When white light shines on the bubble, it acts as a prism. While some of the white light bounces back off, some of them pass through the prism forming fringes of colors. But then, they too bounce off the inner part of the film. So one set of light rays shine into a soap bubble, but two sets of rays come back out again. When they emerge, the waves that bounce off the inner film have traveled a tiny bit further than the waves that bounced off the outer film. So, we have two sets of light waves. After that, the waves starts merging (Just like the ripples in the pond) . Some add together while some cancel out. That is why we see the pattern of colors repeating in the thin film soap.
