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The source that illuminates the double-slit in 'double-slit interference experiment' emits two distinct monochromatic waves of wavelength 500 nm and 600 nm, each of them producing its own pattern on the screen. At the central point of the pattern when path difference is zero, maxima of both the patterns coincide and the resulting interference pattern is most distinct at the region of zero path difference. But as one moves out of this central region, the two fringe systems are gradually out of step such that maximum due to one wavelength coincides with the minimum due to the other and the combined fringe system becomes completely indistinct. This may happen when path difference (in nm) is

Asked in JEE Main Online 2013 · Fringe width and fringe position

• (1) 2000
• (2) 3000
• (3) 1000
• (4) 1500

Step-by-step solution

Why the other options are wrong

• (1) 2000 nm is a whole number of 500 nm but 3.33×600 nm, neither a clean maximum nor minimum of 600 nm.
• (2) 3000 nm is a whole multiple of both, so both are maxima there.
• (3) 1000 nm gives 2×500 but 1.67×600, not a half-integer.

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