Talk:Wavenumber

message: I've seen the quantitiy "circular wavenumber" referred to as "angular wavenumber".  "Angular" seems to be a bit more descriptive and follows the pattern of "angular frequency".  (No one says "circular frequency", but language is not always symmetrical or sensible.)  Also, Google finds more pages with "angular wavenumber" than "circular wavenumber".  Should we retitle the "circular wavenumber" section and mention both quantity names?  Zeroparallax 10:54, 17 March 2006 (UTC)
replies: text:  I think so, that sounds very reasonable. Fresheneesz 06:49, 8 May 2006 (UTC)
thread_title: Angular Wavenumber

message: This page need to specify the conversion factors between wavenumbers (cm-1) and Energy/angular frequency, preferably in terms of fundamental constants if that is possible. —Preceding unsigned comment added by 82.35.34.20 (talk • contribs) 
:Done. Han-Kwang 15:16, 4 May 2007 (UTC)
thread_title: Conversion

message: It would be easier to understand the concept if we use plain language appropriate to an encyclopedia. I suggest replacing:
Wavenumber in most physical sciences is a wave property inversely related to wavelength, having SI units of reciprocal meters (m−1). 
With:
Wavenumber in most physical sciences is a wave property inversely related to wavelength, having SI units of cycles per unit length or radians per unit length, where the unit length is stated and is typically meters, centimeters, nanometers etc. The dimensions are L-1.
Also:
"The energy corresponds to a wavenumber of 300 reciprocal centimeters (or inverse centimeters or per centimeter)"
would be more clearly: "The energy corresponds to a wavenumber of 300 cycles per centimeter" 
Any comments? GilesW (talk) 07:30, 22 April 2009 (UTC)
replies: text: Agreed!  To mere mortals (indeed, even to mortals with some physics background) it fails to give a feel for what the concept is, or to give the reader a way of envisaging it.  I'll attempt a little reworking.  The lead should attempt to explain to a novice the overall idea, so I'll add a sentence there to that effect.  Equally the lead should avoid being overloaded with technical detail, so I'll move some of its mathematical detail down into the article itself. Feline Hymnic (talk) 22:29, 12 October 2012 (UTC)
thread_title:  Obscurantism 

message: In the context of RF coils etc., please define "radial wavenumber". It is said to be "all important" [], (see second page between expressions (3) and (4)).
I cannot find a definition. GilesW (talk) 07:56, 8 May 2009 (UTC)
thread_title:  Radial wavenumber  

message: This edit:
http://en.wikipedia.org/w/index.php?title=Wavenumber&diff=prev&oldid=33599618
seems inconsistent with E=h\nu=\hbar\omega.  The equation, as it presently appears, is:
:k \equiv \frac{2\pi}{\lambda} = \frac{2\pi\nu}{v_p}=\frac{\omega}{v_p}=\frac{E}{\hbar c}, 
but should (I think) be:
:k \equiv \frac{2\pi}{\lambda} = \frac{2\pi\nu}{v_p}=\frac{\omega}{v_p}=\frac{E}{\hbar v_p}.
Comments?  —Preceding unsigned comment added by 70.234.243.222 (talk) 02:14, 11 April 2010 (UTC)
thread_title:  Energy vs frequency 

message: I was curious as to what the connection is between wave number and curvature.  Why would you have a reference to curvature on this page?  —Preceding unsigned comment added by 65.199.189.6 (talk) 15:02, 14 July 2010 (UTC)
replies: text: Other than because wavenumber and curvature both have dimensions of inverse length, I can't think of a relation. Miguel (talk) 08:31, 15 April 2013 (UTC)

text: Sounds a reasonable question.  The curvature article doesn't mention wavenumber, so there's no obvious link.  I'll remove the this "see also" item. Feline Hymnic (talk) 22:34, 12 October 2012 (UTC)
thread_title:  See also 

message: "For electromagnetic radiation in vacuum, wavenumber is proportional to frequency and to photon energy." Ok so what about if we're not in vacuum? Apparently when a photon goes from vacuum (index of refraction 1.0) to glass (index of refraction 1.5) the wavenumber would increase by the same amount (1.5 divided by 1.0). Does that imply its energy increased by the same amount? And when it leaves the glass would it lose that energy?
I can't find answers to these questions anywhere on the internet. Odd...
24.162.242.96 (talk) 13:32, 14 June 2013 (UTC)
thread_title:  wavenumber / energy equivalence in other medium besides vacuum? 

message: In the introduction it is currently written 
"1 cm−1 of energy is the amount of energy in a single photon with a wavelength of 1 cm"
This seems to imply a linear connection between wavenumber and energy, which is however inverse.
I would change it into:
"10 cm−1 of energy is the amount of energy in a single photon with a wavelength of 1/10 cm"
Thoughts?
Further I suggest to link a energy converter, like this one:
130.83.182.68 (talk) 14:41, 15 February 2017 (UTC)
thread_title:  Inverse dependence of the energy of the wavenumber 

message: As well as I know it, wavenumber was used in spectroscopy from the days before an accurate speed of light. Wavenumber can be directly measured with a diffraction grating, so it is more like temporal frequency can be determined from wavenumber, assuming a certain speed of light. Gah4 (talk) 20:16, 5 April 2018 (UTC)
replies: text: I was about to write this again, but I see that I already did.  Any thoughts since last year? Gah4 (talk) 03:47, 10 June 2019 (UTC)
thread_title:  ''In optical spectroscopy, it is often used as a unit of temporal frequency assuming a certain speed of light.'' 

message: I suggest removing the 'Definition' section entirely as there are two inconsistent definitions of wavenumber and the current section is mixed up between them. Instead let's just have the two sections "In wave equations" and "In spectroscopy" that each address their own internally consistent logic.
Wavenumber definition 1 (general wave equations):
* A measure of waves per unit length, i.e., the inverse of wavelength.
* Units can be anything that is inverse of length (1/m, 1/cm, 1/micron, 1/nm)
* Changes when a wave propagates into a different medium (red light has 1.6 μm^-1 wavenumber in air; red light has 2.1 μm^-1 wavenumber in water).
* Can be complex.
Wavenumber definition 2 (spectroscopy):
* A measure of energy quantum as scaled by hc (e.g., "The spacing between the two orbitals is 50 cm^-1.").
* A measure of frequency as scaled by c (e.g., "The laser has bandwidth of 10 cm^-1").
* Units are typically inverse centimeter (cm^-1), also called "wavenumbers" in speech (e.g., "The energy levels are spaced by 50 wavenumbers.").
* Does not change when a light wave propagates into a different medium (an atom emitting red light emits light with wavenumber ~16000 cm^-1 regardless of whether it's in diamond or vacuum)
* In other words, it only equals the inverse of wavelength for the special case of light waves in vacuum.
* Seems to be always real-valued.
Thoughts? --Nanite (talk) 22:06, 5 April 2018 (UTC)
replies: text: No comment on removing or not the definition section.  The conversion is hc for spectroscopy wavenumber and {\hbar}c for radians/length.  I am not sure how spectroscopists consider the index of refraction. I will guess that they correct for it. Monochromators are usual in air, and not other media.  Physics uses k in wave equations, and that could be in any media. Gah4 (talk) 03:47, 6 April 2018 (UTC)
replies: text:  Ah thanks, you are right on the factor of c being inverted -- I've corrected my post. --Nanite (talk) 19:29, 6 April 2018 (UTC)

text: This is one of those "you know it when you see it, but otherwise it is hard to explain" cases. I believe that spectroscopists use it in place of frequency, from the early days before there were good measurements of c. From a diffraction grating, you measure wavelength but not frequency. Many equations in spectroscopy work better with a quantity that is inverse wavelength, especially in diffraction. Otherwise in QM the quantity that you want is the one that goes with \omega, and so should not change with index of refraction. As you note, it is energy with hc factored out. Gah4 (talk) 19:59, 6 April 2018 (UTC)

text: In physics, wave vector is commonly used, especially in the form e^{i({\mathbf{k\cdot x}}-\omega t)}. First, this \mathbf k is 2\pi times the form used by spectroscopists, but also is a vector. However, the non-vector k might be used for the magnitude of the vector. In this form, it is used for the solution of wave equations, where it describes the spatial part of propagating waves. Also, in absorbing materials it is complex.  The imaginary component, along with the i in the exponent, forms a decreasing exponential. My first thought is that this is not called a wave number, maybe magnitude of wave vector, but I am not yet convinced of that. Then the phase velocity of a wave is \omega/k and the group velocity d\omega/dk, again these can be complex. Gah4 (talk) 19:59, 6 April 2018 (UTC)
thread_title:  Remove Definition section. 

message: As far as I know, the historical reason for spectroscopy to use wavenumber instead of frequency is that measurements were already more accurate than the known speed of light. In many places, such as acoustics, it is easier to describe waves in terms of frequency, but that wasn't the case here.  Only later when the connection through quantum mechanics to photon energy came about, did this distinction become important.  Gah4 (talk) 13:54, 31 October 2018 (UTC)
thread_title:  historical 

message: An editor has asked for a discussion to address the redirect Orders of magnitude (wavenumber). Please participate in the redirect discussion if you wish to do so.  Utopes (talk / cont) 22:54, 7 April 2020 (UTC)
thread_title:  "Orders of magnitude (wavenumber)" listed at [[Wikipedia:Redirects for discussion|Redirects for discussion]]