# Why does moving the source-slit closer to the double-slit plane decrease the sharpness of the interference-pattern?

Thomas Young used a single-slit plane before the two-slits plane in order to make the light source coherent.

My book (NCERT Physics II) presents the above query and writes:

Let $s$ be the size of the source-slit and $S$ its distance from the plane of two slits. For interference fringes to be seen, the condition $$\dfrac{s}{S}\lt \dfrac{\lambda}{d}$$ should be satisfied; otherwise, interference patterns produced by different parts of the source overlap and no fringes are seen. Thus, as $S$ decreases (i.e. the source slit is brought closer), the interference pattern gets less and less sharp, and when the source is brought too close for this condition to be valid, the fringes disappear.

I've totally failed to comprehend what the book is saying. I also don't know how it deduced the inequality relation. Can anyone please explain me what is the reason for this phenomenon? Also, can anyone tell me how the inequality relation was deduced? What does it mean by sharpness of the interference pattern?