[" Q "27/180" In a double slit experiment,two parallel "],[" slits are illuminated first by light of wavelength "400],[" nm and then by light of unknown wavelength.The "],[" fourth order dark fringe resulting from the known "],[" wavelength of light falls in the same place on the "],[" screen as the second order bright fringe from the "],[" unknown wavelength.The value of unknown "],[" wavelength of the light is "],[[900nm,700nm],[" None of these "]]