I am quite aware of what the intensity patterns will be on the screen for a single slit and a double slit experiment. However I have a hard time grasping the consequence of the intensity patterns when a source of light goes through a single slit first and then a double slit. Could anyone kindly clarify this for me?
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Related - Why must the single slit in a double slit experiment be narrow? – Farcher Aug 22 '19 at 18:14
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Have in mind that for an interference pattern to show on the screen during a double-slit experiment, you need a coherent source of light. If you use a high coherence laser, you can directly illuminate the double-slit; but if you lack a coherent source of light you first need to make one. That's when the first slit becomes useful as it makes light spatially coherent.
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Assuming that the source is coherent in nature, the intensity of the interference pattern will be same as that of a ' only ' double slit interference and ' a single slit and then double slit ' interference. This is so because the single slit acts again as a new source of light with same intensity, hence producing same intensity fringes as that of the 'only' double slit experiment.
Orion 73
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