A beam of light in air with refractive index \(n = 1.00\) is incident perpendicular to a silicon substrate (\(n = 3.50\)) coated with a thin dielectric film (\(n = 1.50\)). A spectrophotometer measures the reflected intensity as a function of wavelength \(\lambda\) in air, producing the graph shown with two consecutive reflection minima at \(\lambda = 700\text{ nm}\) and \(\lambda = 420\text{ nm}\). Based on the graph, what is the thickness of the dielectric film?