A linear aperture whose width is $\mathrm{0.02\; cm}$ is placed immediately in front of a lens of focal length $\mathrm{60\; cm.}$ The aperture is illuminated normally by a parallel beam of wavelength $5\times {10}^{-5}\mathrm{ }\mathrm{c}\mathrm{m}$. The distance of the first dark band of the diffraction pattern from the centre of the screen is