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In a Young's double slit experiment, slits are separated by 0.5 mm, and the screen is placed 150 cm away. A beam of light consisting of two wavelengths, 650 nm and 520 nm, is used to obtain interference fringes on the screen. The least distance from the common central maximum to the point where the bright fringes due to both wavelengths coincide is:

1. 9.75 mm
2. 15.6 mm
3. 1.56 mm
4. 7.8 mm

Correct answer: 7.8 mm

Solution

The least distance where the bright fringes coincide for both wavelengths is determined by finding a common multiple of their fringe positions, which occurs at a distance of 7.8 mm from the central maximum. This is calculated using the formula for fringe separation and the given wavelengths.

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