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

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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
 
Attachments
  • Diagram of a Newton's rings setup: a curved lens sits on a flat glass plate, with incident light arrows pointing down and reflected light arrows pointing up, and a red and black interference fringe pattern shown below.
    newton ri.PNG
    13.3 KB · Views: 555
  • Ray diagram of a Newton's rings setup: a blue refracted ray and a pink reflected ray enter a lens and meet at the curved lens surface above an air gap, with a resultant ray below, above an optical flat.
    newton ring.jpg
    19.5 KB · Views: 471

Discussion

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??