Suppose that the central diffraction envelope of a double slit diffraction pattern contains 11 bright fringes and the first diffraction minima eliminate (are coincident with) bright fringes. We have to find the bright fringes that lie between the first and the second minima of the diffraction envelope.
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Solution: Click For PDF Version Let the width of each slit be a and the separation of their midpoints be d.The condition for the first and the second diffraction minima are
It is given that there are 11 bright fringes within the central diffraction envelope, and that the first diffraction minima eliminate bright fringes, therefore the sixth Young’s double-slit bright fringe on either side of the first diffraction envelope should coincide with it. This condition implies that
We now obtain that
We conclude
that between the first and second diffraction minima there will lie
5 bright fringes, which correspond to
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