A broad beam of light of wavelength 558 nm is incident at 90 on a thin, wedge-shaped film with index of refraction 1.

A broad beam of light of wavelength 558 nm is incident at 90° on a thin, wedge-shaped film with index of refraction 1.50. An observer intercepting the light transmitted by the film sees 14 bright and 13 dark fringes along the length of the film. By how much does the film thickness change over this length?