A double slit is illuminated with monochromatic light of wavelength $600.0 \mathrm{~nm}$. The $m=0$ and $m=1$ bright fringes are separated by $3.0 \mathrm{~mm}$ on a screen $40.0 \mathrm{~cm}$ away from the slits. What is the separation between the slits? [Hint: Is the small angle approximation justified?]