In baptism experiment, the distance between source and eyepiece is $1.2\mathrm{}\mathrm{m},$ the distance between two virtual sources is $0.84\mathrm{}\mathrm{m}\mathrm{m}.$ Then, the wavelength of light used if eyepiece is to be moved transverselythrough a distance of $2.799\mathrm{}\mathrm{c}\mathrm{m}$ to shift $30$ fringes is