Two optically 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 index of the glass plates is 1.55. The plates are illuminated at normal incidence with monochromatic light, and fringes are observed.

Q1: The monochromatic light has a wavelenght of 580 nm. The number of bright fringes which are visible is closest to:

a) 70
b) 90
c) 100
d) 80
e) 60

Q2: Using the same problem, if the spacing of the dark fringes is 2.0 mm, the wavelenght of the monochromatic light, in nm, is closest to:

a) 475
b) 425
c) 525
d) 725
e) 675