Do we need any assumption when using Newton ring's formula ?

[Outrageous]

Homework Statement

2t=nλ, t=thickness , n=the number of rings counted, λ=wavelength.
Which diagram is correct?

Homework Equations

The Attempt at a Solution

I think the vertically down and up is correct for the using 2t=nλ, but the curved surface should make the incident angle different with the angle of the reflected ray . If so, the other diagram should be wrong unless we can make the reflected light(glass) must be parallel with the reflected light(lenses) .

please help , thank you

 

[Outrageous]

The center of the pattern is black. Here the two glass surfaces are in intimate contact and there is no reflection because it is as if there were no surface.
What is the meaning of no surface??