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Young’s double slit experiment is first performed in air and then in a medium other than air. It is found that 8th bright fringe in the medium lies where 5th dark fringe lies in air. The refractive index of the medium is nearly

1. 1.59
2. 1.69
3. 1.78
4. 1.25

Correct answer: 1.59

Solution

In Young's double-slit experiment, the fringe position is proportional to the wavelength of light in the medium. The 8th bright fringe in the medium coinciding with the 5th dark fringe in air implies that the ratio of wavelengths in air and the medium is 8:7. Since the refractive index is inversely proportional to the wavelength, the refractive index is approximately 8/7 = 1.59.

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