Immerse the ring in the cup of shampoo and then blow steadily but slow through it. Everyone of us must have made such bubbles when we were young. But have you watched carefully how they shimmer with all the colours of rainbow?
This is called light interference on thin films. If a one-coloured light beam falls on the surface of the film, it partly reflects and partly filter through the film undergoing refraction. Some of the beams that managed to filter through escape through the other side of the bubble. Some of those get reflected and are refracted on the opposite surface. Which colour is created and which not depends on the thickness of the film.
These phenomena have always fascinated people not only with their beauty, but also with the fact that they can be observed in short-lasting objects like bubbles, oil spills on water, oil slicks on a wet road, insects' wings, mother-of-pearl and peacock's tail.
The recipe for giant soap bubbles is following: 6 parts of water, 2 parts of a good plate-washing liquid, 1 part of glycerine. Let it rest for a few hours.
The colours of the rainbow form in a bubble thanks to the superposition of reflected rays, i.e. thanks to the interference. Our eye registers both rays reflected from the internal and the external part of the bubble liquid film. In this way the light ray measures the thickness of the bubble wall. The intensities sum-up if the difference between the light pathways equals to an integer multiple of wavelength. In this case both reflected rays have the same phase and the interference is constructive. If the phases are opposite then the interference is destructive and a given colour is not observed. If the light beam incises perpendicularly to the bubble surface and a given colour is observed, the wall thickness d is a multiple k of the half wavelength λ/2, corresponding to this colour (the light travels twice inside the liquid film):
But this reasoning is somewhat more complicated. When the light propagates inside the water film and is being reflected from the surface separating water from air, its phase changes by 180o. In this way, another λ/2 is to be added to the path of light inside the bubble film and therefore the film thickness is: d=(2k+1)'/4n
