Thin-film optics: light reflecting from top and bottom surfaces interferes. Path difference Δ = 2nh·cosθ (n≈1.34 for soapy water). Constructive interference when Δ = mλ. As the film drains gravitationally (Marangoni + gravity), colors shift through Newton's rings sequence: black, gray, white, yellow, orange, red, violet, blue, green...