Two Very Flat Glass Plates,16 Cm Long,are in Contact at One

Multiple Choice

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