A ray of light hits the surface at point X. Some of the light is reflected, but some travels through the bubble wall and is reflected at the other side. When light directed from low index material strikes a high index material (air to film), there is a 180 degree phase shift just from the reflection (a hard reflection). So the film thicknesses discussed for red and blue light in the panels to the right are incorrect by half a wavelength.

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A ray of light hits the surface at point X. Some of the light is reflected, but some travels through the bubble wall and is reflected at the other side. When light directed from low index material strikes a high index material (air to film), there is a 180 degree phase shift just from the reflection (a hard reflection). So the film thicknesses discussed for red and blue light in the panels to the right are incorrect by half a wavelength.