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Asked in GSEB Board March 2020 · Intensity and coherent sources
Given: the single source slit in front of Young's double slit is made wider.
Idea: the two slits act as coherent sources only while they are lit by a small enough source; the fringes survive while s/S<λ/d, where s is the width of the source slit and S its distance from the double slit.
A wide source slit is a row of many narrow sources that are mutually incoherent.
Each of them produces its own set of fringes, displaced slightly from its neighbour's.
Those sets overlap, so the bright fringes are filled in from the sides and the dark fringes are lightened.
The pattern therefore gets less and less sharp, and a wide enough source slit washes it out altogether.
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