Bubbles. Layer of bubbles on water produced by washing up liquid. The colours, produced just before the bubbles burst, are due to light inter- ference. Certain wavelengths are extinguished when the incident white light reflected from the back of the film interferes with (cancels out) certain wavelengths of that reflected from the front of the film. The colours depend on the thickness of the film, which varies across the bubble; black where the film is very thin indeed, silver colours at about 70 nanometers thick, gold about 100 nano- meters thick, green about 150 nanometers thick, & red as thickness increases. The bubbles' shapes result from the balancing of surface tensions.