Question

Why does the very thin region at the top of a soap bubble sometimes appear black?

1. It is too thin to reflect light. 2. Inverted light waves and non-inverted light waves reflecting from the two surfaces cancel each other. 3. Both reflected waves are inverted or else neither is inverted. 4. No light passes through them.5. The rest of the film creates a shadow that obscures the colors we would otherwise see.

Answer 1

Option 2 is correct.

2.Inverted light waves and non-inverted light waves reflecting from the two surfaces cancel each other.

The colors of a bubble are dependent on the thickness of the film. A bubble becomes thinner and thinner as it dries out (due to evaporation), before finally popping. As the surface film of the bubble becomes increasingly thinner, a change in overall color can be seen. Thick walls cancel out longer wavelengths in the red range. As the bubble film gets thinner, yellow wavelengths are cancelled out. As it gets even thinner, green light is lost. Beyond this point, even shorter wavelengths in the blue wavelength range disappear. The resulting colors are a combination of the colors that do not undergo destructive interference and their degrees of constructive interference. Blue-green colors dominate in thicker films and yellow hues in thinner films. Eventually, the film becomes too thin to create interference of visible wavelengths, as all wavelengths are cancelled out. At this point the bubble appears colorless. Against a black background the bubble surface could appear black.

Eventually, the film becomes too thin to create interference of visible wavelengths, as all wavelengths are cancelled out. At this point the bubble appears colorless. Against a black background the bubble surface could appear black.