What causes the colorful swirls in soap bubbles?

Answer 1

When an incoming ray of light strikes the outer surface of a bubble, part of the light ray is reflected immediately, while the other part is transmitted into the soap film. After reaching the inner surface of the film, this transmitted light ray is reflected back toward the outer surface. When it leaves the bubble, it travels in the same direction as the ray that was immediately reflected and is, therefore, parallel to that ray. If these two rays of light are reflected back so that their wavelengths are "out of phase" with each other, the second ray will partly cancel out the reflection of the first ray. This is called destructive interference, which results in a reduction of color intensity. If, however, the wavelengths of the two reflected rays are "in phase," they will enhance each other. This is called constructive interference. The light rays that are reflected off the inner surface of the bubble travel further than the light rays that are reflected off the outer surface. Some wavelengths will interfere destructively and others constructively, depending on the extra distance traveled by a transmitted-and-reflected ray. Whether the reflected rays are in or out of phase with each other depends on the extra distance (through the film and back) that the second ray must travel before rejoining the first ray. This distance depends on the angle of the incident light and the thickness of the film.

White light is made up of different colors, corresponding to specific wavelengths. As the film thickness changes, the extra distance the ray must travel changes. Interference is constructive when the total extra distance matches a specific wavelength of light, and is destructive when it is half a wavelength. So if white light shines on a bubble, the film reflects light of a specific hue, and this hue changes with the film's thickness.

The iridescence of a soap bubble, which seems to contain a wealth of changing color, stems from light striking the bubble from varied angles. The path length varies with the angle of incident light, giving varying path differences for the internally and externally reflected rays at different points on the bubble. This means that, even if the soap film is of uniform thickness, different colors can be seen. Light entering the bubble directly travels a shorter path than light entering at a wider angle. This allows different wavelengths to undergo constructive and destructive interference, so different colors are perceived.

Answer 2

Because of refraction of the light