Two Flat Glass Plates Are in Contact Along One End

Multiple Choice

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