We wish to construct a double-pinhole setup illuminated by a uniform, quasi-monochromatic, thermal slit source of incoherence light of mean wavelength 600 nm and width b, a distance of 1.5 m from the aperture screen. If the pinhole is 0.4 mm apart, how wide can the source be if the visibility of the fringes on the plane on observation is not less than 90%.

We wish to construct a double-pinhole setup illuminated by a uniform, quasi-monochromatic, thermal slit source of incoherence light of mean wavelength 600 nm and width b, a distance of 1.5 m from the aperture screen. If the pinhole is 0.4 mm apart, how wide can the source be if the visibility of the fringes on the plane on observation is not less than 90%.