# Talk:Total internal reflection

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## Untitled

Nice! jimfbleak 13:32 22 May 2003 (UTC)
Thanks :-) Theresa knott 13:39 22 May 2003 (UTC)

## Possible problem?

Quote from current article:

The critical angle ${\displaystyle \theta _{c}}$ is given by:

${\displaystyle \theta _{c}=\arcsin \left({\frac {n_{2}}{n_{1}}}\right),}$

where ${\displaystyle n_{2}}$ is the refractive index of the less dense medium, and ${\displaystyle n_{1}}$ is the refractive index of the denser medium.

I think that ${\displaystyle n_{2}}$ and ${\displaystyle n_{1}}$ have been labelled incorrectly here. Surely ${\displaystyle n_{1}}$ should be the refractive index of the initial medium, and ${\displaystyle n_{2}}$ the angle of the medium which is being entered? Otherwise, how could this situation occur (also quoting from article):

If the fraction: ${\displaystyle {\frac {n_{2}}{n_{1}}}}$ is greater than 1, then arcsine is not defined--meaning that total internal reflection does not occur even at very shallow or grazing incident angles.

So the critical angle is only defined for ${\displaystyle {\frac {n_{2}}{n_{1}}}\leq 1}$.

That is to say, how could a situation arise in which ${\displaystyle n_{2}}$ is greater than ${\displaystyle n_{1}}$, if we define ${\displaystyle n_{1}}$ to be the denser substance (i.e. the one with the larger refractive index)?

Also consider that the critical angle at an air-Lucite boundary should be given by sin^-1(1÷1.50), whereas the critical angle at a Lucite-air boundary should be given by sin^-1(1.50÷1) (that is to say, undefined - TIR can't occur on the air side of the boundary).

Birchlabs (talk) 18:52, 29 May 2009 (UTC)

## Applications

total internal reflection is very important in the total internal reflection fluorescence microscope (funnily enough!). Would be nice to put a link in.

## Small mistake here in the diagram...

Dear Theresa, nice stuff, I used it as a base to create the french version of it, but I noticed that the diagram is slightly off - when you have TIR it means that you go from a less refringent medium to a more refringent one, that is n2 > n1, thus the ray in medium 2 should be farther away from the normal, not closer! If it was not the case how the refracted angle could ever exceed 90 degree :-) Thanks!

Your right! thanks, I'll change it.Theresa knott 07:50, 20 Sep 2003 (UTC)

A couple of points.

1. Why have you created a french version here on the engilsh wiki. Since neither diagram contains words why do you need a french version anyway.
2. Please sign you comments so we don't have to check the page history. If you just type ~~~~ the softwhere will do it for you, Saves typing.
3. Keep you fingers away from the minor edit check box. This box is tell people they don't need to bother looking at what you have done e.g. you've corrected a spelling mistake or fixed a broken link. You certainly do want people to read comments on talk pages. Theresa knott 07:50, 20 Sep 2003 (UTC)

Sorry I'm new to it :-) I did not understand that the image were split between the different langages, and I'll delete the one I did on the english site - feel free to use it of course! (thanks for the signature trick :-) Aveclafaux 06:44, 23 Sep 2003 (UTC)

## Alternative diagram

Hands up who thinks this diagram is nicer! :) Oliver P. 22:34, 25 Sep 2003 (UTC)

It's not a question of which is nicer. The one that Aveclafaux drew is accurate whilst my one contains a mistake! So i'm going to change. I'll delete my version in about a week, unless anyone objects. Theresa knott
Ah, good point! I'll explain that on Wikipedia:Votes for deletion. -- Oliver P. 21:24, 26 Sep 2003 (UTC)

## Dispersion

Perhaps a brief description of dispersion would be appropriate.

## FTIR in relation to fingerprint scanners

I arrived on this page after a redirect on 'Frustrated Total Internal Reflection' because I was looking into how FTIR is used in fingerprint and other touch sensing application. After reading this article I am none the wiser on how they do it so I'm off to search of a more accesible article - perhaps the workings of the fingerprint scanners could be added to the FTIR section as an additional example in the future. -- 17 February 2006‎ Doctus

Fingerprints on a glass of water made visible by frustrated total internal reflection

I had added this picture of a glass of water to the article to illustrate frustrated total internal reflection, but it was later removed with no reason given. Would it be useful to have it in the article? Olli Niemitalo (talk) 22:07, 9 January 2012 (UTC)

No opinions voiced plus I found a reference to cite so putting it back. Olli Niemitalo (talk) 11:27, 9 February 2012 (UTC)

## Merger proposal

I have proposed merging in critical angle because that page has no relevance outside of a description of total internal reflection. -- The Photon 04:34, 31 May 2006 (UTC)

Yes, do it. Both subjects will benefit from being treated together. Just make sure critical angle appears in bold as close to the top of the article as possible, to prevent confusion for readers coming in via a link to that term.--Srleffler 05:35, 31 May 2006 (UTC)

Musing about a question. Are hot-roadbed mirages an example of total internal reflection? I've seen many examples of mirage explanations, but I've never seen any mention of internal reflection. It seems to me that if the light is bent as it enters the warm air layer, it should only refract. On the other hand, if internal reflection is an issue, then whenever light strikes the hot air layer at more than the Critical angle, it should reflect 100%. Better question: when light strikes a hot air layer, does it split into a refracted beam and a reflected beam? And if the angle is varied, does the refracted beam vanish when the angle exceeds the Critical angle? If so, then all the explanations of mirages I've encountered are extremely misleading if not outright incorrect, since they totally miss the possibility that mirages are an example of TIR. --Wjbeaty 07:06, 30 October 2006 (UTC)

Consider a so-called "inferior mirage", where the air is hot near the ground, like in the desert or a hot, shimmering road. Model this by assuming that the refractive index is a smooth function of the height above the ground, i.e. take n=n(z) where z is height above the ground. To embody our assumptions, assume n(z)= a for z<z_1, n(z)=b for z>z_2, where 1<a<b, z_1<z_2. The assumption that a is less than b corresponds to having hot air below. Also, to fix ideas, assume that n(z) increases smoothly and monotonically from a to b, i.e. dn/dz>0 for a≤z≤b. So n(z) looks roughly like a step function from a to b.
Then by solving using Leibniz least time principle (or is that Euler? Lagrange?) a light ray z=z(x) (where x is distance along the road) will have d^2n/dz^2(x)>0 (concave up) in the region where dn/dz>0, i.e. in the slab between z=a and z=b. If the angle of incidence is close enough to horizontal, this will cause it to turn around and go upwards due to refraction.
If the transition region z_1<z<z_2 is sharp enough, this looks a lot like internal reflection, and in the limit, it will become internal reflection. The critical angle and all phase shifts can be computed by doing this limit process carefully (I assert).
So there is a continuum of possibilities connecting refraction and reflection. See Mirage#Heat_haze for more clues. 84.227.237.33 (talk) 15:08, 3 April 2014 (UTC)
Another reference: Refractive_index#Inhomogeneity. 84.227.237.33 (talk) 20:29, 3 April 2014 (UTC)

## TIR not limited to optics

This article focuses on just the optical spectrum. It should be noted somewhere that TIR applies to electromagnetic waves outside of the optical spectrum, for example radio transmissions experiencing TIR in the ionosphere. ChrisSerrano 14:36, 13 April 2007 (UTC)

also this phenomena is not limited to light. It also occurs for sound and water waves, and is a general wave phenomena. This should be noted.Brokencalculator (talk) 02:35, 13 March 2008 (UTC)

## Turtle picture?

Why is there a picture of a turtle to the right? What does it have to do with the article? 165.124.138.210 (talk) 23:46, 22 April 2008 (UTC)

i disagree. It is clearly showing an example of internal reflection. If you think otherwise then submit a proposal for a picture change Metroid476 (talk) 00:17, 11 January 2009 (UTC)

I also think the mirror image of the turtle on the water surface is a beautiful example of total internal reflection. Eranus (talk) 13:27, 2 June 2009 (UTC)

## Phase shift on total internal reflection

I added a section on the phase shift of the reflected light under total internal reflection. I would appreciate if someone would better word it. I think it is a very basic subject known for centuries that few people (even graduate optics students) know. I will add later a mathematical description of the phase shift, this is a bit problematic because the actual value depends on definiton. Born and Wolf gives a good treatment of the subject. There is also a good OPN on the subject. I will referance them both at a later stage.Eranus (talk) 13:27, 2 June 2009 (UTC)

## Applications

Where did you find that CSM is the largest user of total internal reflection, and that the government has spent a bunch of money to make sure the campus is invisible from space? I'm pretty sure that's a bunch of BS. —Preceding unsigned comment added by 128.111.119.177 (talk) 18:41, 25 August 2010 (UTC)

## Absorption in the low-index material?

I was recently told that, to a small extent, the optical properties (aside from n) of the low-index material matter. In particular, if the low-index material absorbs the wavelength of interest, you can't expect 100% reflection. The context of the question was "cladding" an unclad glass fiber with epoxy. I was told that this won't work well because a little energy will transfer from the evanescent wave into the epoxy. This seems consistent with the mechanism of total internal reflection microscopy, in which the evanescent wave excites a fluorophore, and with frustrated TIR, in which this article describes photons as tunneling to another high-index material. If the epoxy absorbed by scattering or by fluorescent absorption, it seems correct that a little absorption could happen and that reflection might drop to %99.9 which, for a wave guide, would be a big problem. Could someone comment on this? If true, this page should discuss this. —Ben FrantzDale (talk) 22:47, 13 December 2011 (UTC)

i am not understanding — Preceding unsigned comment added by 49.205.222.52 (talk) 14:54, 5 September 2012 (UTC)

## "Examples in Everyday Life"

"This is different phenomenon from reflection and refraction. Reflection occurs when light goes back in same medium. Refraction occcurs when light travels from different mediums. Here both are not happening. This is due to both and a mixture of both."

This sentence needs to be rewritten and clarified by an editor. — Preceding unsigned comment added by 23.16.126.200 (talk) 22:08, 19 January 2013 (UTC)

Removed. Olli Niemitalo (talk) 10:14, 20 January 2013 (UTC)

## At the critical angle

Light is shown propapagating along the interface when incident at exactly the critical angle. But doesn't this violate the principle of reversibility? Instead, shouldn't the light should be reflected if incident at exactly the critical angle? -- cheers, Michael C. Price talk 07:21, 20 January 2013 (UTC)

## Tightened wording of example

I tightened the wording of the example and inserted the sentence "(and the drain has disappeared!)" This is my interpretation of a hard-to-follow sentence in a previous edit. Is it correct?

Also, does this real-life example always work, or only usually, depending on the typical geometry of a sink and the bottom of a glass? I didn't do any experiments (my understanding is that this would defeat the purpose of Wikipedia). 84.227.237.33 (talk) 14:24, 3 April 2014 (UTC)

I removed the example because it is overly complicated (e.g. compare to looking through a water surface from below), unsourced, and I don't think it works. Ulflund (talk) 17:43, 30 August 2014 (UTC)