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An oil film floating on water shows colours due to interference of light. The approximate thickness of the oil film for this effect to be observable is of the order of:

1. 10,000 Angstrom
2. 1 cm
3. 1 mm
4. 100 Angstrom

Correct answer: 10,000 Angstrom

Solution

Thin-film interference requires the film thickness to be of the order of the wavelength of visible light (roughly a few thousand Angstrom). 10,000 Angstrom = 1 micrometre, which is of this order and gives visible coloured interference. 1 cm and 1 mm are far too thick, and 100 Angstrom is far too thin.

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