The iridescent colours observed in soap bubbles are a result of the interference of light waves. When light strikes the thin soap film, it reflects off both the outer and inner surfaces. These two reflected waves interfere with each other. Due to the slight difference in the path length traveled by the waves (determined by the thickness of the film and the angle of incidence) and the phase change upon reflection, certain wavelengths of light are constructively interfered (enhanced), while others are destructively interfered (cancelled). This results in the appearance of vibrant colours, which vary depending on the local thickness of the soap film.