Question

A plano-convex glass lens is ground to a special non-spherical surface. The lens is placed on an optically flat glass plate for
testing. The equation for the curved surface of the lens is y = 600 x |r^3| where y and r are both in m. Newton's rings are
observed by reflection when the lens is illuminated normally from above, with monochromatic light whose wavelength is
600 nm. Do not consider the center spot as a ring.
3) In Figure 35.3, the radius of the second dark ring is closest to:

Answer 1

(solved already)

I forgot that 2y, twice the thickness of the air gap between the convex glass and the flat glass is the path difference and that it can be equaled to m*lambda, in which case m would be 2.