Why central fringe in Newton's ring is dark ?

We know,

 Path difference between two consecutive reflected ray is, ∆ = 2μtcosr + λ/2

      Here r = 0

      I.e cosr =1

For central fringe t = 0

  So  ∆ = λ/2     i.e path difference is the odd multiple of λ/2 therefore central fringe in Newton's ring is dark.