Thus, different colors can be seen by holding the shell at different angles. Interference of light from the upper and lower surfaces of the soap film is taking place. If the film is thick, the part of the wave that reflects from one surface will be displaced from the part that reflects from the other surface. No interaction, no cancellation, no interference colors. For thin films, the two parts of the wave coincide as they recombine.
Interference occurs when light from a single source divides and recombines.
Each colored ring represents a particular thickness of oil film, just as the lines on a surveyor's contour map represent equal elevations. Polarization is a property of transverse waves. Unlike light, sound is a longitudinal wave and can't be polarized. Whether a wave can be polarized or not, in fact, is one of the tests to distinguish transverse waves from longitudinal waves. Nonpolarized light has as many components along the polarization axis as it has perpendicular to that axis.