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Initially interference is observed with the entire experimental set up inside a chamber filled with air, Now the chamber is evacuated. With the same source of light used, a careful observer will find that.

1. The interference pattern is almost absent as it is very much diffused
2. There is no change in the interference pattern
3. The fringe width is slightly decreased
4. The fringe width is slightly increased

Correct answer: The fringe width is slightly increased

Solution

Fringe width in an interference setup is proportional to the wavelength of light in the medium. Evacuating the chamber increases the wavelength slightly compared with air, so the fringe spacing increases slightly.

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