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Asked in CBSE AIPMT 1999 · Thin film interference
Given: the coloured bands seen on a thin soap film.
Idea: a thin film reflects light from both of its surfaces, and the two reflected beams can superpose.
Part of the incident light reflects at the top surface, part travels through the film and reflects at the bottom.
The second beam covers an extra path of about twice the film thickness, so the two arrive with a path difference.
For a given thickness that path difference favours some wavelengths and cancels others.
The surviving wavelengths are what the eye sees, so the colours are due to interference.
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