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 $0·20^\circ$. To increase . the fringe angular width to $0·21^\circ$ (with same $\lambda$ and D) the separation between the slits needs to be changed to