Question

Consider a two-slit interference pattern, with monochromatic light of wavelength ?.

(a) What is the path difference ?? for the third bright fringe? Give your answer in terms of the wavelength of the light.
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(b) What is the path difference ?? for the third dark fringe above the central bright fringe? Give your answer in terms of the wavelength of the light.
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Answer 1

In the experiment of double slit interference, the expression for path difference can be written as,

Part (a): The condition for bright fringes is,

Therefore, path difference for third bright fringe, that is m = 3, is,

  

Part (b): The condition for dark fringe is,

  

Therefore the path difference for third dark fringe above the central bright fringe, that is m = 2, is,