Two Flat Glass Plates Are in Contact Along One End

Two flat glass plates are in contact along one end and are separated by a sheet of tissue paper at the other end. A monochromatic source of wavelength 490 nm illuminates the top plate. If 20 dark bands are counted across the top plate, what is the paper thickness? (1 nm = 10 ^{- 9} m) Note that a dark band occurs where the plates contact.

A) 2.7 *10 ^{- 6} m
B) 3.4 * 10 ^{- 6} m
C) 4.7 *10 ^{- 6} m
D) 4.9*10 ^{- 6} m