Newton's Rings and plano-convez lens

[simplicity12]

Homework Statement

A plano-convez lens (flat on one side, convez on the other) rests with its curved side on a flat glass surface. The lens is illuminated from above by light of wavelength 521 nm. A dark spot is observed at the centre, surrounded by 15 concentric dark rings (with bright rings in between). How much thicker is the air wedge at the position of the 16th dark ring than at the centre?

Homework Equations

There were no equations introduced in this section.

The Attempt at a Solution

The answer says 3.90 X 10^-7m. By multiplying the wavelength by 15 and then dividing it by 2, i get this answer.. by why? Is there actually an equation for these type of problems?

 

[Mentz114]

It's interference, and there is a formula. Have a look at this -
http://en.wikipedia.org/wiki/Newton's_rings