Light interference – The Thales circle – interference on thin layers

A small part of the red light is reflected in each layer. As the layer thicknesses δ are of equal size, the amplitudes that is, the lengths of the arrows are always of the same size.
The frequencies of the rotating wheels representing the reflected partial waves are also identical. Just as with the two acoustic waves from Station 2, we recognise that only the resulting angle that arises between the first vertical arrow and the second arrow changes as the wave propagates a little through the soap film. The two reflected partial waves add up, in vector addition, to the resulting reflected light wave √(I_R ) at this point.

As the thickness d of the soap wall increases, the number of layers with thickness δ also grows. We see that the incident light is reflected in all layers. Based on the phase shift between the individual layers, the individual amplitudes of the light wave move along the circumference of a circle. We can thus describe the reflection and transmission of a given light wave on a soap film using a Thales circle. The resulting reflection and transmission amplitudes and are always perpendicular to one another.

With the increasing thickness of the soap film towards the bottom, the Thales circle is traversed several times. This corresponds to the periodic sequence of minima and maxima of reflection or transmission of red light.