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Question

A very thin transparent film of soap solution of negligible thickness is seen under reflection of white light. Then the colour of the film appear to be :

A) blue

B) black

C) red

D) yellow

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Solution

For very thin films the distance travelled inside the film is insignificant and so the two reflected waves are almost exactly out of phase with each other (due to the phase change at one surface); they interfere destructively and the film appears `black`. For this reason when a soap film goes black it is about to burst

Hence B is the correct option.

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