A Fresnel' biprism was used in an attempt to produce monochromatic light of wavelength  is used to generate an interference fringe pattern When a thin, clear glass sheet with a high refractive index is introduced, 1.50 At a thickness of 7 microns in the direction of one of the interfering beams, the pattern's centre fringe is displaced to the position of the sixth brilliant fringe. The wavelength of the monochromatic light employed is then close to: