(a) A beam of light consisting of two wavelengths $400\text{ nm}$ and $600\text{ nm}$ is used to illuminate a single slit of width $1\text{ mm}$. Find the least distance of the point from the central maximum where the dark fringes due to both wavelengths coincide on the screen placed $1.5\text{ m}$ from the slit. OR (b) In a Young's double-slit experimental set-up with slit separation $0.6\text{ mm}$ a beam of light consisting of two wavelengths $440\text{ nm}$ and $660\text{ nm}$ is used to obtain interference pattern on a screen kept $1.5\text{ m}$ in front of the slits. Find the least distance of the point from the central maximum where the bright fringes due to both the wavelengths coincide.