In Young's double slit experiment the separation d between the slits is 2 mm, the wavelength  $\lambda$ of the light used is 5896 Å and distance D between the screen and slits is 100 cm. It is found that the angular width of the fringes is 0.20°. To increase the fringe angular width to 0.21° (with same  $\lambda$ and D) the separation between the slits needs to be changed to