In a biprism experiment, the fringes are observed in the focal plane of the eye piece at a distance of 1.2m from the slits. The distance between the central bright band and 20th bright band is 0.4 cm. When a convex lens is placed between biprism and the eyepiece, the distance between the two vertical magnified images is found to be 0.9 cm. Determine the wavelength of light used.