Two sources of light $0.6 \mathrm{~mm} apart and screen is placed at a distance of 1.2 \mathrm{~m} from them. A light of wavelength 6000\,\mathop A\limits^o used. Then the phase difference between the two light waves interfering on the screen at a point at a distance 3 \mathrm{~mm}$ from central bright band is