In a normal situation the light arriving from double slits are assumed to be of the same intensity. Constructive interference : A + A = 2A,Destructive interference :A - A = 0. The effect of CI and DI is bright and dark fringes .
In a Young's double slit expt, one light source passing a single slit and then a double slit will therefore ensure that the intensity are the same from both slits to produce observable interference.
Further from the central bright fringes , the intensity of light arriving from the two slits will not be the same as one will be closer than the other. CI and DI will not be as stated above as there will not be complete cancellation of the amplitudes from the 2 slits. The fringes will be less contrasting as the dark fringe will not be completely dark.
