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As a soap bubble evaporates, it appears ...

As a soap bubble evaporates, it appears black just before it breaks. Explain this phenomenon in terms of the phase changes that occur on reflection form the two surface of the soap film.

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As the soap bubble becomes very thin, the thickness of the bubble approaches zero, Since light reflecting off the front of the soap surface is phase- shifted `180^(@)` and light reflecting off the back of the soap film is phase-shifted `0^(@)`, the reflected light meets the conditions for a minimum. Thus, the soap film appears blacks.
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