Soap Film; Constructive Interference of Light Reflected. Bands of color are created by white light shining on a film of soap. Some of the light reflects off the surface of the film, while the rest of the light travels through the film and reflects off the back of the film. The reflected light waves interfere with each other, either constructively (colored bands) or destructively (black bands). Different colours are seen due to varying film thickness and viewing angles.