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