6 ms·

Why soap bubbles are colourful and windowpanes are not

118 points by yomritoyj 8y ago

zsellera 8y ago
Thanks, great. Slight addition: visible light spectra is between 400-700nm; sure you can still see 390nm and 730nm, but the sensitivity is rapidly decreased there.
osrec 8y ago
Not sure why, but I see the raw markup rather than a formatted article. Interesting content nonetheless, but difficult to read on mobile.
- peralmq 8y ago
  It's an IPython Notebook that you can use with http://jupyter.org/ http://jupyter.org/
- uranusjr 8y ago
  Formatted from the gist, served on nbviewer.jupyter.org: https://nbviewer.jupyter.org/gist/jmoy/4dda9b8b8e2b3eb27666bd6ebe208ea3 https://nbviewer.jupyter.org/gist/jmoy/4dda9b8b8e2b3eb27666b...
  - osrec 8y ago
    Thank you!
  - jtbayly 8y ago
    I know this is a techy sort of place, but this article isn’t about the code. I think the original link should be replaced with this one since it is useful without additional steps.
    - ehsankia 8y ago
      It seems like an issue with Gist on mobile. On desktop, Gist properly renders the notebook exactly as that other link.
- cup-of-tea 8y ago
  If this was on github.com it would be rendered but obviously they don't render them on gist.
- afranchuk 8y ago
  If you request the desktop version of the page it'll render the notebook. For some reason they don't attempt rendering the notebooks on mobile.
deleted 8y ago
[deleted]
saagarjha 8y ago
Well, because the thickness on the soap bubble is on the order of the wavelength of visible light, which causes interference that we can see. Windowpanes are far too thick for this to occur.
- janekm 8y ago
  Actually as the article explains it's a little more interesting. Naively even in a thick window pane we would expect interference patterns from light reflected from the front and the back of the window pane. The article explains why we don't.
- yomritoyj 8y ago
  OP here. Why thickness should matter is precisely the question. If you look at the treatment of this on Wikipedia or a physics text it is the phase difference beteween the front and back reflections which matter and not the total path length. Even for a windowpane there will be some wavelengths where the two waves are in phase and others where they are out of phase. The true explanation is that for a windowpane these two kinds of wavelenghts are clustered densely together and therefore even for tiny wavelength ranges the interference effect averages out.
  - chopin 8y ago
    For the effect the phase difference is the thing which matters. However, the wave reflected from the back and the wave reflected from the front run through different path length (twice the thickness). For the effect to work the waves need to be in phase along that distance which is a few µm for sunlight (look up coherence length). The different wavalengths make only up for color. Some wavelength will have destructive interference and go missing. As the eye is quite sensitive to that you will see a colored surface, whichs color changes with view angle (as different wavelengths will cancel for different angles).
- y04nn 8y ago
  I would have explained it by the irregularities at the surface of windowpanes. Huh, is it what is said in a more mathematical way in the link?
braythwayt 8y ago
Having read QED early on, my understanding is that while treating a windowpane or mirror as a front and back surface is a useful simplification in most cases, it is not for interference. The "sum of the histories" explanation is that light interacts with all of the molecules at every depth, and once the depth becomes larger than the "wavelength," the interactions statistically can el each other out, and the probability of an interference pattern appearing beco,es infinitesimal. I have also heard that while "sum of the histories" was useful for explaining QED to undergrads, it wasn't an effective way to calculate results and didn't yield any predictions that more math-heavy approaches coukdn't produce, so it was discarded. For all I know it has been shown to be wrong for some observed phenomena. Anyhow... Is that explanation the correct "sum of the histories" explanation? And if so, is ot considered useful to think of it in these terms for laypersons who don't want to dive into the math?
- joshvm 8y ago
  You can still get interference if the medium is thicker than a wavelength, but it needs to be close. This effect is quite often seen in CCD detectors used in infrared spectroscopy, as well as certain detectors used for visible spectroscopy. It's called etaloning. But as you suggest, in thicker media there's much less chance of it happening. http://www.andor.com/learning-academy/optical-etaloning-in-charge-coupled-devices-technical-article http://www.andor.com/learning-academy/optical-etaloning-in-c...
- abecedarius 8y ago
  It sounds like you're thinking of Feynman's popular book QED (which I'd agree ought to be great for people who want an actual idea of what's going on without taking a college course) -- but in chapter 3 he shows how "we can get the correct answer for the probability of partial reflection by imagining (falsely) that all reflection comes from only the front and back surfaces... what is really going on: partial reflection is the scattering of light by electrons inside the glass." This comes from adding up the coherent effects of that scattering -- he wasn't saying the interference goes away (becomes incoherent). It's not my field, but my impression was that path integrals (sum over histories) did initially figure mainly in discovering the Feynman rules for QED, without getting used much directly by others, but later did find more applications.
razimantv 8y ago
This treatment is missing an important thing: Sunlight is not coherent over such long distances such as window pane thicknesses. Basically the two reflections cannot even interfere in the case of sunlight because they do not have a phase relationship.
- raverbashing 8y ago
  You're right, but soap bubbles in the sun also appear colorful. You only care about those that come at a specific angle and are reflected to an observer. For most angles it doesn't matter, remember that lakes polarize their reflection (as seen by an observer).
  - razimantv 8y ago
    Soap bubbles appear colourful in the sun in spite of low coherence because the soap film is thin. Soap films can be as thin as tens of nanometers meaning that light has to be only coherent for a femtosecond for interference to happen -- and this is very comparable to the cycle time of the light itself so that coherence is practically assured. But the round trip time in a 5mm window pane is 50 picoseconds, which is thousands of times larger making coherence important.
    - raverbashing 8y ago
      Ah I see your point. I guess a similar thing happens when you look at plastic with polarized sunglasses, then you see different colors (though there's a chemical issue with plastic as well that affects the polarization of light) You could do a 5mm(+/-10nm) glass pane for it to reflect/absorb a specific wavelength but it would be hard.
      - deleted 8y ago
        [deleted]
      - chopin 8y ago
        Most plastic sheets are birefringent due to the way they are produced. In the right circumstances, that can make a very colorful appearance when viewed in polarized light. As the difference in refraction index is normally rather small this works also for thicker sheets as no long coherence length is required for the effect to work. The effect should be strongest if you hold up the sheet to the clear sky with the sun behind you (the light coming from there is polarized horizontally). Sunglasses have the polarization vertically (to block exactly this light). As the birefringence turns the polarization the patch viewed through sheet becomes brighter. If you add in dispersion, you can get colored patches. It's the same effect mineralogists use with polarizing microscopes.
knolan 8y ago
The colours of a soap bubble can be understood with a Michel-Levy chart and thin film interference of a birefringent medium. https://www.itp.uni-hannover.de/fileadmin/arbeitsgruppen/zawischa/static_html/twobeams.html https://www.itp.uni-hannover.de/fileadmin/arbeitsgruppen/zaw... You can observe the same colours in thick plastics under crossed polarisers.
- greeneggs 8y ago
  Yes, there is also a simple derivation in Boys's classic book, "Soap Bubbles: Their Colors and Forces Which Mold Them", p. 136. [1, 2] [1] pdf link (it is out of copyright): http://www.arvindguptatoys.com/arvindgupta/soap-bubbles-boys.pdf http://www.arvindguptatoys.com/arvindgupta/soap-bubbles-boys... [2] google books link (free preview), go to the very last page for the color plate
code_duck 8y ago
It’s great to read an explanation for this. Glassblowers can see iridescence when we blow ‘bubble trash’, which happens when you blow glass so thin it can float away in the air. If one could get it on thicker bubbles without using chemicals such as tetraisopropyl titanite, that would be fantastic.
asafira 8y ago
I think there are some big misunderstandings here unfortunately, and it's likely because coherence is rarely defined well. yomritoyj claims that the lack of coherence shouldn't impact whether or not destructive or constructive interference occurs. That is, if a monochromatic light source is impinging on a layer of material, one will ultimately still get that the returning electromagnetic wave is the sum of the wave that hit the front surface and reflected, and the wave that hit the back surface and reflected some time earlier. For white light, one could simply say that you could decompose it into many separate wavelengths that behave this way (a continuum of wavelengths). The missing point here is the following: imagine the above is true, and you can absolutely draw plots as is given by the notebook above. Now, let's make the analysis a little more general: assume that in the time that the light hit the back surface of the layer of material, something happened to the incoming light and it shifted in phase. That is, your final sum-of-two-fields (as described above) E_returning = E_incoming (2 * thickness/lambda) + E_incoming(0) is NOT that simple, but instead written as E_returning = E_incoming (2 * thickness/n) + E_incoming(phi) where phi is some extra nasty angle. It should be clear this happens, for example, from this first result for "incoherent light" on google [1]. Now, we haven't proven yomritoyj's conclusions to be wrong --- there is still interference. Now, however, let's add two details: 1) phi depends on wavelength. if phi depends on the wavelength, then the plots he drew could have a random extra phase added at each wavelength. This would destroy any interesting features in the plots, and you'd get some basically random reflection from each wavelength. 2) phi changes over time. if phi changed in time, you now not only get a random reflection, but the amount of light reflected at a certain wavelength will change to something else sometime later. This time is usually very quick for incoherent light like the sun, and your eye is constantly averaging over many different intensity reflections over time for each wavelength. Lastly, given the above, why the hell does this work at all for soap bubbles then? Well, for soap bubbles, the light is not so terrible (so incoherent) that it gets a chance to have that extra "phi" phase to include in the interference --- that's because the wave reflecting from the back surface comes back so quickly! (soap bubbles are so thin!) I encourage people to plug in the speed of light to get a feel for these timescales --- this is the sort of thing physics phd's get used to =). [1] http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wave%20properties/text/Coherent%20and%20incoherent/index.html http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wa...
deleted 8y ago
[deleted]
mistercow 8y ago
IIRC, this is also exactly the reason that anodized metals create a rainbow effect. In that case, the thin oxide layer creates the necessary interference.
kgwgk 8y ago
Feynman's explanation for iridiscence: https://books.google.ch/books?id=2o2JfTDiA40C&pg=PA33 https://books.google.ch/books?id=2o2JfTDiA40C&pg=PA33 See also: https://en.wikipedia.org/wiki/Thin-film_interference https://en.wikipedia.org/wiki/Thin-film_interference
olliej 8y ago
Um, thin film interference. It even has a nice succinct name. It’s a simple byproduct of basic optics :-/

118 points by yomritoyj 8y ago

Thanks, great. Slight addition: visible light spectra is between 400-700nm; sure you can still see 390nm and 730nm, but the sensitivity is rapidly decreased there.

osrec 8y ago

Not sure why, but I see the raw markup rather than a formatted article. Interesting content nonetheless, but difficult to read on mobile.

peralmq 8y ago
It's an IPython Notebook that you can use with http://jupyter.org/ http://jupyter.org/
uranusjr 8y ago
Formatted from the gist, served on nbviewer.jupyter.org: https://nbviewer.jupyter.org/gist/jmoy/4dda9b8b8e2b3eb27666bd6ebe208ea3 https://nbviewer.jupyter.org/gist/jmoy/4dda9b8b8e2b3eb27666b...
- osrec 8y ago
  Thank you!
- jtbayly 8y ago
  I know this is a techy sort of place, but this article isn’t about the code. I think the original link should be replaced with this one since it is useful without additional steps.
  - ehsankia 8y ago
    It seems like an issue with Gist on mobile. On desktop, Gist properly renders the notebook exactly as that other link.
cup-of-tea 8y ago
If this was on github.com it would be rendered but obviously they don't render them on gist.
afranchuk 8y ago
If you request the desktop version of the page it'll render the notebook. For some reason they don't attempt rendering the notebooks on mobile.

deleted 8y ago

[deleted]

saagarjha 8y ago

Well, because the thickness on the soap bubble is on the order of the wavelength of visible light, which causes interference that we can see. Windowpanes are far too thick for this to occur.

janekm 8y ago
Actually as the article explains it's a little more interesting. Naively even in a thick window pane we would expect interference patterns from light reflected from the front and the back of the window pane. The article explains why we don't.
yomritoyj 8y ago
OP here. Why thickness should matter is precisely the question. If you look at the treatment of this on Wikipedia or a physics text it is the phase difference beteween the front and back reflections which matter and not the total path length. Even for a windowpane there will be some wavelengths where the two waves are in phase and others where they are out of phase. The true explanation is that for a windowpane these two kinds of wavelenghts are clustered densely together and therefore even for tiny wavelength ranges the interference effect averages out.
- chopin 8y ago
  For the effect the phase difference is the thing which matters. However, the wave reflected from the back and the wave reflected from the front run through different path length (twice the thickness). For the effect to work the waves need to be in phase along that distance which is a few µm for sunlight (look up coherence length). The different wavalengths make only up for color. Some wavelength will have destructive interference and go missing. As the eye is quite sensitive to that you will see a colored surface, whichs color changes with view angle (as different wavelengths will cancel for different angles).
y04nn 8y ago
I would have explained it by the irregularities at the surface of windowpanes. Huh, is it what is said in a more mathematical way in the link?

braythwayt 8y ago

Having read QED early on, my understanding is that while treating a windowpane or mirror as a front and back surface is a useful simplification in most cases, it is not for interference. The "sum of the histories" explanation is that light interacts with all of the molecules at every depth, and once the depth becomes larger than the "wavelength," the interactions statistically can el each other out, and the probability of an interference pattern appearing beco,es infinitesimal. I have also heard that while "sum of the histories" was useful for explaining QED to undergrads, it wasn't an effective way to calculate results and didn't yield any predictions that more math-heavy approaches coukdn't produce, so it was discarded. For all I know it has been shown to be wrong for some observed phenomena. Anyhow... Is that explanation the correct "sum of the histories" explanation? And if so, is ot considered useful to think of it in these terms for laypersons who don't want to dive into the math?

joshvm 8y ago
You can still get interference if the medium is thicker than a wavelength, but it needs to be close. This effect is quite often seen in CCD detectors used in infrared spectroscopy, as well as certain detectors used for visible spectroscopy. It's called etaloning. But as you suggest, in thicker media there's much less chance of it happening. http://www.andor.com/learning-academy/optical-etaloning-in-charge-coupled-devices-technical-article http://www.andor.com/learning-academy/optical-etaloning-in-c...
abecedarius 8y ago
It sounds like you're thinking of Feynman's popular book QED (which I'd agree ought to be great for people who want an actual idea of what's going on without taking a college course) -- but in chapter 3 he shows how "we can get the correct answer for the probability of partial reflection by imagining (falsely) that all reflection comes from only the front and back surfaces... what is really going on: partial reflection is the scattering of light by electrons inside the glass." This comes from adding up the coherent effects of that scattering -- he wasn't saying the interference goes away (becomes incoherent). It's not my field, but my impression was that path integrals (sum over histories) did initially figure mainly in discovering the Feynman rules for QED, without getting used much directly by others, but later did find more applications.

razimantv 8y ago

This treatment is missing an important thing: Sunlight is not coherent over such long distances such as window pane thicknesses. Basically the two reflections cannot even interfere in the case of sunlight because they do not have a phase relationship.

raverbashing 8y ago
You're right, but soap bubbles in the sun also appear colorful. You only care about those that come at a specific angle and are reflected to an observer. For most angles it doesn't matter, remember that lakes polarize their reflection (as seen by an observer).
- razimantv 8y ago
  Soap bubbles appear colourful in the sun in spite of low coherence because the soap film is thin. Soap films can be as thin as tens of nanometers meaning that light has to be only coherent for a femtosecond for interference to happen -- and this is very comparable to the cycle time of the light itself so that coherence is practically assured. But the round trip time in a 5mm window pane is 50 picoseconds, which is thousands of times larger making coherence important.
  - raverbashing 8y ago
    Ah I see your point. I guess a similar thing happens when you look at plastic with polarized sunglasses, then you see different colors (though there's a chemical issue with plastic as well that affects the polarization of light) You could do a 5mm(+/-10nm) glass pane for it to reflect/absorb a specific wavelength but it would be hard.
    - deleted 8y ago
      [deleted]
    - chopin 8y ago
      Most plastic sheets are birefringent due to the way they are produced. In the right circumstances, that can make a very colorful appearance when viewed in polarized light. As the difference in refraction index is normally rather small this works also for thicker sheets as no long coherence length is required for the effect to work. The effect should be strongest if you hold up the sheet to the clear sky with the sun behind you (the light coming from there is polarized horizontally). Sunglasses have the polarization vertically (to block exactly this light). As the birefringence turns the polarization the patch viewed through sheet becomes brighter. If you add in dispersion, you can get colored patches. It's the same effect mineralogists use with polarizing microscopes.

knolan 8y ago

The colours of a soap bubble can be understood with a Michel-Levy chart and thin film interference of a birefringent medium. https://www.itp.uni-hannover.de/fileadmin/arbeitsgruppen/zawischa/static_html/twobeams.html https://www.itp.uni-hannover.de/fileadmin/arbeitsgruppen/zaw... You can observe the same colours in thick plastics under crossed polarisers.

greeneggs 8y ago
Yes, there is also a simple derivation in Boys's classic book, "Soap Bubbles: Their Colors and Forces Which Mold Them", p. 136. [1, 2] [1] pdf link (it is out of copyright): http://www.arvindguptatoys.com/arvindgupta/soap-bubbles-boys.pdf http://www.arvindguptatoys.com/arvindgupta/soap-bubbles-boys... [2] google books link (free preview), go to the very last page for the color plate

code_duck 8y ago

It’s great to read an explanation for this. Glassblowers can see iridescence when we blow ‘bubble trash’, which happens when you blow glass so thin it can float away in the air. If one could get it on thicker bubbles without using chemicals such as tetraisopropyl titanite, that would be fantastic.

asafira 8y ago

I think there are some big misunderstandings here unfortunately, and it's likely because coherence is rarely defined well. yomritoyj claims that the lack of coherence shouldn't impact whether or not destructive or constructive interference occurs. That is, if a monochromatic light source is impinging on a layer of material, one will ultimately still get that the returning electromagnetic wave is the sum of the wave that hit the front surface and reflected, and the wave that hit the back surface and reflected some time earlier. For white light, one could simply say that you could decompose it into many separate wavelengths that behave this way (a continuum of wavelengths). The missing point here is the following: imagine the above is true, and you can absolutely draw plots as is given by the notebook above. Now, let's make the analysis a little more general: assume that in the time that the light hit the back surface of the layer of material, something happened to the incoming light and it shifted in phase. That is, your final sum-of-two-fields (as described above) E_returning = E_incoming (2 * thickness/lambda) + E_incoming(0) is NOT that simple, but instead written as E_returning = E_incoming (2 * thickness/n) + E_incoming(phi) where phi is some extra nasty angle. It should be clear this happens, for example, from this first result for "incoherent light" on google [1]. Now, we haven't proven yomritoyj's conclusions to be wrong --- there is still interference. Now, however, let's add two details: 1) phi depends on wavelength. if phi depends on the wavelength, then the plots he drew could have a random extra phase added at each wavelength. This would destroy any interesting features in the plots, and you'd get some basically random reflection from each wavelength. 2) phi changes over time. if phi changed in time, you now not only get a random reflection, but the amount of light reflected at a certain wavelength will change to something else sometime later. This time is usually very quick for incoherent light like the sun, and your eye is constantly averaging over many different intensity reflections over time for each wavelength. Lastly, given the above, why the hell does this work at all for soap bubbles then? Well, for soap bubbles, the light is not so terrible (so incoherent) that it gets a chance to have that extra "phi" phase to include in the interference --- that's because the wave reflecting from the back surface comes back so quickly! (soap bubbles are so thin!) I encourage people to plug in the speed of light to get a feel for these timescales --- this is the sort of thing physics phd's get used to =). [1] http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wave%20properties/text/Coherent%20and%20incoherent/index.html http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wa...

deleted 8y ago

[deleted]

mistercow 8y ago

IIRC, this is also exactly the reason that anodized metals create a rainbow effect. In that case, the thin oxide layer creates the necessary interference.

kgwgk 8y ago

Feynman's explanation for iridiscence: https://books.google.ch/books?id=2o2JfTDiA40C&pg=PA33 https://books.google.ch/books?id=2o2JfTDiA40C&pg=PA33 See also: https://en.wikipedia.org/wiki/Thin-film_interference https://en.wikipedia.org/wiki/Thin-film_interference

olliej 8y ago

Um, thin film interference. It even has a nice succinct name. It’s a simple byproduct of basic optics :-/