White light reflects at normal incidence from the top and bottom surfaces of a glass plate (n

Question:

White light reflects at normal incidence from the top and bottom surfaces of a glass plate (n = 1.52). There is air ahove and below the plate. Constructive interference is observed for light whose wavelength in air is 477.0 nm. What is the thickness of the plate if the next longer wavelength for which there is constructive interference is 540.6nm?