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In Young’s double slit experiment, the fringe width is found to be 0.4 mm. If the whole apparatus is immersed in water of refractive index 4/3, without disturbing the geometrical arrangement, the new fringe width will be

1. 0.30 mm
2. 0.40 mm
3. 0.53 mm
4. 450 microns

Correct answer: 0.30 mm

Solution

The fringe width in Young's double-slit experiment is directly proportional to the wavelength of light. When the apparatus is immersed in water, the wavelength decreases by a factor equal to the refractive index of water (4/3). Thus, the new fringe width is (0.4 mm) × (3/4) = 0.30 mm.

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