Angular Width

The angular width of central maximum of a Fraunhofer diffraction fringe formed by a narrow slit is measured. For this, the slit is illuminated by a light of wavelength 6000 Å. When the slit is illuminated by light of another wavelength, it is found that the angular width decreases by 30%. Find the wavelength of the light used in the second case. Now, in the first case, the total set-up of the experiment is immersed in a liquid. It is found that, the angular width of central maximum decreases by same amount. Calculate the refractive index of the liquid.