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 paper 5.0 × 10−⁶ m thick at the other end.The top plate is illuminated by a monochromatic light source of wavelength 399 nm.How many dark parallel bands will be evident across the top plate? (1 nm = 10−⁹ m)

A) 13
B) 23
C) 12
D) 25
E) ​21