can be expressed in another way. Four is two squared (power 2), eight is two cubed (power 3), sixteen is two to the power 4, thirty-two is two to the power 5 an so on. The power or exponent in each case is increasing by one unit, although the numbers they represent start to jump in very big steps. The term logarithm (log) and exponent are interchangeable to some extent."   Nimur (talk) 20:22, 25 March 2011 (UTC)


 * OP Here. Thanks for your assistance. At first I was considering that log 7 would mean 77 -- as Edison points out, a mantisa is not given. What's remarkable is that in the several research papers I'm reading, the same type of unit is given over and over again -- always "X log cfu/g" 220.244.35.181 (talk) 18:59, 25 March 2011 (UTC)
 * There is such a thing as a logarithmic unit—decibels are an example. Never having seen the unit "cfu" before, I would have guessed (correctly, as it turns out) that "log cfu" is a logarithmic unit with (for example) 7 log cfu = 107 cfu. It's probably safe to assume that the base of the logarithm is 10 in this sort of situation. -- BenRG (talk) 21:30, 25 March 2011 (UTC)
 * OP here again. Thanks 220.244.35.181 (talk) 05:01, 26 March 2011 (UTC)

Emulating squinting
My question basicly is since squinting improves vision why people haven't come up with artifical means to emulate it, so one wouldn't have to? As the eye is optic system in theory it shoud be possible. To do so first one would need to know what it does tto eye. I found two theories why squinting works. There is idea that it squeezes the eye (which may be valid because some refraction errors are caused by eye beeing elongated, but when you squint you move eyelids, so I'm not sure if muscles used for squinting acctualy squeeze the eye). Secondly I did a search on reference desk archives and found some suggestions that it changes aperture (but the pupil can naturaly get smaller - so why dosen't that happen and one has to squint instead?). Secondly there is the question on how to acctualy emulate it. For example, aperture might be hard to reduce with glasses (I know about pinhole glasses, dosen't look ike a practical solution to me), but I suppose contacts might work (i.e. you coud paint an opaque ring on them) Xil  (talk) 17:26, 25 March 2011 (UTC) Contacts DO work! —Preceding unsigned comment added by 165.212.189.187 (talk) 17:39, 25 March 2011 (UTC)


 * I assume you're thinking this would be advantageous because it would be one-size-fits all - that is, no need for tailoring pinhole lenses to the defective eyes? (By the way, why don't pinhole glasses produce upside-down images?) 213.122.56.174 (talk) 18:32, 25 March 2011 (UTC)


 * I believe they do, as do regular lenses, but your brain "flips them back over". StuRat (talk) 21:06, 25 March 2011 (UTC)


 * That's acctualy good question - the eyes themselves work like camera obscura and it is brain that flips the image. So either the image gets fliped twice (or it might be the opposite - the brain dosen't boder seeing how it is correct) or maybe it dosen't because it is very close to the eye and perhaps the fliping effect occurs only in darkness. At any rate I've never seen inage fliped when looking trough any small holes (you can try yourself with a dark sheet of paper). And to answer your question - No, it is not that one size fits all, but that I believe it actualy offers better correction, according to pinhole occluder (assuming it works the same) it compitely removes effects of refractive errors Xil  (talk) 22:41, 25 March 2011 (UTC)


 * The light going through the pinholes takes the same path it normally would (unless you make them so small the wave-properties of light start messing with it). It's just that you don't have all the other light. Normally, the light is random when it hits the lens of your eye, and then focuses as an inverted image on your retina. With pinhole glasses, it would make an inverted image on your eye, which pretty much ends up traveling straight and making an inverted image on your retina. Your brain is capable of flipping an inverted image, but it takes a while for it to do it. It's not something that will happen so fast you don't even notice it. — DanielLC 05:59, 28 March 2011 (UTC)


 * The problem with pinholes is that they don't allow enough light through, so they make everything darker. Also, they eliminate peripheral vision.  StuRat (talk) 21:07, 25 March 2011 (UTC)


 * The "reducing the aperture" advantage basic gets you to pinhole glasses, so that's not much of an option, as noted above. As for deforming the shape of the lens and changing the distance between the lens and the cones and rods on the back of the eye, that could work, but sounds rather complicated and uncomfortable.  I don't see how that's better than glasses, contacts, or corrective eye surgery. StuRat (talk) 21:12, 25 March 2011 (UTC)


 * If it is the squeezing, the only way I see it reproduced is by surgicaly inserting implants that squeeze the eye to correct shape or something along those lines. Also I myself think pinhole glasses is BS, but why not pinhole contacts? I'm thinking that they are so close to eye that effects on vision (and looks) would be minimal Xil  (talk) 22:41, 25 March 2011 (UTC)


 * Contacts could fix the appearance, sure, if the "mask" was black to match the pupil. However, the fundamental limitations of pinholes, which are a lack of light and peripheral vision, still remain.  So, regular contacts are a far better choice. StuRat (talk) 22:55, 25 March 2011 (UTC)


 * Contacts squeeze - that is, they distort the cornea, which accounts for "approximately two-thirds of the eye's total optical power". 213.122.59.22 (talk) 07:35, 26 March 2011 (UTC)


 * Incidentally, I'm not sure if you're making this mistake, but many do, so I will mention it: The advantages of a pinhole lens only works for a single pinhole (or one for each eye). You can't just poke lots of holes in something and have it work, collectively, as a giant pinhole lens.  The reason is, that while the light coming through each pinhole is focused, the light from each pinhole is not in focus with the light from the others.  Thus, you would need a normal lens to adjust all these different focused images into a single one.


 * Perhaps, in the future, a digital image from each pinhole could be combined with the others to provide a nice, focused "collective" image, with an infinite focal depth. I believe that the compound eyes of some insects work something like that. StuRat (talk) 23:05, 25 March 2011 (UTC)


 * I figured so, I was acctualy surprised to see what pinhole glasses look like (this would be yet another proof of it being BS :) wonder why some people think they work?) How about reducing transparency of the lens, instead of making it complitely opaque ? Need be, you would turn to look, even if you hadn't reduced peripheral vision, so all you need is vague notion that you should do that Xil  (talk) 23:20, 25 March 2011 (UTC)


 * Well, that would lose some of the benefit of the pinhole, since some unfocused light would get in, making it slightly blurry. Meanwhile, the image the eye sees would still be somewhat darker and have somewhat reduced peripheral vision.  So, not a very good compromise.  Perhaps a pinhole contact in one eye and a regular contact in the other might be a somewhat better compromise.  This would give you normal vision in the normal contact eye, while providing an infinite depth, focused image of bright objects in the pinhole contact eye, but without any peripheral vision there.  This might work similar to how jewelers and some others wear special glasses with one microscope lens and one normal lens.  I expect that this would work best in bright light, like when working in Antarctica in the summer.  It would be good advice to swap which eye gets the pinhole contact each day, as permanently using such a lens might have some bad effects on the eye (you'd likely need different pinhole lenses for each eye, as the shapes of the corneas vary).  StuRat (talk) 23:32, 25 March 2011 (UTC)


 * I did an image search for "pinhole occluder", and found they have many holes (at one end) and look very much like the pinhole glasses. I hesitate to draw any conclusion from that, though, since I don't know exactly how they're used. (They also look a lot like large plastic serving spoons; appearances can be deceptive.) 213.122.59.22 (talk) 07:30, 26 March 2011 (UTC)


 * I have 20/10 vision when I squint things get blurry. so I attributed it to the distortion of my eyelashes and the additional liquid that the eyelids channelled to the center of the eye directly over the pupil, the I never thought about the squeasing also affects it.  —Preceding unsigned comment added by 98.221.254.154 (talk) 00:51, 26 March 2011 (UTC)


 * Maybe you could conduct a survey to find correlations between "how" the person squints (cheek forehead muscles tight, loose, eyes almost totally closed or almost totally open), and what type of vision they have nearsighted; farsighted and severity of each, even eyelash color.98.221.254.154 (talk) 01:34, 26 March 2011 (UTC)
 * That would work as long as the person has only one eye condition which often is not the case. Therefore I think that artifical model is more acceptable, which is why I am asking it here - probably people with knowledge in optics or human anatomy can answer about causes and effects Xil  (talk) 04:23, 26 March 2011 (UTC)
 * I think that making a pinhole with one's fingers is more effective and less uncomfortable than squinting for any prolonged use. Wnt (talk) 15:46, 26 March 2011 (UTC)
 * It reduces field of view much more than squinting. Xil  (talk) 02:33, 27 March 2011 (UTC)

Reverse osmosis versus evaporation for concentrating an aqueous solution
How do RO and evaporation compare in terms of energy use for concentrating an aqueous solution? How about in terms of capital investment? (If it matters, I am not thinking about huge scale, maybe 100's of liters a day). ike9898 (talk) 17:34, 25 March 2011 (UTC)
 * Partly answering my own question - | this paper addresses the comparison in the context of concentrating milk. The bottom line answer seems to be "it depends".  I'm still interested in your insights ReFDeskers. ike9898 (talk) 18:14, 25 March 2011 (UTC)

vacuum space
I don't understand how vacuum space has the ability to "expand." Was all the vacuum space that we observe in the current universe "inside" the big bang or "among" it? The word expand makes me think of two objects being "forced" apart by something between them growing, instead of them simply moving apart based on their inertias which results in more space between them. —Preceding unsigned comment added by 165.212.189.187 (talk) 17:45, 25 March 2011 (UTC)


 * Milne universe with CMBR.svg is a convenient fiction; spacetime is closer to reality. Spacetime doesn't expand, or change in any way, since it covers all of history at once. "Space" is a slice of spacetime. Any given slice is different from any other slice, so there's an element of arbitrariness in any attempt to match certain points of one slice with certain points of another. If you have a triangular piece of spacetime like the one in this diagram, you can slice it horizontally and say that space is globally expanding (in the sense that each successive slice has a larger total area) and the galaxies have relative velocities that carry them apart. Or you can slice it into hyperbolas, which are all infinite, and say that space is always infinite; but the hyperbolas are nested in such a way that it's useful to say that each one is locally larger than the one before it, and you can even attribute the separate of the galaxies to this local expansion. But general relativity doesn't care about any of this. From the perspective of general relativity, the motion of the galaxies is the same as any other motion. It's fine to say that it's due to inertia. (And gravitation.) -- BenRG (talk) 21:21, 25 March 2011 (UTC)
 * Serious:Can anybody tell if this answers the question? —Preceding unsigned comment added by 98.221.254.154 (talk) 00:55, 26 March 2011 (UTC)
 * Yes, that was a good answer for the question except that it might go over the head of the person that asked the question but such is physics. Dauto (talk) 03:26, 26 March 2011 (UTC)
 * Yes, it was a good answer, but most of us struggle to understand the complexities. I thought the expansion was the result of inertia (reduced by gravity) and also dark energy, but BenRG is the expert on this.    D b f i r s   10:43, 26 March 2011 (UTC)
 * I'm really not an expert on anything but the simplest aspects of cosmology. And if my last reply sounds profound then I've failed. I want to write answers that are so easy to understand that everyone can see how trivial they are. General relativity is just lines on a curved surface—nothing to it. I hope the original poster will return to say whether my reply was helpful at all. "Expansion is the result of inertia (reduced by gravity) and also dark energy" sounds perfectly correct to me, by the way. -- BenRG (talk) 08:07, 27 March 2011 (UTC)

OP here, So some parts of the universe are expanding but others are not? Just because 2 objects are moving away from each other means that the universe is expanding? Am I expanding? —Preceding unsigned comment added by 98.221.254.154 (talk) 13:35, 27 March 2011 (UTC)
 * We all tend to expand a bit with age, but the expansion of the universe seems to be an expansion of the space between galaxies. The space within galaxies and especially within solar systems seems to be prevented from expanding by gravity and the presence of matter. Metric expansion of space gives some detail.   D b f i r s   16:27, 27 March 2011 (UTC)

Yes helpful. Could dark matter be a result of matter "trapping" empty space within its atoms, which can then be moved separately from the rest of empty space? —Preceding unsigned comment added by 98.221.254.154 (talk) 01:47, 28 March 2011 (UTC)
 * It's not just the space within atoms that doesn't expand, it's also the space between planets in a solar system, and the space between stars in a galaxy. The reason might be the proximity to matter, but I'm not sure that anyone really knows.  ( On the other hand, if all space were expanding at a uniform rate, how could we tell? )    D b f i r s   17:35, 28 March 2011 (UTC)

Electromagnetic waves
Electromagnetic waves are often produced by accelerated charged particles. but accelration is a relative quantity.does it mean that wether there is a wave or not depends on the observer? —Preceding unsigned comment added by 178.18.17.91 (talk) 19:08, 25 March 2011 (UTC)
 * Motion is relative, but acceleration isn't. If you and I are each in rockets and we look out of our windows and see that we are accelerating towards each other, we can tell which of us is actually accelerating by which of us is being pressed into our chair. --Tango (talk) 19:29, 25 March 2011 (UTC)

Bird Identification - Junco?
Can you help identify this bird? I photographed it in late October in the San Francisco Bay area of California. I believe it is a Dark-eyed Junco, per my field guide, but before I add this photo to any article, I would like one or more experienced second-opinions. While we're at it, here's another one from the same day, unidentified bird that a friend tells me might be a flycatcher. Ideas? Nimur (talk) 20:31, 25 March 2011 (UTC)


 * The second bird looks like the western form of the yellow-rumped warbler, but I'm not entirely sure. --Dr Dima (talk) 03:32, 26 March 2011 (UTC)

Flammable plastic
Hi. I am looking for the name of a flammable plastic. Specifically it is commonly used in blinds in front of windows and has a flash point temperature lower than that of ordinary paper. Any ideas? Thanks. ~ A H  1 (TCU) 22:23, 25 March 2011 (UTC)
 * Flammable plastics with low flashpoints makes me think of nitrocellulose and Celluloid. -- Jayron  32  23:10, 25 March 2011 (UTC)
 * (ec)Nitrocellulose. See also Cellulose acetate film, Cellulose acetate, and Cellulose triacetate for the plastic that replaced it. Ariel. (talk) 23:12, 25 March 2011 (UTC)

red/blue. Exactly same speed?
Given that white light and all electromagnetic particles/waves travel at the same known speed in vacuum/space. (relative measurement uncertainty 4 parts per billion), and that light is slowed as it passes through another medium such as air,and I'm assuming that a massive gravitational field is another medium, and also that since white light can be easily seperated therefore is not truly homogenous. Is it possible that the red light and the blue light might be slowed and or deflected at slightly different rates, such as when it passes close to a galaxy. If that is possible and if the red light is then traveling slightly faster than the blue light then any end target(earth)would receive red at a greater rate than blue. any light receiver (camera) operating on a long time exposure would then receive more red than blue within the given time frame. If this were so (lots of if's here)then the resulting photo. would show predominance of red which might be misinterpreted as red shift indicating rapidly receding light source. We know that red and blue are refracted at different rates. perhaps the speed is effected similarly. Has anybody done any work on that?Phalcor (talk) 22:35, 25 March 2011 (UTC)
 * A gravitational field is not a medium in that sense. The reason media slow down light is because the light is continuously being absorbed and re-emitted. That doesn't happen in a gravitational field. What does happen, is the light is curved and gains or loses energy (gaining energy makes it bluer, losing energy makes it redder). It doesn't change speed. --Tango (talk) 22:56, 25 March 2011 (UTC)
 * Tango, the picture where light is absorbed and reemitted in medium is inaccurate. That's a more apt description for what happens inside the sun where indeed a photon gets absorbed and reemitted as it finds its way out and follows a random walk and exchanges energy with the medium changing its frequency. When light goes through a medium like glass, air, or water, none of those things happen. It follows a straight path and keeps a constant frequency. A better description would be that light in vacuum and light in a medium are different kinds of animals because light in a medium will also include oscillation of the medium particles along with the oscillation of the electromagnetic filed. Dauto (talk) 00:38, 26 March 2011 (UTC)
 * Yes, red light and blue light do move at different speeds when going through a medium such as water or glass. That's called dispersion. Dauto (talk) 00:42, 26 March 2011 (UTC)


 * Adding to what I said above. While red light and blue light move at different speeds on a medium, all light moves at the same speed through empty space whether there is some gravitational field or not as Tango correctly explained above and even if red light were to move faster that would not lead to red-shift because red-shift is not measured by comparing the amount of red and blue light. Red shift is measured by the shift of the spectral line of Hydrogen. Dauto (talk) 13:51, 26 March 2011 (UTC)


 * Do the half-remembered phrases "phase velocity" and "group velocity" bear on this? Edison (talk) 01:02, 26 March 2011 (UTC)
 * Yes. phase velocity is related to the angular frequency and the wavenumber through the equation
 * $$v_\mathrm{p} = \frac{\omega}{k}.$$,
 * while group velocity is given by
 * $$v_\mathrm{g} = \frac{d \omega}{d k}.$$.
 * If there is no dispersion then the phase velocity is constant and identical to the group velocity, but if dispersion is present, they are different from each other and vary as a function of the frequency. Dauto (talk) 01:33, 26 March 2011 (UTC)