I think there are some big misunderstandings here unfortunately, and it's likely because coherence is rarely defined well.<p>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).<p>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)<p>E_returning = E_incoming (2 * thickness/lambda) + E_incoming(0)<p>is NOT that simple, but instead written as<p>E_returning = E_incoming (2 * thickness/n) + E_incoming(phi)<p>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].<p>Now, we haven't proven yomritoyj's conclusions to be wrong --- there is still interference. Now, however, let's add two details:<p>1) phi depends on wavelength. if phi depends on the wavelength, then the plots he drew could have a random extra phase added <i>at each wavelength</i>. This would destroy any interesting features in the plots, and you'd get some basically random reflection from each wavelength.<p>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.<p>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!)<p>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 =).<p>[1] <a href="http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wave%20properties/text/Coherent%20and%20incoherent/index.html" rel="nofollow">http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wa...</a>