Talk:Snell's law

message: Consistency problem: 
In the top explanatory figure n2>n1 is explicitly stated.
In section "Total internal reflection and critical angle", however, the following must hold true (see also the corresponding figure): n1>n2. As in the text there is no indication of this inversion, this leads to confusion. We should either make the relationship consistent throughout, or use different names for the refractive indices in the different examples.
145.64.134.221 (talk) 11:53, 1 October 2009 (UTC)
thread_title: Old thread

message: The image shows that the normal is the boundary between the media, rather than being horizontal to the boundary. Also, \theta_1 and \theta_2 are wrongly labelled as a result. The image disagrees with the text. See ScienceWorld  for how the image should be corrected.
The image is misleading and confusing, and should be rectified as soon as possible; however, as I lack experience with graphics, I request someone to upload a corrected version as soon as possible.
*Um, no. The image is correct, and correctly labelled. The image in the article has the two media on the left and right sides of the diagram; the interface runs vertically. The image at Scienceworld has the two media at the top and bottom of their diagram; their interface runs horizontally. Try rotating one of the diagrams by 90°, and you'll see how they match. -- Bob Mellish 18:25, 26 September 2005 (UTC)
thread_title:  Image wrong? 

message: I agree it is Snel and not Snell's law. Also in English. The 'discoverer' (not getting in the historical issue here, just the spelling bit) of the quantitative law of refraction was 'Willebrord Snel van Royen', thus one l, and there is no fluidity to this spelling as far as I know. The two-l-spelling has nothing to do with 'Snel' spelled differently in the English language but with incorrect de-latinazation of Snellius.
:FWIW, in the Netherlands, up to the present, the person has always been known as Snellius, not Snel.  I think, most Dutchmen who have heard about him would be surprised to learn that he wasn't born as Snellius.  Iterator12n  Talk  05:29, 11 July 2007 (UTC)
replies: text: Also FWIW, the Dutch encyclopedia Winkler Prins, under "Snellius", shows "Snell van Royen" as the original family name - with two l's.  Finally, with Snellius goes "Willebrordus", not "Willebrord."  This is all you're going to get from me on the name of Snellius!  Iterator12n  Talk  20:07, 14 July 2007 (UTC)
thread_title: Snell's law spelling 

message: I've seen this formula used quite a lot in some establishments as an expansion of the original law for wavelength...
\frac {\lambda_0}{\lambda_1} = \frac{v_o}{v_m} = \frac{c / n_1}{c / n_2} = \frac{n_2}{n_1} to...
\lambda_1 sin\theta_1 = \lambda_2 sin\theta_2
James S 00:12, 11 December 2006 (UTC)
replies: text: Those ratios of velocities, wavelengths, indices of refraction, etc. are fine, but they are not Snell's law.  Snell's law needs to have the sines of the angles in it.  We seem to have forgotten to state the law near the top of the article anywhere.  I'm working on it... Dicklyon 20:04, 22 December 2006 (UTC)

text: OK, I fixed the lead, putting the law equation and illustration into it; and I added a book page of history.  I don't really understand the point of what someone was trying to do with these relations in the Explanation section, so I'll leave it for now.  But as I said, without the angles, or some measurement proportional to their sines, it's not Snell's law. Dicklyon 20:40, 22 December 2006 (UTC)
thread_title:  Other formulae 

message: I propose to merge Angle of refraction into Snell's law, since it covers exactly the same material.  Please support, oppose, or otherwise comment here. Dicklyon 08:36, 23 December 2006 (UTC)
replies: text: support The Photon 03:46, 24 December 2006 (UTC)

text: Expecting no objection, I went ahead and incorporated a few bits from there that we didn't have here, and converted it to a redirect. Dicklyon 05:30, 24 December 2006 (UTC)
thread_title: Merge from Angle of refraction?

message: Here's an 1803 book that explains that Snel did the same thing that Ibn Sahl had done.  Nowhere does the velocity of propagation or the index of refraction enter into his observation that the ratio of sines is a constant for a given pair of media.  Later, when it was realized that light speed varies in different media, it was realized that the law of sines was in agreement with a principle of least time, or Fermat's principle; that's where velocity and index started to come into the equation, via their ratio.  Let's not get the cart before the horse on this. Dicklyon 01:42, 5 January 2007 (UTC)
replies: text: This 1803 book nowhere mentions Ibn Sahl. It does discuss though, p. 295, that Snellius conducted "a series of numerous and delicate experiments.". Roshdi Rashed in his discussion of Ibn Sahl's work nowhere mentions any experiment conducted - he simply relabels the sin/sin-ratio. --Gerard1453 (talk) 16:51, 24 October 2017 (UTC)
thread_title: The original form of the law

message: Snell's law is not just limited to propagation of light but can also be used to explain the propagation of sound waves across different medium where the speed of sound changes. The article fails to mention this. 
I think this law can be used for explaining other kind of wave propagation too, though I am not sure. But at least we sound waves should be mentioned in the introduction of the article. -- Myth (Talk) 05:50, 16 April 2007 (UTC)
replies: text: It says "light or other waves, passing through a boundary between two different isotropic media".  Is that not sufficient to encompass sound? Dicklyon 06:05, 16 April 2007 (UTC)
replies: text:  I agree, but the article focuses heavily on propagation of light waves and the importance is not evident in relation to other kinds of waves. A reader not aware of the applicability of the law in other cases, won't realize it easily. -- Myth (Talk) 06:21, 16 April 2007 (UTC)
replies: text:  That's because light is the usual application.  Sound diffracts so much that Snell's law is seldom a useful approximation to sound wave behavior.  If you have other applications, or relevant sources, please do bring them up. Dicklyon 14:58, 16 April 2007 (UTC)
replies: text:  Snell's law is very important in the study of underwater sound.  See, for example:  Robert J. Urick, "Principles of Underwater Sound (2nd edition)."  New York:  McGraw-Hill, 1975, p.116.  Ephesians 2:10 (talk) 23:10, 8 September 2009 (UTC)
thread_title:  Applicability to sound waves 

message: See WP:BRD.  If you make a change (e.g. adding "by scientists") and someone reverts it, bring it up on the talk page if you care; don't just make it again. Dicklyon 05:26, 1 August 2007 (UTC)
thread_title: How to reach consensus

message: The image on the right (:Image:Snells law.svg) is quite nice, but I think in the literature the interface between media is usually horizontal (like between air and water). I have rotated the image, see :Image:Snells law2.svg (also on the lower right). I propose to replace the original with this rotated version. Comments? Oleg Alexandrov (talk) 00:58, 25 December 2007 (UTC)
replies: text: In optics, rays are most often traced from left to right.  But I agree it looks good going top to bottom. Dicklyon (talk) 01:59, 25 December 2007 (UTC)
replies: text:  Done. Rotated, the image shows the details better while taking less real estate. Oleg Alexandrov (talk) 04:54, 1 January 2008 (UTC)
thread_title:  Proposal to rotate the image 

message: I removed a section from the article, for the following reasons:
# The informational content of the section is minimal, it repeats what is already stated both in the article and the diagram
# Then name of the section is misleading, that section is not about calculating refractive indeces
# The analogy with the car going from highway to the mud causing it to change angle is I think misleading, it is not as if a ray is a pair of parallel waves and one of them reaches the seconc medium first and forces the second wave to turn. Besides, is mud denser than asphalt?
Comments? Oleg Alexandrov (talk) 04:34, 10 January 2008 (UTC)
thread_title:  Explanation for section removal 

message: I'm confused about the formula. If the angle of incidence is 0, then how does the formula work? Or does it not work for maxima and minima of the sine wave? STYROFOAM☭1994TALK 22:58, 29 January 2008 (UTC)
replies: text: The equation n_1\sin\theta_1 = n_2\sin\theta_2 is satisfied with θ1 = 0 and θ2 = 0 for any indices of refraction.  That is, if the ray comes in perpendicular to the surface, it stays that way. Dicklyon (talk) 00:09, 30 January 2008 (UTC)
thread_title:  Formula confusion 

message: Um, Snell's law has been disproven/broken/shattered for the better part of a year now... when is this article going to be updated to reflect that? Light can be bent at left angles using metamaterials, and the light traveling "backwards" in a vacuum exceeds the "normal" limit of light speed (the only thing (other than tachyons)) that travels faster than light is still light... just light traveling backwards). 68.185.167.117 (talk) 15:02, 26 November 2008 (UTC)
replies: text: That work has not bothered Snell's law at all, but if you'd like to add a bit about it, citing a good source, that might be useful. Dicklyon (talk) 16:44, 4 December 2008 (UTC)
thread_title:  Metamaterials? 

message: I have removed a sentence from the main text in which it was claimed that Al Haythem had known the sine law of refraction and which gave A.I. Sabra's Theories of Light From Descartes to Newton as source for this claim. This is not true Al Haythem did not have knowledge of the sine law of refraction and this is clear from the following passage from Sabra(page97): Now let us suppose that Ibn al-Haythem moved one step further and assumed the increase... (there follow a series of mathematical deductions)... In other words the sines of the angles of incidence and refraction are in a constant ratio, which is the geometrical statement of the law of refraction .(...) He did not, however, take that step... Sabra then goes on to argue that this might have been the route taken by Descarte who knew Al Haythem's work in discorvering the law of refraction. What we have here is a hypothetical argument concerning the possible route of discovery taken by Descarte and not the claim that Al Haythem had discovered the law himself, which he had not.Thony C. (talk) 13:12, 4 December 2008 (UTC)
thread_title: Al Haythem, A.I.Sabra and the Sine Law of Refraction

message: This was from the Faster-Than-Light article: This is influenced by man-made metamaterials, which allow light to be bent backwards; the discovery of these shattered the now defunct Snell's Law, an old "law of physics".
Shouldn't something to that effect be added to this article, then?  SineSwiper (talk) 01:52, 18 December 2008 (UTC)
replies: text: Negative-index metamaterials still obey Snell's law, just with a negative index of refraction. — Steven G. Johnson (talk) 04:41, 5 August 2009 (UTC)
thread_title: Snell's Law Obsolete?

message: I think the formula for finding the actual speed of light (n=c/v) should be added into the section. This is the only appropriate place on Wikipedia to put it on. However, I cannot find a reliable source to cite. If someone can find this, I think it will help a great many people. Fireedud (talk) 18:14, 3 January 2009 (UTC)
thread_title:  Finding the speed of light 

message: The image, in the article at "Explanation#Derivations", is incorrect in the lower part, below the interface.  It shows the wavefronts as becoming hyperbolic, so that the portions at large distances from the central axis asymptotically become straight lines.  However, the refracted rays below the interface would then not appear to diverge from a point, and that is not the case in reality.  The point source position is shifted (upwards, in the figure), but the light source seen from below the interface still appears as a point.  These means the wavefronts must continue to diverge from a point, and thus remain segments of a sphere, with only the center of the sphere being shifted.  The figure needs to be corrected by someone with a facility in graphics animation.  A few other editors should verify and confirm my conclusion, but it is really quite obvious, and easily apparent visually so that it needs to be fixed quickly.  Thanks, Wwheaton (talk) 15:53, 23 September 2009 (UTC)
replies: text: I believe it is correct.  The refracted rays will NOT appear to come from a point, as is well known; there will be spherical and chromatic aberrations, as is the case when putting a prism behind a lens that's designed to image through air.  The rays far from the source will approach the critical angle. Dicklyon (talk) 16:02, 23 September 2009 (UTC)

text: I'm going to have to disagree with you as well. According to Snell's Law, \theta_2 = \arcsin\left(\frac{n_1}{n_2}\sin\theta_1\right). Note that n1 (air) = 1 and n2 (water) = 1.33 (and thus n2/n1 = 0.75), and that \sin\theta_1 = \sin\left(\arctan\frac{x}{h}\right), where x is the horizontal distance from the central axis and h is the height of the source over the water. For my purposes, I'll just say h=2 meters (the height and the units are arbitrary and unimportant). So we get that \theta_2 = \arcsin\left(0.75\sin\left(\arctan\frac{x}{2}\right)\right). Due to some lovely trigonometry, \theta_2 is not just the angle of refraction from vertical; it's also the angle of the underwater wave (which is perpendicular to the angles of refraction) from horizontal (i.e. the angle of inclination of the wave). So, we can plot \theta_2, as defined above, in WolframAlpha. You'll see that \theta_2 changes close to zero (the central axis), but that it quickly approaches an asymptote on both the positive an negative ends. (Near-)constant \theta_2 (angle of inclination) far from the central axis means (near-)straight line. That's what the image depicts, so I think the image is correct. Tell me if you think I made an error in my calculations and explanation; they were hastily done. -- tariqabjotu 20:38, 23 September 2009 (UTC)
replies: text: Your argument seems plausible to me at the moment, though I need to verify it when I have more time.  The fact that objects under water appear sharp and unblurred led me to believe that the refraction of a plane surface does not affect the spherical character of the waveform of light from a point source, but I see now that the small-angles approximation may indeed account for this, and the existence of critical refraction supports your argument.  Thanks for (probably) correcting me, and apologies for the false alarm.  Wwheaton (talk) 14:25, 25 September 2009 (UTC)
replies: text: If you look at objects underwater, or embedded in glass or plastic, from an angle, they actually appear with significant blur and color fringing.  See my patent on how such aberrations can be approximately corrected, in the case of an optical system designed to focus in air being used through glass. Dicklyon (talk) 03:46, 26 September 2009 (UTC)
thread_title:  "Today's Featured Picture" for 9/23/2009 incorrect !! 

message: About 20 years ago, I remember being told in college that Snell's Law is not a law because
# It can be derived (which has since made me wonder about the nature of laws)
# It is an approximation
If there's any truth to this, I think it would be good to include. (If there's wide misconception about this, I think it'd be even better to attend to it) Googling for '"snell's law" "not a law"' has been inconclusive. AngusCA (talk) 23:35, 13 June 2010 (UTC)
replies: text: Without a source, there's not much you can do.  Lots of things are called laws; I wonder where those criteria come from. Dicklyon (talk) 05:25, 15 June 2010 (UTC)
replies: text: I can't. I haven't seen that teacher in 20 years. I don't even remember his name, which is fortunate, because he was a jackass. But he seemed to know what he was talking about. AngusCA (talk) 02:39, 16 June 2010 (UTC)
thread_title:  Is it really a law? 

message: Did anybody get a chance to check Wolf's paper to see what it says about history of mathematics? Tkuvho (talk) 11:08, 3 June 2011 (UTC)
thread_title:  Wolf's paper 

message: The second picture in the article(the one that shows the incident, refracted and reflected wave) has an inaccuracy in the Refracted Wave.  The inaccuracy occurs where the angle of reflection is labeled.  I have already made an image that I believe contains more relevant information to the less informed and does not contain the inaccuracy in the only two angles that are relevant to Snell's Law and will be uploading it as the successor shortly.  — Preceding unsigned comment added by Chapin J (talk • contribs) 21:07, 26 September 2013 (UTC)
thread_title:  Second Picture is Incorrect 

message: Every few months, we get an uncommented edit or two to change a sign or two in the "Vector form" section.  I tried to check the source but the relevant pages aren't showing up in gbs.  The ref came in in this uncommented edit, which didn't change the equations, so I don't really know if the equations came from there, or were consistent with the ref at that time, or what.  Someone should check, and put a comment in the wikisource about staying true to the ref.  Or work it out and make sure it's right.  Part of the problem is that the direction of the surface normal is not defined, so people may be trying it different ways. An appropriate figure would help.  Dicklyon (talk) 18:33, 26 January 2014 (UTC)
thread_title:  Vector form 

message: Most Wikipedia pages reference the use of their subject in popular media, this one is referenced in the game Simpsons Tapped Out, specifically when the new Lard Lad statue is unveiled. It should at least get a small mention. 47.32.35.106 (talk) 06:31, 1 October 2016 (UTC)
thread_title:  Popular media  

message: I don´t quite see how Huygens derivation acts. Why choosing this propation direction and not others in each wavelet? According to Doctor Lincoln from Fermilab Why does light bend when it enters glass?, this is not a good explanation. Fermat's approach seems more the description of what happens rather than why. --Xareu bs (talk) 11:49, 30 August 2019 (UTC)
thread_title:  Huygens derivation