YDSE: Film, Medium & Two Wavelengths
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:
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Answer: (d)
- Need a path difference that is a whole number of one wavelength and an odd number of half-waves of the other.
- (bright for 500 nm) and (dark for 600 nm).
- Others fail: 1000 and 2000 are not half-integral multiples of 600; 3000 is bright for both.
- So the answer is (d) 1500.