A Thin Film of Soapy Water (N = 1

A thin film of soapy water (n = 1.37) is held vertical. Light of wavelength 600 nm is reflected from the film and horizontal dark lines appear. What is the difference in thickness of the soap film from one band to the next?

A) 600 nm
B) 438 nm
C) 300 nm
D) 219 nm
E) 900 nm