In most textbook/internet explanation of light coherence, it gives two conditions 1) monochromatic light, same frequency. 2) light in phase or constant phase difference. Only with the two conditions can light interference pattern be observed in the double slit experiment. But this cannot be correct. Constructive and destructive interference can occur only if the both sources have the same polarization plane. Having the E-field plane differing by π would mean the original in phase waves to be out of phase by π. Is it because the narrow slit of Young's experiment select light with polarization planes parallel to the slit. But this has never been mentioned.