Effect of source slit in Young's Double Slit Experiment

[Raghav Gupta]

What is the effect on the interference fringes in Young's Double slit experiment when the source slit is moved closer to the double slit plane?
I have seen you people @DrChinese , @Cthugha , @bhobba helping in these kind of topic before.
Can you help here?
It would be great if others also can help.

 

[jtbell]

That single slit is used to produce spatial coherence in the light that arrives at the double slits:

http://electron6.phys.utk.edu/optics421/modules/m5/Coherence.htm

As you move that single slit closer to the double slits, it "appears" bigger from the point of view of the double slits, and the light arriving at the double slits becomes less spatially coherent.

This is a matter of classical wave optics, by the way. It's not specific to quantum optics.

 

[Raghav Gupta]

Thanks. I have only one last question related to this.
If the size of source slit is s and the distance of single slit(source slit) from double slits is S, then how the condition s/S <= λ/d must be satisfied for the fringes to appear?
Here λ is the wavelength of the source light and d is the distance between 2 double slits.