Why Is the Center of Newton's Rings Dark?

[sheld]

Why the center of Newton's ring is dark? though I am thinking the path difference in the center is zero between two reflected rays...
so,I will be glad if you give the correct information about that...

 

[Orodruin]

As long as the surfaces are in contact the path difference in the middle is zero. If you move them slightly apart you can change this.

 

[ehild]

Why the center of Newton's ring is dark? though I am thinking the path difference in the center is zero between two reflected rays...
so,I will be glad if u give the correct information about that...

The phase difference between the two reflected ways counts, and it is not zero, even in the case when the path difference --->0.