Practice portal › Wave Optics › Interference and Young's Double Slit Experiment
Asked in GSEB Board July 2021 · Intensity and coherent sources
Given: the source slit that illuminates Young's double slit is made wider.
Idea: the two slits behave as coherent sources only while the light reaching them comes from a small enough source; the usual condition is s/S≤λ/d, with s the width of the source slit and S its distance from the double slit.
Treat the wide slit as many narrow slits side by side, independent of one another.
Each of them produces a complete fringe pattern, shifted slightly from its neighbour's.
Adding patterns that are shifted against each other lifts the minima and lowers the maxima.
So the fringes lose contrast: the pattern gets less and less sharp, and finally disappears into uniform illumination.
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