Question 6

  When sunlight reflects from a thin film of soapy water, the film appears multicolored, in part because destructive interference removes different wavelengths from the light reflected at different places, depending on the thickness of the film. As the film becomes thinner and thinner, it looks darker and darker in reflected light, appearing black just before it breaks. The blackness means that destructive interference removes all wavelengths from the reflected light when the film is very thin. Explain why.

SOLUTION:
Light reflected from the top of the film (smaller to larger index of refraction) is phase-shifted by 180^o. Light reflected from the bottom of the film is not phase-shifted (larger to smaller index of refraction). Hence, as the film becomes thinner and thinner, the two reflected wavefronts become closer and closer to being completely out of phase. The out of phase waves destructively interfere, so the film appears dark.