Considering interference between two sources of intensities ' I ' and ' 4 I ', the intensity at a point where the phase difference is $\pi$ is $(\cos \pi=-1)$

The phase difference between two waves giving rise to dark fringe in Young's double slit experiment is ( n is the integer)

How is the interference pattern affected when violet light replaces sodium light?

In Fraunhofer diffraction pattern, slitwidth is 0.5 mm and screen is at 2 m away from the lens. If wavelength of light used is $5500\mathop A\limits^o$, then the distance between the first minimum on either side of the central maximum is ( $\theta$ is small and measured in radian)