Two Very Flat Glass Plates, 16 Cm Long, Are in Contact

Two very flat glass plates, 16 cm long, are in contact at one end and separated by 0.020 mm at the other end. The space between the plates is occupied by oil with index of refraction 1.45. The
Refractive index of the glass plates is 1.55. The plates are illuminated at normal incidence with
Monochromatic light, and fringes are observed. If the spacing of the dark fringes is 2.0 mm, what is
The wavelength of the monochromatic light?

A) 525 nm
B) 475 nm
C) 725 nm
D) 425 nm
E) 675 nm