The formation of interference patterns by electrons is best explained by which statement?

The very top of the film shows no reflected light, indicating that the thickness is less than 1/4 the wavelength of visible light within the film, since the phase change on reflection for the beam from the first surface of the film is π radians and the phase change on reflection from the second surface of the film is 0 radians, making the interfering rays mostly out of phase from that part of the film. given this information, what is the approximate thickness of the film at the first bright fringe counting from the top of the film, in terms of the wavelength of light in vacuum λ, and the refractive index of the film n?