Not sure why, but I see the raw markup rather than a formatted article. Interesting content nonetheless, but difficult to read on mobile.
Feynman's explanation for iridiscence: https://books.google.ch/books?id=2o2JfTDiA40C&pg=PA33
See also: https://en.wikipedia.org/wiki/Thin-film_interference
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/zaw...
You can observe the same colours in thick plastics under crossed polarisers.
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.
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?
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.
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.
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/Wa...
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.
Um, thin film interference. It even has a nice succinct name.
It’s a simple byproduct of basic optics :-/
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.