Talk:Wavelength

message: The two references to the topic of general waveforms so far do not actually describe how these calculations are done, but provide only few words of description. 
The stress upon a periodicity in time in the article in preference to the propagation of a waveform seems to me misguided. For example, if one makes the analogy with a performer blowing large soap bubbles in a park, the bubbles are launched as huge spheres, and as they are carried in the wind they enlarge and become ellipsoids. At a location near the launch one sees a periodic appearance of spheroids at the period of launch T. At a remote position one sees a periodic appearance of enlarged ellipsoids with a period T. Just how long the period T is between arrivals, or between repetitions of what happens periodically in time a a fixed location, as described by a Fourier series in time, is not so interesting as the process of transformation as the spheroids change to enlarged ellipsoids, that is, the propagation phenomenon. Changing the period and spacing the bubbles differently is not really essential.
So I think what is needed is a more interesting discussion with some more detailed references tying what happens to the dispersive nature of the medium. Emphasis upon the more-or-less incidental period between events is not the really interesting point. Brews ohare (talk) 06:11, 2 May 2012 (UTC)
replies: text: I agree the section remains rather unsatisfying.  I did my best to find a sensible way to incorporate the Fourier series into something to do with wavelength, cobbling what was there; but it's still a bit of a misfit.  Most sources that talk about dispersion and Fourier analysis don't do in the context of periodic waves, and usually do it in terms of wavenumber, not wavelength.  And their analysis doesn't usually conclude anything related to wavelength or to repetition in space.  Probably we should just simplify the section, since there are better articles for covering these other concepts of waves in linear dispersive media.  As for the concept of wavelength being applied to other than approximately sinusoidal waves, it's unusual at best; discussing it can easily be misleading, or spiral into contradictions, like when you get into claims that it's well-defined for arbitrary periodic functions.   Dicklyon (talk) 05:58, 3 May 2012 (UTC)
replies: text: Hi Dick: Quite possibly the easiest approach is to use wavevector, or maybe to use a simple example instead of trying the general case. As you know, however, I do not agree in the slightest that application of wavelength to a periodic wave in space of general form is in any way misleading, although the occurrence of such waves in nature is not general, but restricted to particular media. From a conceptual point of view, wavelength is what Fourier series is about in space, with the simple interpretation of the general argument ξ as x instead of angle, or time. From this stance, as I have pointed out by direct quotations from at least three sources, many authors do exactly that. Brews ohare (talk) 21:09, 3 May 2012 (UTC)
replies: text: Not that many authors do that.  And none of them seem to reveal any reason for doing a sinusoidal decomposition of the spatial pattern.  At least in the case of the dispersive linear system there's a reason to decompose into sinusoids.  Dicklyon (talk) 03:52, 4 May 2012 (UTC)
replies: text: Hi Dick: Perhaps it is just argumentative, but here's a question: why do authors use Fourier series when argument ξ is interpreted as time, or angle, or whatever? Why is it automatically a wasted effort only when ξ is interpreted as a spatial variable? Why is period T more significant as a time period than λ is a a spatial period? Could it be that in fact there is no difference at all? Brews ohare (talk) 20:40, 5 May 2012 (UTC)
replies: text: It's not unusual to use wavenumber (or reciprocal wavelength) in a Fourier transform, as a way to get a sinusoidal decomposition, especially for wave packets.  But periodic-in-space waves are a relatively rare corner, seldom encountered where a sinusoidal decomposition would be helpful.  If they're also periodic in time at the same time, in a linear system, the system is nondispersive, and has a trivial wave equation, for which a sinusoidal decomposition is not needed; it adds nothing to the understanding of the system.  If they're in a nonlinear system, the sinusoidal decomposition is not particularly illuminating either.  Dicklyon (talk) 22:27, 5 May 2012 (UTC)

text: Here's another question: when an oboe plays a note, it sounds different than when a violin plays the same note. Could it be that the difference can be expressed as a difference in the Fourier series expressing the wavelengths of vibration supported by the general waveform in the oboe's air column for that note compared to the wavelengths present in the general waveform on the violin string when the same note is present? Would that be an interesting enough example of Fourier expansion of the general waveform in space to warrant mentioning the use of Fourier series for spatial analysis of waveforms? Or, perhaps, an article Wavelength (music) is needed? Brews ohare (talk) 20:52, 5 May 2012 (UTC)
replies: text: The sound difference has much more to do with the waveform in air; this is what propagates and carries the pattern.  The physics within the instrument gives rise to different modes, and to a temporal near-periodicity from how the signal interacts with the reed or the bow, but I haven't seen an analysis like you're describing, where the composite signal is analyzed into space-domain sinusoids.  Of course it could be done.  Dicklyon (talk) 22:27, 5 May 2012 (UTC)

text: Once it's left the instrument the waveform is not uniform, i.e. there is no 'general waveform', even allowing for distance attenuation. The sound you hear an inch from a source is very different from the sound you hear three feet or thirty feet from it: the best example is the human voice as we're most familiar with it: often you can tell how far away someone is quite accurately by the sound of their voice. This is less noticeable with an instrument because its pure note dominates and so it depends on the particular frequency of the instrument. And of course in most cases performers don't want you to hear different sounds depending on how far away you are, and will go to great lengths to minimise effects of distance (modifying the building to compensate for example).--JohnBlackburnewordsdeeds 22:57, 5 May 2012 (UTC)

text: To pursue this matter further, harmonics are produced on the guitar by deliberately introducing a node on the guitar string, a direct application of wavelength considerations. Brews ohare (talk) 21:05, 5 May 2012 (UTC)
replies: text: That doesn't produce harmonics, but kills the fundamental and certain other harmonics.  Yes, it is described in terms of wavelength on the string, or the modes of the instrument.  But not in terms of a sinusoidal decomposition of a periodic-in-space pattern.  Dicklyon (talk) 22:27, 5 May 2012 (UTC)

text: Dick & Blackburne: You might find some interest in reading a source or two on this subject instead of relying on your recollections. Something interesting can be done here. Brews ohare (talk) 20:51, 10 May 2012 (UTC)
replies: text: I have plenty of good books on the physics and psychophysics of music and musical instruments; it's not clear what you see as relevant in the page you've linked in that college physics text.  Dicklyon (talk) 21:24, 10 May 2012 (UTC)
replies: text: Dick: Figures 14.26 and 14.27 of this source compare waveforms for a pure note on a tuning fork with the same note on a clarinet and a flute. The point, of course, is that the characteristic voice of the instrument is expressed in its peculiar waveform, which is in each case periodic with the same wavelength but of different shape. Accordingly, the differences between voices is sought in the different harmonics of the fundamental found in each. This difference can be expressed in time or in space, although the latter requires some expression of the characteristics of the medium, which cannot be unduly dispersive. Obviously, instruments usually operate in air, and the slight dispersion of sound in air is no impediment to applying a spatial analysis. The design of an instrument is perhaps even more clearly related to wavelength, as the dimensions of the instrument determine how an excitation of a particular spatial mode will be related to its various harmonics; for example, how the standing wave on a violin string is connected to the various resonances of the instrument. Brews ohare (talk) 13:04, 15 May 2012 (UTC)
thread_title:  More general waveforms references 

message: The sentence "For example, in sinusoidal waves over deep water a particle in the water moves in a circle of the same diameter as the wave height, unrelated to wavelength" should be edited to mention that this is true for a particle on the surface, particles below the surface moving in smaller circles.  — Preceding unsigned comment added by 64.134.138.137 (talk) 17:30, 1 March 2013 (UTC)
replies: text: Fixed.--Srleffler (talk) 00:14, 2 March 2013 (UTC)
thread_title:  Introduction edit 

message: Why on earth is there a citation to "Paul A. LaViolette (2003). Subquantum Kinetics: A Systems Approach to Physics and Cosmology" (citation for "the notion of a wavelength also may be applied to these wave packets"). That book is utterly pseudoscientific (and the author is known for some way out fringe ideas), and does not belong in a science article in my opinion. Aren't there better references to use for wave packets? Rolf Schmidt (talk) 23:42, 1 January 2015 (UTC)
replies: text: Fixed. Thanks for pointing that out.--Srleffler (talk) 05:01, 2 January 2015 (UTC)
thread_title:  Citation to "Subquantum Kinetics" pseudoscience 

message: When was lambda first used for wavelength? I noticed Fresnel used λ in 1819 in 'Memoire on the diffraction of light'  and Herschel used λ = v T in 1828 in 'On the Theory of Light'.  Ceinturion (talk) 09:49, 23 October 2015 (UTC)
thread_title:  First use of lambda 

message: Presently the wikilink Vacuum wavelength goes to this article, but it is not clarified. I would suggest adding something like the following blurb.
(More or less?)
--Nanite (talk) 16:40, 21 July 2016 (UTC)
replies: text: Seems like a good idea.--Srleffler (talk) 03:26, 22 July 2016 (UTC)
thread_title:  Vacuum wavelength 

message: The first sentence of the lede implies that wavelength is a characteristic of a sine wave only. Later in the lede it says: oh by the way, it's a characteristic of any periodic wave. The lede should be written so that it is in compliance with MOS:FIRST (...If its subject is definable, then the first sentence should give a concise definition). The first sentence should be something like, "In physics, a wavelength is the distance between any two successive parts of a periodic wave that are in phase, i.e., that are at idential points of its cycle."  Sparkie82 (t•c) 06:12, 23 March 2018 (UTC)
replies: text: That's better , thank you. Sparkie82 (t•c) 15:54, 7 April 2018 (UTC)
thread_title:  Lede doesn't provide a concise definition