Wikipedia:Reference desk/Archives/Science/2015 December 6

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December 6

Does light accelerate?

When light goes through glass and air, does it accelerate and decelerate? Light speed is c / 1.5 (≈ 200000 km/s) in glass, and 299700 km/s in air.

If we could measure the speed of light at smaller and smaller distances, would we see light accelerating? Can we measure the speed of light at very short distances at all? Maybe it's just the case that light needs a really short distance to reach full speed.

All explanations I found so far about a hypothetical 'acceleration of light' were that it does not happen, because light has an absolute speed. This is kind of circular thinking to me --3dcaddy (talk) 02:33, 6 December 2015 (UTC)

It has an absolute speed in a vacuum but a different speed in a medium. Bubba73 ^{You talkin' to me?} 02:37, 6 December 2015 (UTC)

Yes, the question is what happens between those mediums. How can both options be tested? Does it accelerate to the higher speed or just jump to another speed without the intermediate speed? Intuitively, it's difficult to imagine that something has not to pass through the intermediate speeds.--3dcaddy (talk) 02:46, 6 December 2015 (UTC)

Well, electrons jump from one orbital to another without going in between. It seems to me that the same could happen to the speed of the photon. Bubba73 ^{You talkin' to me?} 03:35, 6 December 2015 (UTC)

For most 'normal' things, infinite acceleration is impossible because F=m.a - Force equals mass times acceleration. If acceleration is infinite and the object has any non-zero mass, then the force required would be infinite. But photons have zero rest-mass, so even the tiniest force could result in an infinite acceleration - so this kind of weirdness isn't impossible.

However, I think the mechanism that causes the change of speed is a lot more subtle than that...I believe it's easier to understand using the wave-aspect of light than the particle-like aspect. SteveBaker (talk) 04:04, 6 December 2015 (UTC)

See our article on the speed of light, which does not directly answer your question in the terms you've phrased it in, but should provide all of the necessary context for understanding the answer. In brief, no the light does not pass through intermediate speeds (unless it passes through multiple physical mediums sequentially). That is to say, as soon as the photon passes through the space occupied by the material in question, it instantaneously again achieves the speed of 299,792,458 m/s, a universal constant upon which many basic principles of the physical universe depend. As you note, this can seem counter-intuitive if you are expecting a photon to operate as would an object, but, as Steve notes, photons have zero intrinsic mass. Depending on the semantics you choose to apply to the verbs accelerate and decelerate (that is, whether it reflects a change in absolute speed or the process of achieving a given speed), you might say that light does not accelerate at all but rather moves at the constant c and at other constants when it must propagate through a material. That is, the very existence of the phenomena of acceleration as it acts upon any object (which must have a non-zero rest mass) is an artifact that results from the principal of mass-energy equivalence. Snow ^{let's rap} 06:24, 6 December 2015 (UTC)

One way to look at the answer is to say that light neither accelerates nor decelerates as it only ever travels at one speed - the speed of light. It just happens that that speed is different in different mediums. So, on passing from vacuum to air to plastic, the speed in each is different but it changes from the speed of light in a vacuum to the speed of light in air, etc, instantaneously on changing media. As noted above, it is like the situation with an electron being promoted to a different energy state, it ceases to exist in one and simultaneously starts existing in the other. This is intuitively bizarre when considered from a particulate point of view, but more comprehensible from a wave perspective. As an analogy, consider a graph of y = x on 0 ≤ x ≤ 1 and y = 2x − 1 on x > 1. The graph is continuous (like the path of light) but its gradient is discontinuous at x = 1 (like the speed of light at a boundary). We can say the graph has slope of 1 for x < 1 and slope of 2 for x > 1 but at x = 1 it is not sensible to say it increases from 1 to 2 going through slopes like 1.5; rather, it is one slope and then a different one. Snow Rise has offered another way of looking at the situation by noting the consequence of a lack of rest mass. That light has its own speed which is a constant but changes with medium is odd, I agree, but has profound consequences. EdChem (talk) 11:57, 6 December 2015 (UTC)

The OP has asked an excellent question. Can we be so sure light jumps from one speed to another at a boundary interface? I don't think so.

All real mediums have surfaces that at atomic-level distances are very rough, and surface roughness extends in size up to what can be seen in light microscopes. I find that the concept that a medium boundary is a straight line of perfect smoothness is a theoretical (you might say false) concept that can only be applied for electromagnetic energy (light is just EM energy at certain wavelengths) at very long wavelengths, such as applies with radio emission in the broadcast bands.

As actual medium boundaries are irregular on a scale that is comparable to at least the shorter wavelengths of light/EM radiation, they cannot necessarily be considered an abrupt transition, thus light does not, in general undergo an abrupt change in speed. If boundary roughness is multiples of the wavelength then the speed must change with some profile over that distance.

For very short wavelengths (beyond visible light, e.g, x-rays, gamma rays), the wavelenth may be comparable or long compared to inter-atomic spacing. Then the boundary certainly cannot be regarded as abrupt, even if the medium was a perfectly formed "straight sided" crystal.124.178.173.1 (talk) 16:47, 6 December 2015 (UTC)

I think you are confusing the issue. When the 'rays' strike the 'boundary' a new array of point sources are initiated, the combined wave front of which forms the light wavefront in the new material. By definition, the boundary is where the apparent acceleration occurs. It has a physical location, but may be either side of the local physical boundary between the two materials but will be within a wavelength of it. Greglocock (talk) 21:11, 6 December 2015 (UTC)

I may be confusing myself. But I think you have added to it. If you accept that the speed change boundary may be up to one wavelength away from the "physical boundary" (your "definition"), then you must also accept that the physical boundary is, on the wavelength scale, undefined. At wavelength scale, what we think of as a perfect plane surface may have hills and valleys multiple interatomic distances high. In fact there may be, at long light/EM wavelengths, hundreds or thousands of such hills and valleys.

At short wavelengths, multiple wavelengths may fit within adjacent atoms, so even with a "plane surface" there is no flat plane boundary. I'm not standing on a boundary if if there is nothing but an "atom" far to the left of me, and another far to my right, far below me, etc. So where is the step acceleration located, when the interatomic disance of the medium is of the same order or larger than a wavelength? I think there is no such location. At the very least, it is "undefined".

It is usually misleading to try to apply classical reasoning when you are working on the scale of interatomic distances. 60.228.161.183 (talk) 00:28, 7 December 2015 (UTC)

You should not confuse the phase speed of light with the speed of the actual photons. The former can change abruptly while the latter does not change at all. Ruslik_Zero 03:39, 7 December 2015 (UTC)

I recall a similar explanation...I'm sure this is the key here.

That said, it's always bothered me that the speed of light in air is not the same as the speed in vacuum - yet from the perspective of a photon, air is mostly vacuum, with an occasional molecule zipping past. Air molecules are (on average) about 4 nanometers apart - and the radius of a Nitrogen molecule is something around 0.1 nanometers (although the concept of "size" is a bit hard to define at the atomic level). It's like saying that a room containing 100 basketballs is full of basketballs. If the photon doesn't come close to any molecules - why should it travel at any speed other than 'c'? I don't get it.

But yes - phase speed and particle speed aren't the same thing. SteveBaker (talk) 03:50, 7 December 2015 (UTC)

Photons in the air do not propagate freely. They are elastically (and inelastically) scattered by molecules. The scattering cross-section may significantly exceed the size of molecules themselves. Each scattering event introduces a delay and therefore slows energy/momentum propagation. So, strictly speaking, the photons that enter the air are different from those that exit it, but are actually indistinguishable because the scattering in coherent. As to the phase speed or the group speed for that matter, they are simply some abstract macroscopic parameters that describe the average classical electromagnetic field. These speeds are generally not related to the energy/momentum propagation. Ruslik_Zero 13:28, 7 December 2015 (UTC)

So should one imagine the photons taking a subtly zigzag course through the material at "the speed of light in vacuum" - but because of the slightly indirect route, they are taking longer? Of simply that when a photon gets close enough to be scattered, it is actually absorbed and re-emitted after some delay? Or are both of those also naive over-simplifications?

Either way - the OP's question about acceleration is moot. SteveBaker (talk) 16:08, 7 December 2015 (UTC)

What does the VIIA of periodic table stand for?

I would like to understand what the letter A stands for, in all these columns. 12:15, 6 December 2015 (UTC) — Preceding unsigned comment added by 92.249.70.153 (talk)

It doesn't stand for anything, but signifies that the column is on the left-hand side of the table. VIIB is on the right. Note that this labelling scheme is no longer recommended, with modern usage preferring a simple numbering from group 1 to group 18. You can read much more at Group (periodic table) article. Bazza (talk) 12:28, 6 December 2015 (UTC)

Thank you for your comment and for the beautiful article that you linked to. 92.249.70.153 (talk) 12:30, 6 December 2015 (UTC)

You can also look at Periodic table#group and History of the periodic table. The A/B originates back with Mendeleev's first periodic table. 'A' was used if the outermost electron in a group was in the 's' or 'p' orbitals, 'B' if it was in a 'd' orbital. EdChem (talk) 12:31, 6 December 2015 (UTC)

Also, unfortunately, the A/B designation differed depending on the source. Some periodic tables numbered the first 10 rows "IA-VIIIA" (the cobalt-nickle-iron triad was considered one "group") and then the next 8 rows "IB-VIIIB" starting with the Copper group through the noble gases.(former IUPAC system) Other systems did as you describe above: the "A" groups were the tall main group elements and the "B" groups as the transition elements.(CAS system) Eventually, the IUPAC said "screw it all" and just designated all groups from 1-18, with arabic rather than roman numerals (so as to avoid confusion with the two conflicting older systems). But you will still find modern periodic tables with all three systems, either alone or in some combination. This is all covered in the Group article cited above. I personally prefer the CAS system, because it separates the chemically distinct main group from transition elements, and the Roman numerals match many of the common valences of the elements.--Jayron32 21:37, 6 December 2015 (UTC)

Element can lose or gain more than three electrons?

I have a doubt, because I have read two things in the same book (chemistry for dummies) that look me as a paradox. 1) "but an element doesn't lose or gain more than three electrons" (p.56) 2) it's written in the same book in one of the tables there "Sn^4- so we see against the sentence that it could be that element will lose more that three electrons. Maybe I don't understand in the book or in chemistry? 12:28, 6 December 2015 (UTC) — Preceding unsigned comment added by 92.249.70.153 (talk)

I'd say the book is wrong. Titanium dioxide, TiO₂, is ionic and consists of Ti⁴⁺ cations, at least formally. It is trying to be simple but in this case is way too simple. EdChem (talk) 12:33, 6 December 2015 (UTC)

Could it be a typo? (I mean that instead of 4 it's written 3)? 92.249.70.153 (talk) 12:37, 6 December 2015 (UTC)

My opinion is that it is a statement made thinking about factors way beyond the level you are at and is more likely to cause confusion than to be helpful. I would ignore it. PS: I think you meant Sn⁴⁺ above, the tin(IV) cation is well known (it is the stablest of tin's ions) and the anion Sn⁴⁻ would be exceptionally unstable. EdChem (talk) 12:52, 6 December 2015 (UTC)

Oxidation state can run from -5 to +9 (at least according to the article, +10 remains hypothetical). However, it is such an approximation that it doesn't really mean that much. I have no idea how much positive or negative charge you can 'really' have on an atom in a stable compound, if you could somehow 'look' at it, and would welcome any insight! As for the book... without context I really don't know what they're suggesting. Wnt (talk) 13:26, 6 December 2015 (UTC)

@Wnt: the IP who asked the question has also asked about statements that ionic bonds form between metals and non-metals and that metals form cations and non-metals anions, so he is early in his chemistry studies and I doubt will have yet encountered oxidation state. Certainly my thinking was of cases like the chromate anion CrO₄²⁻ with a formal chromium(VI) centre actually having given up less than three electrons worth of density due to the covalency of the Cr=O interactions, which is why I said the statement is considering factors way beyond the IP's level. The other problem is that cations Mⁿ⁺ can be formed for high n in extreme circumstances - looking at the ionisation enthalpies, for instance - so the statement also runs into problems with over-generality (as have other statements the IP has quoted). If it is meant as an introductory text and restricted to the s and p blocks, then ±4 makes some sense as a limit, but I still think skipping over the statement is wisest. Does that make sense?

Oops... I forgot to sign this. EdChem (talk) 00:47, 7 December 2015 (UTC)

I never actually took a formal Inorganic Chemistry class, and I really don't know the usual order of instruction. But oxidation state is a really, really crude concept - simpler than VSEPR, let alone molecular orbitals or band theory - so I think it is as relevant to a beginning student as anyone else. It also tends to have a certain arbitrariness about it, special rules and "it can be viewed that way" answers. The article starts off saying it describes chemical compounds, rather than ions in vacuum, though it makes sense you can extend it to that, and if you do then yes, you should be able to have very high oxidation states. The system is defiantly contrafactual: as described at electronegativity, there really is no such thing as a pure ionic bond, and differences of about 1.7 correspond to 50% ionic character. So Ti-O or Cr-O bonds are slightly more than 50% ionic, in theory... I don't think it's quite that simple to come up with a result for the total charge on the metal though, since each partial positive charge will tend to repel the others. But it's definitely less than the formal charge. The main reason why I wanted to mention oxidation state at all is that it is one model in which atoms actually can lose electrons, per the OP's description. In reality I think there's always some kind of molecular orbital density around the electropositive atoms, and even a cesium bound to fluorine will get more time with its electron pair than an American family court would usually allot. Wnt (talk) 20:45, 6 December 2015 (UTC)

I am guessing the IP is at the mid-high school level of learning basic chemistry, which means (in Australia, at least) that oxidation state, crude a concept though it is, has yet to be covered. Like many concepts, oxidation state provides a useful tool but only when its limitations are recognized and respected, and I think it is ill-suited to a declarative statement as to a maximum number of transferrable electrons for some of the reasons Wnt mentions. EdChem (talk) 00:53, 7 December 2015 (UTC)

DIY fabrication of articles using resilient silicone material

I've seen ice cube trays and oven mitts made of some resilient silicone material. I wonder if it's feasible to make things with that type of material as a DIY project. I'm thinking about something like custom protective cases for electronics and custom grips for handles. The product needs to stand up to at least moderate use. If it's feasible DIY project, how do you do it? — Preceding unsigned comment added by 123.136.11.156 (talk) 18:51, 6 December 2015 (UTC)

See Silicone rubber and Injection molding of liquid silicone rubber. A Google search on "silicone rubber molding kit" might also be useful - there are plenty of firms out there who sell kits suitable for the hobbyist, although the process of designing the mould may not be trivial for complex shapes. Tevildo (talk) 21:50, 6 December 2015 (UTC)

I'm under the impression that the kind of kits you mentioned are for making molds that themselves will be used to reproduce solid shapes by casting. The silicone molds are typically the end products. Do you know how durable the material is? --123.136.11.157 (talk) 00:27, 7 December 2015 (UTC)

Most of the hobbyist sites I've looked at cater for both resin casting using silicone moulds and making articles out of silicone. For durability, I'm afraid I can only go on my experience of everyday silicone-rubber objects, which is probably similar to yours. I've not made any myself. Tevildo (talk) 21:56, 7 December 2015 (UTC)

There is also this Sugru stuff that's quite popular, it is silicon based but it's more like modelling clay than rubber, so maybe not for protective cases or ice cube trays, but definitely for "solid" things like custom grip handles.. Vespine (talk) 21:53, 6 December 2015 (UTC)

Radios in trucks?

I'm currently writing a novel about a young woman running a farm in the American south in the early 1950s. I've been trying to be as authentic as possible where it comes to all the stuff about the geography of the area, farming methods used then, famous people of the period, etc. I'm having trouble with a scene where the main character is driving a new pickup truck home from a farm sale. I'd like to include them listening to the Grand Ole Opry on the radio (as sort of a tribute to a relative who lived then and loved Hank Williams), but I don't know if cars or trucks had radios back then. The truck manufacturer is not named but would probably be Ford or Chevrolet. So, did trucks come with radios back then? Thanks, White Arabian Filly (Neigh) 19:31, 6 December 2015 (UTC)

1950s? Yes, of course they did. I once owned a 1938 Ford V8 Deluxe, and found an original radio for it and fitted it. But even earlier cars than that had radios. Akld guy (talk) 19:50, 6 December 2015 (UTC)

I would say the average pickup truck came standard with no radio then, but that you could buy an AM radio as an option. An AM/FM radio might not have been available at that time, at least in a truck. (Note that trucks were considered to be strictly practical vehicles then, as this was before the days of Cadillac Escalades and their ilk. So, a radio for entertainment might not have been considered important, but AM radio was considered practical, for telling you about news and weather, for example. Also note that AM and FM were both strictly mono then.) StuRat (talk) 20:10, 6 December 2015 (UTC)

On the other hand, if you want an actual reference instead of what Stu would say, you might look (1) at advertisements of the period, or (2) at books about the trucks of the period. As to 1, some old advertisements (for example, in Life magazine) can be accessed through Google Books, but it's biased toward recent publications, so you have to find the hidden Advanced Search page in order to limit the search by publication date. I didn't find anything relevant but I didn't take much time trying it. As to 2, Google Books found Ford Pickup Trucks by Steve Statham, and showed me a snippet saying that on the 1948 models "an automatic push-button tuning AM radio was optional"; and it found Ford F-100/F-150 Pickup, 1953-1996 by Robert C. Ackerson, in which a table on page 12 says that "Radio (five-tube) with rectifier and single knob control" was an available accessory on the 1953 F-100. So apparently Stu's answer was right. --76.69.45.64 (talk) 21:29, 6 December 2015 (UTC)

It's important to note that any radio back then would be based on vacuum tubes - the transistor radio wasn't commercially available for automotive use until the late fifties (the Mopar model 914HR (1955), to be precise). A tube radio wouldn't really be very practical for a working farm truck, rather than one which would be driven entirely on paved roads - although, as noted above, they were available. Tevildo (talk) 21:57, 6 December 2015 (UTC)

Why do you say a tube radio wouldn't be very practical? If you mean it would be jolted around on rough roads...well, many roads of the day weren't paved, so family cars would have had the same problem and thus the radios were built for those conditions. With a great deal of experience in restoring WW2 military radios, I can say that (glass) tubes are far more rugged than latter day readers might think. Akld guy (talk) 22:18, 6 December 2015 (UTC)

In any case, vacuum tubes operate at lower temperatures and thus are more rugged than incandescent light globes. How often do you have to replace the various light globes in your truck? Not very often. 60.228.161.183 (talk) 00:55, 7 December 2015 (UTC)

The first commercial success of AM radios within in cars actually dates back to 1930 by Galvin Manufacturing (now known as Motorola). It was also came with a hefty price tag at the time being around $130. (ref 1) (ref 2) From that point on car radios become more and more common, still at a premium price though. In regards to your question, Chevy began offering radio as an advanced option in their trucks in 1947. (ref 3) So in summary, yes, there were AM radios in trucks in the early 1950. Aclark05 (talk) 22:44, 6 December 2015 (UTC)

Although not a farmer myself, I come from a farming family. Radios (AM band) have long been considered ESSENTIAL in all types of farm vehicles. Even closed cabin tractors have always had them. In rural areas, radio stations give information of keen interest to farmers - news, weather, fire information, sale yard prices, etc. But one thing that might be important. Farm trucks, especially in the 1950's were built well, but built down to a price. That meant they had higher cabin noise than sedans. And farmers travel outside towns, so their speeds are high ---> more noise. So if you were listening to a voice reading, fine, but for music it was a bit of a strain. Compressed rock and roll not so bad (we turned the volume to max), but any sort of music with dynamic range not good. Classical music in a 1950's farm truck was pretty hopeless. Note that car radios then had only 5 watt mono audio. Today's car radios may have 20 watts per channel or even more. That makes listening to music much less of a strain. 60.228.161.183 (talk) 00:55, 7 December 2015 (UTC)

Thanks to all. Most of the book has to do with horses and the fact that they were just then going big-time with most of the shows (improving the prize money to where you could make a good living training or riding because horses were about the hottest thing going, what with all the cowboy movies inspiring everyday people to get a horse and ride), but I want the non-horse stuff to be accurate as well. (I grew up on a farm, so I know horses and animals, and old tractors to some extent.) Thanks! White Arabian Filly (Neigh) 02:00, 7 December 2015 (UTC)

Aclark05, if you don't mind I changed your refs to inline links. Refs aren't constrained to the "section" so will remain on the very bottom of the reference desk page, making it appear like they belong to the bottom section, rather than this question. Vespine (talk) 02:58, 7 December 2015 (UTC)

Wouldn't it be better to just add a Template:Reflist-talk via {{Reflist-talk}} to this question? If there's any other question with inline references without the template, the template should be added there too. Nil Einne (talk) 05:33, 7 December 2015 (UTC)

As one data point, I tracked down the original purchase order for my 1962 Mini. It cost it's original owner 600 UK pounds. The AM radio was an optional extra and cost an additional 50 UK pounds. That's 1/12th the cost of the car! For comparison, seat belts were also an optional extra - also 50 pounds - and the owner didn't buy them. From this I deduce that the radio was a costly item, but considered sufficiently valuable as to outweigh having seatbelts.

Fast forward to today - my modern MINI cost me $21,000 - and the radio was (of course) standard equipment. But if it had been an optional extra costing 1/12th the price of the car ($1,750) I wouldn't have bought it! So, I think it's certain that a radio would be an optional extra in a truck of your era - I'm not sure how many people would have purchased it. Certainly it would be in the luxury category. SteveBaker (talk) 03:39, 7 December 2015 (UTC)

One of the prime functions of AM radio in rural areas in the 1950s and 1960s was to communicate farm commodity prices and to provide agricultural advice from the county extension agent. It would easily be reasonable for a farmer to have a radio in their truck to get the ag news. I remember substantial parts of the morning AM show in the 1960s devoted to agriculture. I would agree that a truck radio would have been optimized for voice, and that music would have sounded pretty tinny. Acroterion (talk) 03:49, 7 December 2015 (UTC)

But look at it this way, back then your 50 quid bought you the state of the art radio for the day right? I can't find the official prices, but according to this article "you can typically add £1,345 for navigation". That's including the MINIs that cost 11000 pounds. that's more than 1/10th the price of the car. Vespine (talk) 03:53, 7 December 2015 (UTC)

Yes - and it's the same deal. Many (probably most) people won't spend that much on a nav system. (Although, since a $100 cellphone or a $150 Garmin or TomTom offers the same functionality - it's something of a mystery to me why anyone would pay more than that for a built-in nav system these days). SteveBaker (talk) 15:33, 7 December 2015 (UTC)

But keep in mind how critical AM radios were to farmers. They needed to know ASAP when a storm was coming, to get their equipment and their families to safety, etc. And just waiting inside all day by the big radio wasn't an option. A built-in nav system just isn't all that critical. There's the portable nav devices, calling on your cell phone for directions, even the tried-and-true paper map. And the idea of buying technology for your car that may be obsolete in a year or two is also scary. I picture a lot of laughs aimed at people with 10 year old tech in their car or truck. StuRat (talk) 23:02, 7 December 2015 (UTC)

Much credit for invention of the automobile radio is given to Bill Lear who also is credited with development of the Lear jet. See Bill Lear#Radio engineer, although few dates are given. It may be possible to accurately determine some dates by examining the in-line citations. Dolphin (t) 05:29, 7 December 2015 (UTC)

A vacuum tube radio in a truck would work fine on bumpy roads, but if played for an extended time with the engine off would drain the battery, way faster than a transistor radio of comparable output power. Edison (talk) 17:18, 9 December 2015 (UTC)

Yes, but it also warms up the dashboard nicely. StuRat (talk) 17:11, 10 December 2015 (UTC)